JPH09138406A - Surface light source device with polarizing function using polarization conversion element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface light source device with a polarizing function capable of obtaining bright polarization luminous flux and suitable for a back light light source of a liquid crystal device. SOLUTION: The light from a lamp L is led from a light incident surface 2 into a directional outgoing light transmission plate 1 of a wedge type section shape to be emitted from a light emission surface 5. A polarized light separating plate 8 is arranged holding an air layer AR2 therebetween, and a prism sheet 4 is arranged holding the air layer AR3 on the outside of the plate 8. The polarized light separating plate 8 transmits through the majority of a P polarization component, and returns a suitable part of an S polarization component to the light transmission plate 1. The returned light receives a polarizing scramble action in the light transmission plate 1, and is P polarized by a polarization conversion element 10, and the P polarization component of the re-emitted light from the light transmission plate 1 is taken out again to be made lighting light. A polarization prism groove is formed on the input/output surface 11 of the polarization conversion element 10, and a polarization conversion prism groove oriented in the 45 deg. oblique direction is formed on the rear surface 12. The rear surface side of the polarization conversion element may be constituted of a 1/4 wavelength plate whose polarization axis is oriented in the 45 deg. oblique direction and a reflection plate instead of the polarization conversion prism groove.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a function of generating a light beam having a certain direction of polarization from light having no polarization, or a function of generating a light beam having a high polarization from light having little polarization (hereinafter, both of them will be referred to as " It is referred to as a "polarizing function"). More specifically, the present invention relates to a side light type surface light source device with a polarization function using a combination of a directional light guide plate and a polarization conversion element. The surface light source device according to the present invention can be applied to any application requiring a polarized light beam, but is particularly advantageous when applied to a backlight light source of a liquid crystal display device.

[0002]

2. Description of the Related Art A side light type surface light source device, in which a light source element such as a cold cathode tube is arranged on the side of a light guide plate and the light guide plate surface serves as a light emitting surface, has a thin structure and a relatively large cross sectional area. It is widely used for backlights of liquid crystal displays, etc. by taking advantage of the characteristic that it has the luminous flux.

In particular, when a light-scattering light guide (an optical element in which a microscopic refractive index non-uniform structure is distributed inside a transparent optical material to provide light-scattering ability) is used as the light guide plate, a simpler structure is obtained. Thus, a sidelight type surface light source device excellent in light utilization efficiency can be configured. Further, under the condition that the size of the microscopic non-uniform refractive index structure is not excessively reduced inside the transparent optical material (for example, it does not fall below 0.06 [μm]), the light beam emitted from the light emitting surface is clearly directed. You can give them sex. A light guide plate satisfying such a condition will be referred to as a "directional emission" light guide plate. Since the mechanism of the light guide plate having the directional emission property is well known, a detailed description thereof will be omitted here (for details, see, for example, Japanese Patent Laid-Open No. 7-19).
8956).

As the directional light guide plate used in the side light type surface light source device, in addition to the light guide plate using the above-mentioned light-scattering light guide, total reflection is performed on the surface of the light guide plate made of a transparent optical material. It is known that a fine concavo-convex portion is provided for suppressing the above. Such uneven portions are formed by a method of forming a fine uneven shape on the surface of the light guide plate or a method of fixing the transparent fine particles to the smooth surface of the light guide plate with a transparent binder. Can be formed.

The preferential propagation direction (main propagation direction) of the illumination light emitted from the light emitting surface of the light guide plate having such a directional emission property.
Is generally 60 ° to 80 ° with respect to the normal to the light emitting surface.
It is inclined by about a degree (the reason and specific examples will be described later).

As is well known, such inclination can be corrected by disposing a prism sheet on the light emitting surface side of the light guide plate. There are two methods of arranging the prism sheet: the prism surface is arranged inside (the light emitting surface side, the same applies hereinafter) and the prism surface is arranged outside (the side opposite to the light emitting surface, the same applies below). .

FIG. 1 is a schematic view showing the basic structure of a surface light source device adopting the former arrangement method. To briefly explain this, reference numeral 1 is a light guide plate having a wedge-shaped cross section, and is composed of a light scattering light guide in which a modified refractive index material is uniformly mixed and dispersed in a matrix made of polymethylmethacrylate (PMMA). . An end face on the thick side of the light guide plate 1 is a light incident face 2, and a light source element (cold cathode tube) L is arranged in the vicinity thereof.

Illumination light is applied to one surface (light emitting surface) 5 of the light guide plate 1.
And is incident on the prism sheet indicated by reference numeral 4. The other surface 6 of the light guide plate 1 (hereinafter, referred to as “rear surface”) is not used for taking out illumination light, and is made of a specular reflection silver foil sheet or a diffuse reflection white sheet for the purpose of preventing light from being scattered. The reflector 3 is formed.

The prism sheet 4 is a translucent sheet in which a large number of prism rows composed of inclined surfaces 4a and 4b are formed at a fine pitch, and the other surface is a flat light extraction surface 4e. The material is usually polycarbonate. Plastics such as are used. Note that, for convenience of illustration, the interval between the prism sheet 4 and the light emitting surface 5 and the pitch of the repeating prism rows formed in the prism sheet are exaggerated in the drawings and below. When the surface light source device is used as a backlight of a liquid crystal display, a known liquid crystal display panel is arranged outside the prism sheet 4.

The surface light source device shown in FIG. 1 has a tendency that the thickness of the light guide plate 1 becomes thinner as the distance from the light incident surface 2 side increases. It has excellent characteristics in terms of utilization efficiency and brightness uniformity. Details of the effect based on the shape of the light guide plate are described in the specification, drawings, and the like attached to Japanese Patent Application No. 5-349478.

The light sent from the light source element L into the light guide plate 1 is gradually guided to the end face 7 on the thin side while being subjected to the scattering action and the reflection action in the light guide plate 1, and the light emitting face is gradually emitted. It is emitted from 5. As described above, the light emitted from the light emitting surface 5 is provided under the condition that the particle size of the modified refractive index particles mixed in and dispersed in the light guide plate 1 (generally, the correlation distance regarding the non-uniform refractive index structure) is not very small. Has a clear preferential propagation direction 5a, and its inclination angle is between about 60 degrees and 80 degrees with respect to the normal to the light emitting surface 5.

Light emitted from the light emitting surface 5 having the preferential propagation direction 5a enters from the inner side surfaces 4a and 4b of the prism sheet 4 and is emitted from the outer side surface (light extraction surface) 4e in a substantially front direction. Even when the prism surface of the prism sheet 4 is arranged outward, the preferential propagation direction can be corrected by similar reflection and refraction actions.

It can be said that the technical problem of causing the sidelight type surface light source device to emit bright and uniform illumination light in a desired direction by employing the above-mentioned structure is almost solved. However, as pointed out in the specifications attached to Japanese Patent Application Nos. 6-72746 and 6-83717, for example, when considering application to a backlight of a liquid crystal display, only such characteristics are obtained. So I'm not always satisfied.

That is, as is well known, a liquid crystal display panel of a liquid crystal display is provided with polarizing plates that allow transmission of only a polarized component in a predetermined direction with a liquid crystal layer interposed therebetween. Since the illumination light generated in is normal light with little polarization (polarization degree), about half of the light energy of the illumination light is blocked by the polarizer arranged on the light incident side of the liquid crystal layer, and the liquid crystal display Cannot contribute to the display of.

Therefore, the inventors of the present invention have proposed a surface light source device capable of generating uneven illumination light in the above-mentioned prior applications (Japanese Patent Application Nos. 6-72746 and 6-83717). In the surface light source device according to these prior applications, the light guide plate has a light-transmissive polarization separation plate arranged on the light emitting surface (light extraction surface) side, a retardation plate arranged on the back surface side, and a composite prism surface. The above problem is dealt with by imparting a polarization function to the surface light source device by means of a polarization conversion element.

[0016]

With such an improvement, it has succeeded in giving the surface light source device a polarization function to a considerable extent, but regarding the polarization performed by the polarization conversion element on the back side of the light guide plate, There is still room for improvement. That is, the above-mentioned improved surface light source device has a problem that the original polarization function of the polarization conversion element is not sufficiently exerted because the incident angle to the polarization conversion element is greatly inclined.

As described above, the preferential propagation direction of the light emitted from the back surface of the light guide plate having the directional emission property is largely inclined (about 60 to 80 degrees) from the normal direction like the emission from the light emitting surface. Therefore, it is unavoidable that the incident angle on the flat light incident surface of the polarization conversion element is also greatly inclined.

Further, in the surface light source device using the polarization conversion element having the compound prism surface, in addition to this problem, the polarization conversion efficiency due to the existence of the ineffective surface (the region having no polarization effect) in the polarization conversion element. The decrease of light and the loss of light cannot be ignored.

Therefore, a basic object of the present invention is to provide a surface light source device with a polarization function capable of achieving polarization with higher efficiency while suppressing a decrease in utilization efficiency of light energy. . In addition, the present invention intends to provide a surface light source device with a polarization function which can be advantageously applied to a backlight of a liquid crystal display device.

[0020]

According to the present invention, a light guide plate having a wedge-shaped cross section and having a directional emission property, a light supply means arranged in the vicinity of a thick end face of the light guide plate, and a light emitting surface of the light guide plate are provided. Is arranged so as to extend along the light-transmitting polarized light separating means having a reflection characteristic depending on the polarization component, and arranged so as to extend along the surface opposite to the light emitting surface of the light guide plate, A polarization conversion element that allows the light emitted from the back surface of the light guide plate to enter from the input / output surface in which the deflection prism groove is formed, converts the S polarization component into the P polarization component, and then outputs the light from the input / output surface toward the back surface of the light guide plate. The above problem is solved by configuring the surface light source device using.

The deflection prism groove formed on the input / output surface of the polarization conversion element is for converting the propagation direction of the light emitted from the back surface of the light guide plate to the thickness direction of the polarization conversion element. Are formed in parallel with the extending direction of the light incident surface of the light guide plate.

On the back side of the polarization conversion element, as means for converting the S-polarized component into the P-polarized component, (1) it should be oriented in a direction inclined by 45 degrees with respect to the extending direction of the light incident surface of the light guide plate. A number of polarization conversion prism grooves formed in
(2) A quarter-wave plate that is oriented so that its polarization axis is oriented at a direction inclined by 45 degrees with respect to the extending direction of the light incident surface of the light guide plate in order to convert the S polarization component into the P polarization component, and A reflector is provided outside the quarter wave plate.

When a light-scattering light guide is used for the light guide plate, the effective scattering irradiation parameter E [c
The value of m ⁻¹ ] is in the range of 0.5 [cm ⁻¹ ] ≦ E ≦ 50 [cm ⁻¹ ], and the correlation function γ (r) of the non-uniform refractive index structure that gives the light scattering power is γ ( r) = exp [−r / a] (where r is the distance between two points in the light scattering guide), and the value of the correlation distance a [μm] is 0.06 [μm] ≦ a ≦ 35.
It is preferably in the range of [μm]. In addition, when considering application to a backlight of a normal-sized liquid crystal display, it is in the range of 2.77 [cm ⁻¹ ] ≦ E ≦ 9.24 [cm ⁻¹ ] and 0.06 [μm ] ≦ a ≦ 7 [μm]
It is particularly preferable that it is in the range.

When it is necessary to correct the preferential propagation direction of the illuminating light, a prism sheet for modifying the preferential propagating direction of the illuminating light may be appropriately arranged outside the polarized light separating means.

The "light guide plate having a wedge-shaped cross section" referred to in the present invention is not limited to a light guide plate having a single wedge-shaped cross section, and a shape in which two wedge shapes are connected on the thin side ( It also includes a light guide plate having a connection wedge shape. In that case, there are two “thick-walled end faces”, and the light supply means is also arranged in the vicinity of each thick-walled end face, and has a so-called double-lamp type (also referred to as a dual-lamp type).

[0026]

In the surface light source device of the present invention, recycling light is polarized by a combination of a directional light guide plate, a polarization separating means having a reflection characteristic depending on a polarization component, and a polarization conversion element having a deflecting prism surface. The process is achieved with high efficiency. Therefore, it is possible to generate polarized illumination light with high energy utilization efficiency.

Further, since the outgoing light flux has a clear directivity, it is possible to generate a polarized light flux propagating in a desired direction by additionally utilizing a prism sheet utilizing a prism action. I can. Such characteristics are extremely advantageous when used as a backlight of a liquid crystal display device, which significantly improves the display quality of the liquid crystal display and significantly improves the power saving property.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is basically characterized in that a polarization conversion element having an input / output surface having a deflection prism groove for adjusting an incident angle is arranged on the back side of a light guide plate. Then, the present invention proposes two types of characteristic forms as such a polarization conversion element. In the first form of the polarization conversion element, “internal reflection of the inclined prism groove” is used for polarization conversion. Further, in the second embodiment, “the phase difference (retardation) between the polarization components during the round-trip propagation inside the phase difference plate (quarter wave plate)” is used for the polarization conversion.

Therefore, in this specification, first, the entire surface light source device will be described for the first embodiment using the polarization conversion element of the first mode, and then the polarization conversion element of the second mode will be used. The second embodiment will be described focusing on the portion related to the polarization conversion element of the second embodiment.

FIG. 2 is a sectional view showing a schematic structure of the surface light source device according to the first embodiment of the present invention. This surface light source device is an improvement of the conventional structure shown in FIG. 1 so that an efficient polarization function is exhibited, and elements common to both are designated by the same reference numerals as appropriate.

In the figure, reference numeral 1 represents a light guide plate composed of a directional emission light-scattering light guide having a wedge-shaped cross-section, and here, the material thereof is polymethylmethacrylate (PMMA; refractive index 1.492). Silicone resin material particles (made by Toshiba Silicone; Tospearl 120,
A material in which a refractive index of 1.4345) is uniformly dispersed at a ratio of 0.01 wt% is used.

Generally, when a light-scattering light guide is used for the directional emission light guide plate 1, the value of the effective scattering irradiation parameter E is in the range of 0.5 [cm ^-1 ] to 50 [cm ^-1 ]. Preferably there is.

A preferred range for giving sufficient directivity to the light guide plate 1 is that the value of the correlation distance a according to Debye's scattering theory is 0.06 [μm] or more. From the viewpoint of directivity, there is no particular upper limit of the correlation distance a, but one practical value is 35 [μm]. In addition,
The effective scattering irradiation parameter E and the correlation distance a will be supplemented later.

The size of the light guide plate 1 is 180 mm in length in the depth direction (horizontal direction in the figure) when viewed from the light source element L side and 135 mm in width, and the thickness is 2. at the end on the light incident surface 2 side. 5m
m and 0.5 mm at the end portion 7. When the wedge-shaped light guide plate 1 is used, it is practical that the angle formed by the light emitting surface 5 and the back surface 6 is about 0.5 to 6 degrees.

As the light source element 1, a straight tube type cold-cathode tube having a diameter of 2.4 mm (hereinafter, simply referred to as "lamp L") was used, and it was arranged 1 mm away from the incident surface 2. Light is incident from the lamp L to the right and directional light is extracted from the light emitting surface 5. Reference symbol R is a reflector that is appropriately arranged to increase the amount of incident light from the incident surface 2, and here, the silver foil sheet R is arranged so as to surround the light source L from the rear side.

On the back surface 6 side of the light guide plate 1, a thin air layer AR
A double-sided prism type polarization conversion element 10 is disposed with 1 interposed therebetween. The polarization conversion element 10 is a functional element having a function of receiving light emitted from the back surface 6 of the light guide plate 1 as input light and returning the polarization-converted output light toward the back surface 6 of the light guide plate 1. Details of the structure and operation will be described later.

On the other hand, a polarization separation plate 8 is arranged at a position facing the light emitting surface 5 of the light guide plate 1 with the air layer AR2 sandwiched therebetween, and the prism sheet 4 is further provided outside thereof with the air layer AR2 sandwiched therebetween.
Is arranged. Reference numeral 4e is a light extraction surface of the prism sheet 4 and also a light emission surface as a surface light source device.
When this surface light source device is used as a backlight light source of a liquid crystal display device, a liquid crystal display panel is arranged further outside the prism sheet 4.

The polarization separation plate 8 has a polarization component (P polarization component and S component).
P polarization component and S
It is an element that separates polarized components. There are several types of the polarization separation plate 8, but here, first, a plate-shaped body (optical glass; BK-7 [refractive index 1.51
63] and a thickness of 1 mm) will be described. When the refractive index n0 of the air layer AR2 is 1, the Brewster angle of the polarization separation plate 8 is 56.60 degrees. Other examples of the polarization separation plate 8 will be described later.

The detailed structure and function of the surface light source device having such a schematic structure will be described below.
For convenience of explanation, the description is divided into the following two parts. I. Polarization function based on the actions of the polarization separation plate 8 and the light guide plate 1 and the light emission direction correction function of the prism sheet II. Structure and function of the polarization conversion element 10.

[I] Polarizing function based on the action of the polarization separating plate 8 and the light guide plate 1 and the light emitting direction correcting function of the prism sheet. In FIG. 2, the emitted light of the light source L and the reflected light from the reflector R are: A structure with a non-uniform refractive index inside the light guide plate 1 entering the inside of the light guide plate 1 from the light incident surface 2 (here, modified refractive index particles)
The light is guided toward the end portion 7 of the light guide plate 1 while being scattered by. In the process, directional light is gradually emitted from the light emitting surface 5. As described above, the central angle of directivity (the emission angle of the representative ray C0) is generally in the range of about 60 degrees to about 80 degrees measured from the normal line H0 standing on the light emitting surface 5. In this example, strong outgoing light is obtained in the direction of about 65, so the outgoing angle φ of the representative ray C0 representing the outgoing light from the light emitting surface 5 is set to φ = 65 degrees.

FIG. 3 is a diagram for explaining the polarization function of the polarization separation plate 8 using the representative ray C0. As shown in the figure, the representative ray C0 emitted from the light emitting surface 5 of the light guide plate 1 goes straight through the air layer AR2 and the lower surface 8 of the polarization separation plate 8 is shown.
It is split into a light ray C1 which is incident on a and which enters the polarization separation plate 8 and a light ray C2 which is reflected and again travels to the light guide plate 1. The light ray C1 is refracted at the surface 8a, travels straight in the polarization separation plate 8, reaches the upper surface 8b of the polarization separation plate 8, and is split into an external emission light ray C3 and an internally reflected light ray C4. The internally reflected light ray C4 further returns to the lower surface 8a, and is again split into an internally reflected light ray C7 and a light ray C8 which travels to the air layer AR2. The internally reflected ray C7 is the upper surface 8b.
Then, it is again divided into an externally emitted light ray C9 and an internally reflected light ray C10. Thereafter, the same process is repeated for the internally reflected ray.

On the other hand, the light rays C2 and C8 traveling straight through the air layer AR2 toward the light guide plate 1 are the light rays C11, C13 and the reflected light rays C12, C re-incident in the light guide plate 1 at the light emitting surface 5.
Divided into 14 The reflected light rays C12 and C14 head toward the polarization separation plate 8 again and follow the same path as C0.

Through this multiple reflection / transmission process, the light energy of the light ray C0 is also divided and distributed into the reflected light rays and the transmitted light rays at the interfaces 5, 8a and 8b. The polarization component is very different. In addition, the BK having a thickness of 1 mm that constitutes the polarization separation plate 8
The internal light transmittance of the -7 plate is 99.9% or more, and its absorption loss is so small that it can be ignored. Therefore, let the ray C0 be P
Each light ray C in the case where natural light having a polarization degree of 0, which has 100 parts of the energy of the polarization component and 100 parts of the energy of the S polarization component, is used.
The amount of energy for each polarized component of 1 to C14 was calculated, and the results are also shown in the figure.

For example, when the light ray C0 is split into C1 and C2, the transmittance of the P-polarized component is extremely high and 98.8%.
However, the transmittance of the S-polarized component is only 76.6%.
Therefore, the light ray C2 is almost exclusively S-polarized light. That is, BK-7 Brewster angle 56.
It can be said that the Brewster angle condition is approximately satisfied in the range deviated from 60 degrees to 10 degrees, and the S polarization of the reflected ray C2 is extremely high (if the incident angle of C0 is the Brewster angle). If it coincides with 56.60 degrees, the S-polarization rate of the reflected ray C2 is naturally 100%).

The reflectance for each polarization component when the light ray C1 is incident on the upper surface 8b of the polarization separation plate 8 is P polarization component 1.2%, S
The polarization component becomes 23.4%. Therefore, the amount of energy of the light ray C3 emitted to the outside by polarization component is 97.
6, the S-polarized component is 58.7, and it can be seen that it has a considerable degree of polarization. Values obtained by performing the same calculation for C4 and below are shown in the figure. As is clear from these values, the energy amount of each of the P and S polarization components sharply decreases as the ray division is repeated. Therefore, the polarization components of the externally emitted light from the light rays C3 and C9 shown in FIG. Estimating the different amounts of energy, P polarization component = 97.6 and S polarization component = 61.9.

As described above, even when only the light rays shown in FIG. 3 are taken into consideration, it can be seen that a considerably polarized polarized light beam is obtained, but the light beam is returned from the polarization separation plate 8 to the light guide plate. The polarization function is further enhanced by the process in which light (hereinafter referred to as return light) is reused.

In the example of FIG. 3, the return light is almost completely S-polarized, as indicated by the numerical values attached to the rays C11 and C13. The returned light is scattered and reflected by the light guide plate 1 and polarized by the polarization conversion element 10 which will be described later, and most of the returned light is emitted from the light emitting surface 5. Considering the emission directivity at that time, the incident surface 2
It is considered that the weak directivity to the right in FIG. 3 is preserved as a whole, though not as much as when incident from
Therefore, it is considered that the behavior of the re-emitted light from the light emitting surface 5 is similar to the behavior of the representative ray C0 described above to some extent.

By the way, rays C11 (S-polarized component 18.2) and C13 (S-polarized component 10.
In 6), the polarization direction is disturbed by undergoing the processes such as scattering and reflection in the light guide plate. If this is called the polarization scrambling effect, the S polarization degree of the light emitted from the light emitting surface 5 originating from C11 or C13 is considerably lowered by this polarization scrambling effect.

If it is assumed that the polarization scrambling effect is perfect and the light is re-emitted from the light emitting surface 5 without loss,
The amount of energy is the P-polarized component 9.
1, S-polarized component 9.1, and C13-derived component,
The P-polarized component 5.3 and the S-polarized component 5.3 are obtained.

When both are added, the P-polarized component = 14.
4, S-polarized component = 14.4. If this light goes through the same history as C0, then (14.4 / 100) × 9
The energy amount of 7.6 = 14.1 is the upper surface 8 of the polarization separation plate 8.
It is added to the P-polarized component of the light flux emitted from b. Therefore,
The final amount of energy of the P-polarized component estimated approximately from this model is 97.6 + 14.1 = 11.1.
Further, the energy amount of S-polarized component is 61.9+ (61.9 /
100) × 14.1 = 70.6.

That is, a part of the light flux emitted from the light emitting surface 5 of the light guide plate 1 is S-polarized to return light, and at least a part of the light is depolarized in the light guide plate 1 to emit light. A recyclable polarization process in which the light is re-emitted from the surface 5 and is again subjected to the S-polarized light elimination action by the polarization separation plate is introduced. Therefore, as explained in the above case, at least in principle, the P-polarized component included in natural light is reduced to 1
It is also possible to achieve the effect of amplifying to more than 00%.

The enrichment action of the P-polarized component is further enhanced by the polarization conversion action of the polarization conversion element 10 described later. In the description of the first embodiment, the light guide plate 1
The (first) emitted light is represented by C0 with an emission angle of 65 degrees, but the essence of the phenomenon hardly changes even if the conditions of the emission angle change to some extent. FIG. 4 is a graph for understanding this, in which the horizontal axis represents the incident angle to the BK-7 plate (= the exit angle from the light emitting surface 5) and the vertical axis represents the single transmission of each of the P and S polarization components. (Total emission energy from 8b / total incident energy to 8a; re-incident of returning light is not considered).

As can be seen from the graph, the transmittance of the P-polarized component shows a high value over almost the entire range of 65 to 80 degrees.
Moreover, it exceeds the transmittance of the S-polarized component by about 20% or more. Therefore, it is clear that the essential part of the above description does not need to be changed even if there is some spread or deviation in the propagation direction of the light flux emitted from the light emitting surface 5.

By the way, as can be seen from the description of the polarization process, the directivity of the emitted light, which is characterized by a large emission angle (65 degrees in the above example) of the representative light beam C0, from the polarization separation means 8. The emitted light remains strong.
Therefore, when a surface light source device that emits light in the front direction or a direction close to the front direction is required, it is necessary to correct the preferential propagation direction of the emitted light from the polarization splitting means 8.

The prism sheet 4 arranged on the outer side of the polarization separation plate 8 is used when such a demand occurs, and the light exit surface 8 of the polarization separation plate 8 is subjected to the prism action.
It has a function of correcting the propagation direction of the directional light flux emitted from b to the front direction. Hereinafter, FIG. 5 (A), (B)
In addition to the reference diagram, the function of correcting the light emitting direction of the prism sheet will be described.

FIG. 5 (A) is a diagram for explaining a typical structure and arrangement of the prism sheet, and FIG. 5 (B) shows a modified arrangement, both of which are shown in FIGS. The traced paths of the light rays C3 and C9 are also shown in a sectional view in which the peripheral portions of the polarization separation plate and the prism sheet in the arrangement shown are extracted and enlarged. In both figures, the prism sheets 4 and 4'are made of, for example, polycarbonate (PC; refractive index n
pr = 1.59) and has a large number of prism surfaces 4a, 4b (vertical angle θpr) or 4'on one surface.
a, 4'b (vertical angle θ'pr) are formed. The prism forming surfaces of the prism sheets 4 and 4'are used as a light incident surface as shown in FIG. 5A or conversely used as a light extraction surface as shown in FIG. 5B.

First, referring to FIG. 5A, the polarization separation plate 8
C3 and C9, which are drawn so as to be emitted at an emission angle of 65 degrees with respect to the light extraction surface 8b, are derived from the representative ray C0 from the polarization separation plate 8 from the related description of FIG. It can be considered that the emitted light beam is approximately representative. These representative rays C3 and C9 travel straight through the air layer 9 (refractive index n0 = 1.0) and then enter the prism surface 4a of the prism sheet 4 at an angle close to vertical.

Since it can be considered that the ratio of the amount of light incident on the prism surface 4b opposite to the prism surface 4a is relatively small, the representative rays C3 and C9 travel straight to the prism surface 4b and are specularly reflected. The illumination light flux D is incident on the flat light extraction surface 4e of the sheet 4 at an angle close to vertical, and is emitted from the surface 4e at an angle close to vertical.

The inclination angle θa of the prism surface 4a on the incident side is
The light beams C3 and C9 are set so as to be incident almost vertically (here, θa = 25 degrees), and the other prism surface 4b is used.
Is set so that the internally reflected light enters the flat light extraction surface 4e almost vertically (here, θb = 65).
(Degree / 2 = 2.5 degrees), the direction of the illumination light flux D can be more accurately matched to the vertical direction. In this way, it is possible to adjust the directional characteristics of the illumination light flux D by appropriately selecting the formation angle of the prism surface.

Next, in FIG. 5B, the behavior of the representative rays C3 and C9 is depicted in a sectional view in the case where the prism sheet 4'is arranged so that its prism surfaces 4'a and 4'b face outward. Has been. Similar to the case of FIG. 5 (A), the representative rays C3 and C9 have an air layer AR2 (refractive index n0 = 1.0).
After going straight on, the light enters at a slanted angle with respect to the flat surface 4'c of the prism sheet 4'and is refracted upward, most of which is perpendicular to the opposite prism surface 4'a, 4'b. It is emitted as an illumination light flux D ′ at a close angle. The refractive index of the material forming the prism sheet 4'and the prism surface 4'a,
Depending on the values of the inclination angles θa ′ and θ′b of 4′b, an optical path that is emitted from the prism surface 4′a once into the air and then specularly reflected by the opposing prism surface 4′b and heading in the front direction is obtained. It may be used.

Thus, in the arrangement of FIG. 2, as shown in FIG.
(B) Whichever arrangement is applied, the polarization separation plate 8
By appropriately selecting the inclination angle of each prism surface and the like in relation to the directional characteristics of the light emitted from the light source and the material forming the prism sheet, the light emission direction can be controlled over a considerable range.

The prism sheets 4 and 4'are not limited to those in which the prism surfaces are formed in rows as shown in the drawing.
Other types may be used. For example, a film in which protrusions having a triangular pyramid shape or a dome shape are distributed, a plate-shaped element having columnar convex portions having a semicylindrical cross section, and the like are conceivable. It is also possible to stack and use a plurality of sheets.

There are various types of the polarization separating plate 8 other than the single transparent plate described above, and any type may be used in the present invention. Here, some examples of other types of polarization separation plates will be described (for details, the above-mentioned prior application, Japanese Patent Application No. 6-72746).
No., Japanese Patent Application No. 6-83717, see the description and drawings attached thereto).

(1) A form in which two or more transparent plates as described above are arranged in an overlapping manner. For example, two P plates with a thickness of 1 mm
MMA (polymethylmethacrylate; refractive index 1.49
2, Brewster angle = 56.17 degrees; absorption loss of internal transmission is 0.01% or less, which can be ignored as in the case of BK-7. ) Plate-shaped member of 0.5) with an appropriate spacer in between
The ones separated by mm are used as the polarization separation plate 8.

(2) A form in which the cross-sectional shape of the polarization separation plate (particularly, the light incident surface) is formed into a waveform so that the satisfaction of the Brewster angle condition at the time of light incidence is further improved.

(3) A mode in which a multilayer film made of a modified refractive index material is used as the polarization separation plate. In this embodiment, the polarization separation function will be described in some detail with reference to FIG.

FIG. 6 is a diagram for explaining the function of a polarization separation plate using a multilayer film in the same format as FIG. However, in this example, assuming that the emission angle of the emission light from the light guide plate 1 is shifted to a slightly higher angle side (sleeping direction), the state is represented by the representative ray D0 having the emission angle of 70 degrees. The description will be given assuming that the preferential propagation direction is slightly changed depending on the refractive index of the base material of the light guide plate 1 and the value of the correlation distance a.

In the multilayer film type polarization separation plate, transparent materials having different refractive indexes are used for the materials of adjacent layers,
It has a structure in which thin films of several layers (in principle, at least two layers) to several tens layers are laminated. FIG. 6 shows the cross sections of the three layers and the main optical path of the representative ray D0.

The multi-layered polarization separation plate 18 comprises a titanium dioxide (TiO2; refractive index n1 = 2.3) layer 1 in order from the incident side.
81, silicon dioxide (SiO2; refractive index n2 = 1.46)
Layer 182, titanium dioxide (TiO2; refractive index n3 = n1
= 2.3) layer 183, and has a structure in which titanium dioxide layers and silicon dioxide layers are alternately laminated according to the total number of layers. It is also possible to stack layers of materials with different refractive indices (eg zirconium dioxide ZrO2, titanium oxide TiOx with a general oxidation number).
Layers).

The principle of polarization separation of such a multilayer film type polarization separation plate 18 utilizes the difference in the reflection characteristics of the S polarization component and the P polarization component at the interface formed between the modified refractive index materials. In that respect, it is common with the above-mentioned polarization separating plate means of the types (1) and (2).

As shown in FIG. 6, the representative ray D0 representing the light flux emitted from the light guide plate 1 is represented by the air layer AR2.
(Refractive index n0 = 1.0) to S polarization component Is = 100,
The first layer 181 with the relative intensity of the P-polarized component Ip = 100
Incident on the layer 181 produces a light ray D1 and a reflected light ray D2 which enter the inside of the layer 181.

The reflectance at that time is Rs = 51.8% for the S-polarized component, but Rp for the P-polarized component.
= 0.55%, which is extremely small. That is, only about half of the S-polarized component can enter the first layer 181, but almost the entire amount of the P-polarized component passes through the interface and the first layer 181.
To enter.

When this ray D2 reaches the interface with the second layer, it again branches into rays D3 and D4. The reflectance at that time is Rs (S-polarized component) = 9.3% and Rp (P-polarized component) = 1.9%. That is, about 90% of the S-polarized light component enters the second layer 182, while most of the P-polarized light component passes through the interface and enters the second layer 182.

Similarly, at the interface between the second layer 182 and the third layer 183, D5 and D6 are branched, and the reflectance at that time is Rs (S polarization component) = 9.3% and Rp (P polarization). Component) = 1.9%.

As described above, most of the P-polarized light component is propagated to the next layer each time each interface is encountered, whereas a considerable portion of the S-polarized light component is eliminated. Assuming that the effects of multiple reflection and absorption loss are small, the polarization component when entering the third layer in this case is estimated. S polarization component; Is = 39.7, P polarization component; Ip = 95.7.
Is calculated. If the number of layers increases, each value I
It is considered that s and Ip decrease in geometric progression.

If the average common ratios at that time are rs (S-polarized light component) and rp (P-polarized light component), then rs <rp, and rp is slightly below 1. Therefore, it is considered that the “purity” of the P-polarized light component increases as the total number of layers forming the multilayer film type polarization separation plate 18 increases. Such a tendency is not limited to the case where the emission angle of the representative ray D0 is 70 degrees, but is established under a considerably wide range of angle conditions.

FIG. 7 is a graph showing an example of data demonstrating this. This graph shows TiOx on an optical glass plate BK-7 (refractive index 1.5163) with a thickness of 1 mm.
Regarding a polarization separation plate in which a multilayer film made of SiO2 (refractive index 1.46) and ZrO2 (refractive index 2.0) (having a refractive index of about 2.3) is formed by vacuum evaporation, a spectrophotometer U- manufactured by Hitachi, Ltd. 3 shows the result of measuring the polarization separation function using 3200 while changing the incident angle from 55 degrees to 70 degrees.

As can be easily read from this graph,
It can be seen that a high polarization separation function is exhibited over the entire visible light range. Further, the larger the incident angle is, the higher the polarized light separating function tends to be, but the layer 18 of the representative light beam D0.
Even if the incident angle to 1 is deviated from 70 degrees to 10 degrees,
The transmittance of the P-polarized component at each interface remains close to 100%. The transmittance of the S-polarized component is consistently lower than the transmittance of the P-polarized component. From this, it is understood that the P-polarized component purifying function of the multilayer film type polarization separating plate is not established only under special conditions.

Also in this example, the polarization scrambling effect on the return light similar to that described above and the polarization conversion function described below exhibit the enhancement effect of the P-polarized component by re-emission from the light guide plate 1. Needless to say, it is possible to correct the light emission direction by disposing the prism sheet 4 at the subsequent stage of the multilayer film type polarization separation plate 18.

[II] Polarization conversion element 10 (first mode;
Structure and Function of Polarization Conversion by Inner Surface Reflection of Inclined Prism Groove) As described in the above [I], the polarization of the illumination light is considerably increased by the cooperative action of the polarization separation plate 8 and the light guide plate 1. To be achieved. However, after a more detailed examination,
It turns out that there is still considerable room for improvement. That is, as has been clarified in the above description, the conversion of the S-polarized component, which is mostly contained in the return light, into the P-polarized component depends on the polarization scrambling effect of the light guide plate 1 and is not always sufficient. In particular, when it is attempted to meet the recent needs for thinning the surface light source device, a strong scramble effect cannot be expected.

The polarization conversion element 10 arranged along the back surface 6 of the light guide plate 1 provides a means for overcoming such a situation, and the light transmitted through the back surface 6 of the light guide plate 1 has its S polarization. It has a function of converting a component into a P-polarized component with high efficiency. Hereinafter, the structure and function of the polarization conversion element 10 of the first embodiment used in the embodiment shown in FIG. 2 will be described in detail.

As shown in FIG. 2, the polarization conversion element 10 has both front and back surfaces 11 and 12 as prism surfaces, but the orientation directions of a large number of parallel prism grooves forming both prism surfaces 11 and 12 are set. Are not parallel and cross each other. FIG. 8 is a diagram showing the alignment direction of the prism grooves when the polarization conversion element 10 is viewed from the light guide plate 1 side. In the figure, what is drawn by a solid line is a deflection prism groove formed on the input / output surface 11 facing the back surface 6. The deflection prism groove is oriented in a direction parallel to the extending direction of the lamp L (that is, the extending direction of the light incident surface 2 of the light guide plate 1).

On the other hand, what is drawn by a broken line is a polarization conversion prism groove for achieving polarization conversion, which is formed on the surface (rear surface) 12 opposite to the input / output surface. The polarization conversion prism groove is ± 45 degrees with respect to the extending direction of the lamp L (that is, the extending direction of the light incident surface 2 of the light guide plate 1) (because of the symmetry, the inclination direction can be ±). It is formed to be inclined.
In addition, in FIG. 8, the number of each prism groove is greatly reduced and shown.

Next, the deflection action of the deflection prism groove formed on the input / output surface of the polarization conversion element 10 and the polarization conversion action of the polarization conversion prism groove formed on the back side will be described with reference to FIG.
This will be described with reference to FIG. First, FIG. 9 shows the cross section of the polarization conversion element 10 by locally extracting and depicting it
The outline of the optical path of 1 and the deflection conversion process is also shown in perspective. Since the polarization conversion element 10 is for converting the S-polarized component contained in the light emitted from the rear surface of the light guide plate 1 into the P-polarized light, the S light emitted from the rear surface 6 at the emission angle β1 as the representative ray L1. Consider a polarized ray L1. Exit angle β
1 is almost the same as when emitted from the light emitting surface 5, and is generally 60
Between 80 and 80 degrees.

The light ray L1 travels straight through the air layer AR1, enters the polarization conversion element 10 from one prism surface (effective surface) 101 of the deflecting prism groove, and is reflected on the inner surface of the opposing prism surface 102 (usually the entire surface). Reflection), and the direction is changed in the thickness direction of the polarization conversion element 10 (indicated by reference numeral 121). Then, it is incident on one prism surface 111 of the polarization conversion prism groove formed at an angle of 45 degrees, and is turned to the other side (or the front side of an angle of 45 degrees) in the figure (indicated by reference numeral 122). .

The light ray L1 is further reflected by the prism surface 112 which faces the light ray L1.
Is reflected (usually total reflection), is reflected by the inner surface of one prism surface 101 of the deflection prism groove (usually total reflection),
The air escapes from the facing prism surface 112 to the air layer AR1. The output light represented by the escaped light ray L1 ′ returns to the light guide plate 2 again from the back surface 6.

The prism apex angle β 2 of the deflecting prism groove is selected so that the optical path 121 is perpendicular to the extending direction of the polarization conversion element 10 (the extending direction of the back surface 6 of the light guide plate 1). The value of the prism apex angle β 2 selected under this condition is, for example, about 65 degrees. The inclination angles of the prism surfaces 101 and 102 do not necessarily have to be designed to be equal (the shape of the prism groove may be symmetrical or asymmetrical). On the other hand, the prism apex angle of the polarization conversion prism groove is 90 degrees, and the inclination angles of the prism surfaces 111 and 112 are 45 degrees (the shape of the prism groove is symmetrical).

In this process of reciprocating between the front and back of the polarization conversion element 10, S-polarized light is converted into P-polarized light with an efficiency of almost 100%. That is, the representative light beam L1 is changed from the S-polarized state up to the optical path 121 to the P-polarized state in the optical path 122 to the P-polarized state after the optical path 122.

Actually, light whose propagation direction is largely different from that of the representative light beam L1 also enters the polarization conversion element 10 to some extent. Such light is reflected by the prism surfaces 111, 112.
And then dissipate, causing a slight loss of light. Reference numeral 3 (not shown in FIG. 2) is a reflector (for example, a silver foil) that is appropriately provided to prevent light loss due to the dissipation of such transmitted light.

FIG. 10 illustrates a part of a prism groove for polarization conversion formed on the back surface of the polarization conversion element 10 (for convenience, a "unit unit") in order to explain the polarization conversion process in more detail.
Say ) Is an enlarged perspective view of the optical path 1 in FIG.
The polarization states of 21 to 123 are also shown.

In the figure, the unit for polarization conversion is represented by a right-angled isosceles triangular prism ABCDEF, and surfaces ABEF and CDFE correspond to the prism surfaces 111 and 112, respectively. Therefore, EF represents the extending direction of the polarization conversion prism groove. As shown in FIG. 8, the extending direction of the prism groove is inclined by 45 degrees with respect to the extending direction of the deflecting prism groove on the input / output surface side (the lamp parallel direction).

The light emitted from the back surface 6 of the light guide plate 1 includes
It is considered that many S-polarized components are included and the plane of polarization thereof forms 45 degrees with the direction of the side EF. Therefore, here, the representative light ray L1 is assumed to be linearly polarized light having a polarization plane in such a direction (indicated by arrow S1). In consideration of the symmetry of the prism surface, the representative ray L1 is the surface ABCD.
It suffices to consider one that passes vertically through the plane ABCD through any point R0 above.

The representative ray L1 is from the point R0 to the prism surface A.
It is directed to the BEF, reflected at the point R1, and then changed in the propagation direction by 90 degrees to take the optical path 122. The reflection at the point R1 is the reflection from the high refractive index medium side at the interface with the air layer, and is a normal condition (refractive index of the polarization conversion element 10 is about 1.49 to 1.60).
Then it becomes total reflection. During this reflection (total reflection), the plane of polarization is twisted to become linearly polarized light having a plane of polarization in the direction indicated by arrow S2.

The light ray L1 is further directed to the prism surface CDFE, and after being reflected at the point R2, the propagation direction is changed by 90 ° and the optical path 12 is changed.
Take 3 and go to the input / output side. Optical path 123 is optical path 121
Parallel to the opposite direction. The reflection at point R2 is also
Similar to the reflection at the point R1, it is a total reflection under normal conditions. Further, during this reflection (total reflection), the plane of polarization is further twisted to become linearly polarized light having a plane of polarization in the direction indicated by arrow S3.

Thus, through the process of internal reflection on the surface on which the polarization conversion prism groove is formed, the S-polarized light beam L1
The input light, which is represented by
It is converted into polarized light L1 '. Alphabet "G" -shaped patterns 100, 100 ', 100 "express the process of twisting of the polarization plane. The polarization plane of the light beam L1' which has been polarization-converted once is the prism plane of the deflection prism groove. It is preserved at the time of reflection and transmission of 101 and 102,
The P-polarized light beam L1 ′ reenters the light guide plate 1 from the back surface 6.

As a result, the ratio of the S-polarized component is reduced in the light guide plate 1. Accordingly, the proportion of the P-polarized component contained in the light emitted from the light emitting surface 3 is increased, and the polarization separation plate 8,
The amount of P-polarized light that becomes illumination light through the prism sheet 4 increases. That is, the polarization conversion element 10 includes the polarization separation plate 8
From the rear surface 6 to the light guide plate 1 is a significant part of the S polarization component (a portion emitted from the back surface 6) that is P-polarized and then extracted as illumination light (P polarization recycling process). It has an effect.

Here, it should be noted that the efficiency of the polarization conversion is high because almost all the light emitted from the back surface of the light guide plate 1 is effectively incident on the polarization conversion element 10 and is subjected to the polarization conversion action. In addition, the loss of light energy is very small on both the input / output surface and the polarization conversion surface (the difference from the inventions according to Japanese Patent Application Nos. 6-72746 and 6-83717).

Next, with reference to FIG. 11, a second embodiment of the present invention will be described. FIG. 11 is a sectional view showing the schematic structure of the surface light source device according to the second embodiment in the same format as FIG. As described above, in the second embodiment, "the phase difference (retardation) between the polarization components generated during the round trip propagation inside the phase difference plate (quarter wave plate)" is used for the polarization conversion. The feature is that a polarization conversion element is used.

In the surface light source device of the second embodiment shown in FIG. 11, except that the polarization conversion element 50 of the second embodiment is used in place of the polarization conversion element 10 of the first embodiment,
The structures and arrangements of the light guide plate 1, the polarization separation plate 8 and the prism sheet 4 are all the same as those of the surface light source device of the first embodiment shown in FIG. Therefore, the description of the first embodiment can be applied to this embodiment except for the detailed description of the polarization conversion element. Therefore, here, as an alternative to the above term [II], the following term [II ′]
, Only the explanation related to the polarization conversion element 50 is provided,
Repeated description of other parts is omitted.

[II '] Structure and function of polarization conversion element 50 (second mode; polarization conversion by phase difference between polarization components generated during reciprocal propagation inside retardation plate (quarter wave plate)) As shown in FIG. 11, the polarization conversion element 50 arranged along the back surface 6 of the light guide plate 1 in the second embodiment transmits the back surface 6 of the light guide plate 1 as in the case of the first embodiment. It has a function of efficiently converting the S-polarized component contained in a large amount of light into the P-polarized component. As schematically shown in FIG. 11, the input / output surface of the polarization conversion element 50 is a prism surface in which a large number of deflection prism grooves are formed as in the case of the first embodiment.

On the other hand, on the back side of the polarization conversion element 50, a quarter-wave plate 52 covered with a reflection plate (or a reflection layer; the same applies hereinafter) 53 is provided. The quarter-wave plate 52 has a well-known structure and function, and when paired with the reflection plate 53, it functions as a half-wave plate using a round-trip optical path. The optical axis of the wave plate 52 is inclined ± 45 degrees with respect to the extending direction of the deflection prism groove on the input / output surface side.

FIG. 12 shows the alignment direction of the prism grooves and the optical axis when the polarization conversion element 50 is viewed from the light guide plate 1 side in the same format as in FIG. In the figure, what is drawn by a solid line is a deflection prism groove for deflecting the propagation direction of light at the time of input / output. The deflection prism groove is formed by the light guide plate 1.
The lamp L is oriented in a direction parallel to the extending direction of the lamp L (that is, the extending direction of the light incident surface 2 of the light guide plate 1) formed on the surface facing the back surface 6.

On the other hand, in the quarter-wave plate 52 for performing polarization conversion, the direction of its optical axis is the extending direction of the lamp L as shown by the broken line (that is, the light incident surface 2 of the light guide plate 1). It is arranged so as to have a direction inclined by ± 45 degrees with respect to the (extending direction of) (because of the symmetry, the inclination directions can be ±).

Next, the deflection action of the deflection prism groove formed on the input / output surface of the polarization conversion element 50 and the polarization conversion action of the quarter wavelength plate will be described with reference to FIG. FIG. 13 is a cross sectional view of the polarization conversion element 50, which is locally extracted and drawn, and the optical path of the representative light beam L1 and the outline of the polarization conversion process are also shown in perspective. As the representative light ray L1, consider the S-polarized light ray L1 emitted from the back surface 6 at the emission angle β1 as in the case of the polarization conversion element 10. Exit angle β1
Is almost the same as when the light is emitted from the light emitting surface 5, and is generally 6
It is between 0 and 80 degrees.

The deflection action of the deflection prism groove formed on the input / output surface 51 and the polarization conversion action by the quarter wavelength plate will be described with reference to FIG. The deflection effect of the deflection prism groove formed on the input / output surface 51 is the same as that of the polarization conversion element 1.
It is similar to the case of 0.

That is, the light ray L1 goes straight through the air layer AR1,
The prism surface 5 that enters the polarization conversion element 50 from one prism surface (effective surface) 501 of the deflection prism groove and faces the polarization conversion element 50.
The light is reflected by the inner surface of 02 (usually total reflection) and is turned in the thickness direction of the polarization conversion element 50 (indicated by reference numeral 521). Then, it is incident on the quarter-wave plate 52 (reference numeral 52
2), and is reflected at the interface with the reflection plate 53 to change the direction by 180 degrees (shown by reference numeral 523).

The light ray L1 is reflected again by the prism surface 502 (usually total reflection), and escapes from the same prism surface 501 as when entering the deflecting prism to the air layer AR1. The output light represented by the escaped light ray L1 ′ returns to the light guide plate 2 again from the back surface 6.

The prism apex angle β 3 of the deflecting prism groove is selected so that the optical path 521 is perpendicular to the extending direction of the polarization conversion element 50 (the extending direction of the back surface 6 of the light guide plate 1). The value of the prism apex angle β3 selected under this condition is, for example, about 65 degrees. The prism surfaces 501 and 502 do not have to have the same inclination angle (the shape of the prism groove is
Both symmetrical and asymmetrical are possible).

In this process of reciprocating between the front and back of the polarization conversion element 50, S polarized light is converted into P polarized light with an efficiency of almost 100%. That is, the optical path 522 after entering the quarter-wave plate 52
In, the conversion from the S-polarized state up to the optical path 521 to the P-polarized state progresses, and after the optical path 523, it becomes almost complete P-polarized light. The polarization plane of the light beam L1 ′ which has been polarization-converted once is preserved when reflected and transmitted by the prism surfaces 501 and 502 of the deflection prism groove, so that the P-polarized light beam L1 ′ is converted into the back surface 6
Will re-enter the light guide plate 1.

As a result, as in the case of the first embodiment, the ratio of the S-polarized component in the light guide plate 1 is reduced (toward 50%), and the P-polarized component of the light emitted from the light emitting surface 3 is reduced. The polarization separation plate 8 and the prism sheet 4 accordingly.
The P-polarized light component that becomes the illumination light increases. That is, the polarization conversion element 50 makes it possible to easily P-polarize a considerable portion (a portion emitted from the back surface 6) of the S-polarized component included in the return light from the polarization separation 8 to the light guide plate 1 and reuse it as illumination light. There is a remarkable function to do.

The first using the polarization conversion element 10
In the same manner as in the above embodiment, almost all of the light emitted from the back surface of the light guide plate 1 is effectively incident on the polarization conversion element 10 and is subjected to the polarization conversion action. , The loss of light energy is extremely small on any polarization conversion surface (Japanese Patent Application No. 6-72746,
Difference from the invention of Japanese Patent Application No. 6-83717).

By the way, as already mentioned, as a typical example in which the characteristics of the surface light source device with a polarization function of the present invention are utilized very effectively, there is application to a backlight of a liquid crystal display device. That is, by applying the surface light source device with a polarization function of the present invention to the backlight of the liquid crystal display device, only by arranging the polarization direction of the light flux emitted from the backlight light source and the polarization axis direction of the polarizing plate to be parallel, The proportion of light energy that effectively contributes to display can be significantly improved.

FIG. 14 is an element exploded perspective view showing the basic arrangement when the surface light source device with a polarization function according to the present invention is used as a backlight of a liquid crystal display device. In this example, the first or second backlight is used as the backlight.
The same type of surface light source device with a polarization function as that shown in the embodiment (see FIGS. 2 and 11) is used, and common elements are designated by the same reference numerals.

That is, reference numeral 1 is a light guide plate made of a light-scattering light guide having a directional emission property having a wedge-shaped cross section, which is used in the first and second embodiments. Light guide plate 1
The size is designed according to the size of the liquid crystal cell used. Here, the horizontal length in the figure is 180 mm and the width is 135 mm.

As described above, generally, when a light scattering guide is used for the directional light guide plate 1, the preferable ranges of the effective scattering irradiation parameter E and the correlation distance a are each 0.
5 [cm ^-1 ] ≤E≤50 [cm ^-1 ], 0.06 [μm] ≤a
≦ 35 [μm]. Further, when considering the application to a liquid crystal display of a notebook personal computer of a popular size, 2.77 [cm ⁻¹ ] ≦ E ≦ 9.
Particularly preferable results are obtained at 24 [cm ⁻¹ ] and 0.06 [μm] ≦ a ≦ 7 [μm].

Lamp L provided with a silver foil sheet R on the back side
The light that enters from the right side is extracted as directional light from the light emitting surface 5. On the back surface 6 of the light guide plate 1, the polarization conversion element 10 or 50 described above is arranged. Reference numeral 8 denotes a polarization separation plate 8 arranged along the light emitting surface 5 of the light guide plate 1, and the prism sheet 4 is provided outside thereof.
Is arranged. The polarization separation plate 8 is of any type described above. Further, the prism sheet 4 made of polycarbonate (PC; refractive index 1.59) was used, and the prism forming surface was arranged so as to face the polarization separation plate 8.

A liquid crystal display panel including two polarizing plates 60 and 80 and a liquid crystal cell 70 disposed between the polarizing plates 60 and 80 is provided on the light emitting side of the backlight which is composed of the surface light source device with a polarization function including these elements. Will be placed. Polarizer 6 on the light incident side
The direction of the transmission polarization axis of 0 is set to face the horizontal direction in the figure, while the direction of the transmission polarization axis of the polarizing plate 80 on the light emission side is set to face the vertical direction in the figure.

As already described in detail, the light source L, the light guide plate 1, the reflector R, the polarization separation plate 8 and the prism sheet 4,
Since the light beam emitted from the surface light source device with polarization function composed of the polarization conversion element 10 or 50 is strongly P-polarized, the main polarization axis is oriented in the horizontal direction in the figure under the arrangement condition shown. Become. Therefore, the proportion of the amount of light that passes through the polarizing plate 60 in the light that has entered the polarizing plate 60 as the backlight luminous flux is large, and a light transmittance of more than 50% is ensured.

In order to give symmetry to the visual characteristics of the liquid crystal display, the directions of the polarization transmission axes of the polarizing plates 60 and 80 are inclined with respect to the vertical or horizontal direction of the frame on the display surface (usually 45 degrees). Techniques for making them known are known. When this technique is applied, a half-wave plate 90 (illustrated by a broken line) whose optical axis is inclined by a predetermined angle with respect to the polarization axis of the illumination light is provided between the prism sheet 4 and the polarizing plate 60. If further arranged, the direction of the polarization axis of the illumination light can be matched with the direction of the polarization transmission axis of the polarizing plate 60 by the well-known function of the half-wave plate 90. When the inclination angle of the polarization transmission axis of the polarizing plate 60 is 45 degrees, the inclination angle of the optical axis of the half-wave plate 90 is half, that is, 22.5 degrees.

In the above description of the embodiments, it is assumed that a rod-shaped fluorescent lamp is used as the light source L, but the light source in the present invention may be any light supply means in a broad sense. It is not always necessary for the device itself to have a light emitting ability. For example, it may be the emitting end of an optical fiber bundle coupled to another light emitting element. The polarization property is not particularly limited, and even when light having a specific polarization property such as light derived from laser oscillation is supplied, the essential polarization effect itself of the present invention is not impaired.

As described above, the "light guide plate having a wedge-shaped cross section" according to the present invention also includes a light guide plate having a shape in which two wedge shapes are connected on the thin side (connection wedge shape).
The light guide plate 1 in each of the above-described embodiments is replaced with such a light guide plate having a connection wedge shape, and 1 is provided near each thick end face (light incident face).
By arranging the lamps L one by one, a so-called dual lamp type surface light source device is configured. It will be apparent from the above description that the characteristic action of the present invention (the P-polarization recycling process) is also exerted in such a dual-lamp type surface light source device.

Next, when a light-scattering light guide is used as the material of the light guide plate 1, the effective scattering irradiation parameter E and the correlation distance a, which are index values for describing the scattering characteristics, are Deby.
The theory of e will be cited and explained, and further its directional emission property will be briefly explained.

Now, the light of intensity I0 propagates in y (cm) through the light scattering guide made of a medium in which a nonuniform refractive index structure is uniformly formed in a base material having a constant refractive index, and during that time, Considering the case where the intensity is attenuated to I by the scattering of, the effective scattering irradiation parameter E is defined by the following equation (1) or (2).

[0124]

(Equation 1) The above expressions (1) and (2) are expressions of so-called integral type and differential type, respectively, and their physical meanings are equivalent. Note that this E
Is sometimes called turbidity. On the other hand, the scattered light intensity when light scattering occurs due to the non-uniform structure distributed in the medium is as follows in the usual case where most of the emitted light is vertically polarized light with respect to vertically polarized incident light (VV scattering). It is expressed by equation (3).

[0125]

(Equation 2) It is known that, when natural light is incident, the following equation obtained by multiplying the right side of equation (3) by (1 + cos ² Φ) / 2 may be considered in consideration of Hh scattering.

[0126]

(Equation 3) Where λ0 is the wavelength of the incident light, ν = (2πn) / λ0,
s = 2 sin (Φ / 2), n is the refractive index of the medium, Φ is the scattering angle, <η ² > is the mean square of the dielectric constant fluctuation in the medium (hereinafter,
<Η ² > = τ, and τ is appropriately used. ), And γ
(R) is called a correlation function and is expressed by the following equation (6).

According to Debye's theory, when the refractive index nonuniform structure of the medium has an interface and is divided into A phase and B phase and dispersed, the correlation function γ is related to the fluctuation of the dielectric constant.
The following relational expressions (7) and (8) hold for (r), the correlation distance a, the dielectric constant fluctuation root mean square τ, and the like.

[0128]

(Equation 4) Assuming that the non-uniform structure is composed of spherical interfaces of radius R, the correlation distance a is expressed by the following equation.

[0129]

(Equation 5) Using the equation (6) for the correlation function γ (r), the equation (5)
The effective scattering irradiation parameter E when the natural light is made incident on the medium is calculated based on the above, and the result is as follows.

[0130]

(Equation 6) FIG. 15 shows a curve representing a condition for keeping the effective scattering irradiation parameter E constant, where E is 50 [cm ⁻¹ ] and E =, where the horizontal axis is the correlation distance a and the vertical axis is the dielectric constant fluctuation root mean square τ. 100
The drawing is for the case of [cm ^-1 ]. The value of E is an index that is a measure of the "strength" of the scattering power of the scattering light guide medium. The larger the value of E, the stronger the scattering power, and the smaller the value of E, the weaker the scattering power (close to transparent). The tendency arises.
E = 0 [cm ^-1 ] corresponds to the non-scattering state. Therefore,
There is a general theory that a light-scattering light guide with a small E is suitable for the application of a large-sized surface light source with uniform brightness, and a light-scattering light-guide with a large E is suitable for the application of a small-sized surface light source. To establish.
The range of E described above (0.5 [cm ^-1 ] ≤E≤50 [cm ^-1 ]
Alternatively, 2.77 [cm ⁻¹ ] ≦ E ≦ 9.24 [cm ⁻¹ ] takes such a viewpoint into consideration.

On the other hand, the correlation distance a is an amount that is deeply related to the direction characteristic of scattered light in each scattering phenomenon inside the light scattering guide. That is, the above equations (3) to (5)
As inferred from the form of the formula, light scattering inside the light scattering guide generally has a forward scattering property, but the strength of the forward scattering property changes depending on the correlation distance a.

FIG. 16 is a graph illustrating this with respect to two values of a. In the figure, the horizontal axis is the scattering angle Φ
(The traveling direction of the incident ray is Φ = 0 degree.)
The vertical axis represents the scattered light intensity assuming natural light, that is, the value obtained by normalizing the above equation (5) with respect to Φ = 0 degree, Vvh (Φ)
/ Vvh (0) is represented. As also shown in the figure, in the case of a = 0.13 [μm] and 2R = 0.2 [μm] converted to the particle size using the above (9), the normalized scattering intensity of The graph shows a gradual decrease function with respect to Φ. The graph is a function that sharply decreases in the range where Φ is small.

As described above, the scattering generated by the non-uniform refractive index structure in the light-scattering light guide basically shows a forward scattering property, and when the value of the correlation distance a becomes small, the forward scattering property becomes weak. The scattering angle range in scattering tends to be widened. This fact itself has been confirmed experimentally.

The above is a discussion focusing on the individual scattering phenomenon itself due to the non-uniform refractive index structure distributed inside the light-scattering light guide, but the light actually emitted from the light extraction surface of the light-scattering light guide is When evaluating the directional characteristics, the phenomenon of total internal reflection of light incident on the light extraction surface from the inside of the light-scattering light guide and the interface transmittance (escape rate from the light-scattering light guide) at the time of light emission are evaluated. It needs to be considered together.

As is well known from the basic optical theory, the light is guided from the inner side of the light scattering guide having a relatively large refractive index to the light extraction surface to the outside medium (air). Incidence angle α (Here, the direction of a normal line that is erected toward the inside of the light scattering guide with respect to the light extraction surface is α = 0 degree)
However, when the angle exceeds the critical angle αc determined by the refractive index of the medium inside and outside the light scattering guide, the emission (light escape) to the outside (air layer) does not occur.

PMMA (polymethylmethacrylate; which is a typical material of the light-scattering light guide used in the present invention;
With a refractive index of 1.492), α c = 42 degrees.

As will be described later, the refractive index of the resin material suitably used as the matrix of the light-scattering light guide in the present invention is in the range of 1.4 to 1.7, and the corresponding critical angle α c Is in the range of 36.0 degrees to 45.6 degrees.

Therefore, in the case where the light incident surface is provided on the side of the light extraction surface as in the present invention (see each embodiment described later), the light incident from the light incident surface encounters the non-uniform structure and is generated. It is extremely unlikely that the primary scattered light immediately satisfies the critical angle condition and is emitted to the outside from the light extraction surface.

That is, under the conditions premised on the present invention, the effect of multiple scattering inside the light-scattering light guide body, the light extraction surface side and the back surface side interface of the light-scattering light guide body, or the surface facing the interface are arranged. In addition, as a result of the combined effect of the reflection by the reflection member (in the present invention, the reflection type polarization conversion means is arranged. The details will be described later), the light which satisfies the above-mentioned critical angle condition. It can be considered that the phenomenon that the light is emitted to the outside largely contributes to the light emission from the light extraction surface.

If so, when considering light propagating in the direction of the light extraction surface under the condition that the critical angle condition is satisfied, the above-mentioned action that preserves the propagation directionality of the light at the time of incidence from the light incident surface as a whole. The effect of the forward scattering property should be considerably diminished by the combined effect, and the distribution of the light in the propagation direction should be considerably spread.
As a result, the directional characteristics of the light emitted from the light-scattering light guide are largely influenced by the angular dependence of the interfacial transmittance (escape ratio) at the light extraction surface of the light that satisfies the critical angle condition.

Generally, the interfacial transmittance is extremely low under the condition that the critical angle condition (α <αc) is barely satisfied (for example, in the case of the acrylic resin-air interface, the P-polarized component is about 40% and the S-polarized component is 20%. If the incident angle α falls below the critical angle α c, the interfacial transmittance rises sharply and becomes almost constant under the condition of 5 ° to 10 ° or less (in the case of the acrylic resin-air interface, P-polarized light is used). 90% of ingredient
As above, the S-polarized component is 85% or more).

From the above, in the case of acrylic resin (αc = about 42 degrees), the light incident from the inside of the light-scattering light guide to the light extraction surface at an incident angle α = 35 ° to 40 ° is detected. ,
It is considered that it contributes most to the light emission from the light extraction surface of the light scattering guide. The light incident on the light extraction surface at the incident angle of α = 35 degrees to 40 degrees is refracted on the light extraction surface according to Snell's law, and is 60 degrees to the normal line set to the outside on the light extraction surface. Direction within 80 degrees (that is,
The light is emitted in a direction in which it rises about 20 to 30 degrees with respect to the surface of the light extraction surface.

Graph of FIG. 17 In a measurement example demonstrating this,
With respect to the light guide plate 1 used in the first and second embodiments, the results obtained by measuring the intensity of light emitted from the light emitting surface 5 for each polarization component are shown. For the measurement, a luminance meter (LS110 manufactured by Minolta; measurement viewing angle 1/3 degree, with a close-up lens attached) is used under the condition that the center point P (see FIG. 2) of the light emitting surface 5 is always viewed from the distance 203 mm with the line of sight b. Line of sight b
Was carried out while swirling and scanning in the direction perpendicular to the lamp L. Also, plotted on the vertical axis of the graph is COS correction (for compensating for the effect of changing the area of the light-emitting surface that is subject to photometry by scanning φ with 1 / COS φ being a factor). It is the luminance value of the P-polarized component and the S-polarized component after correction).

From this graph, the angles giving the peaks of the luminance values are different by several degrees between the P-polarized component and the S-polarized component, but both show clear peaks in the range of 60 to 80 degrees. The same fact is not limited to this case using the light-scattering light guide body, but is a matter that is confirmed by a general light guide plate having an emission directivity.

It should be noted, however, that when a light scattering guide is used for the light guide plate, if the value of the correlation distance a becomes too small, the forward scattering property of each scattering itself becomes weak, and This directivity is blurred because scattered light in a wide angle range including backscattering is generated only by scattering.

The "directional emission" light guide plate used in the present invention has such a characteristic that such a phenomenon is not significantly exhibited, and the above-mentioned range (0 .06 [μm] ≦ a ≦ 35 [μm] or 0.06 [μm] ≦ a ≦ 7 [μm]), this condition is taken into consideration. When a light scattering light guide in which modified refractive index particles are uniformly dispersed in a polymer matrix is used, the above correlation distance range (0.06 [μm] ≦ a ≦ 35 [μm]) is converted to a range of modified refractive index particle diameters, 0.1 [μm] to 54.
[Μm].

Finally, in the present invention, the material and manufacturing method of the light scattering guide used as the light guide plate will be described. Various polymer materials can be used as the base of the light-scattering light guide that is preferably used as the material of the light guide plate in the present invention. Representatives of these polymers are shown in Tables 1 and 2 below. The light-scattering light guide body based on such a polymer material can be manufactured by the following method.

[0148]

[Table 1]

[0149]

[Table 2] First, one of them is a method utilizing a molding process including a step of kneading two or more kinds of polymers. That is, 2
Polymer materials having different refractive indices of more than one kind (arbitrary shapes are acceptable. Industrially, for example, pellets are conceivable.) Are mixed and kneaded (kneading step),
By injecting the kneaded liquid material into the mold of the injection molding machine at high pressure, and cooling and solidifying the molded light guide plate, the light guide plate with a shape corresponding to the mold shape can be obtained by taking it out of the mold. I can.

Since the kneaded polymers of two or more different refractive indexes are solidified without being completely mixed, nonuniformity (fluctuation) is generated in their local concentration and is fixed,
Uniform scattering power is given. In addition, if the kneaded material is injected into the cylinder of an extrusion molding machine and extruded in a usual manner, the desired molded product can be obtained.

A wide variety of combinations and mixing ratios of these polymer blends can be selected, and the difference in refractive index, the strength and properties of the non-uniform refractive index structure generated in the molding process (scattering irradiation parameter E, correlation distance) a, dielectric constant fluctuation root mean square τ, etc.) may be taken into consideration. In addition,
Typical polymer materials that can be used are as shown in Tables 1 and 2 above.

Another method of manufacturing the material forming the light-scattering light guide is different in the refractive index (0.00
One or more refractive index difference) A particulate material is uniformly mixed and dispersed. Then, one of the methods that can be used for uniformly mixing the particulate material is a method called a suspension polymerization method. That is, when the particulate material is mixed in the monomer and the polymerization reaction is carried out in a state of being suspended in hot water, a polymer material in which the particulate material is uniformly mixed can be obtained.
When this is used as a raw material and molding is performed, a light-scattering light guide having a desired shape is manufactured.

Further, suspension polymerization is carried out for various combinations of particulate materials and monomers (combination of particle concentration, particle diameter, refractive index, etc.), plural kinds of materials are prepared, and these are selectively blended. Then, the light-scattering light guide having various characteristics can be manufactured. Also,
By blending a polymer containing no particulate material, the particle concentration can be easily controlled.

Another method that can be used to uniformly mix the particulate material is to knead the polymer material and the particulate material. Also in this case, kneading / molding (pelletizing) is performed by combining various particulate materials and polymers (combination of particle concentration, particle size, refractive index, etc.), and these are selectively blended to conduct light scattering. By forming and manufacturing a light body, a light-scattering light guide having various characteristics can be obtained.

It is also possible to combine the above-mentioned polymer blending method and the particulate material mixing method. For example, it is conceivable to mix a particulate material during blending / kneading of polymers having different refractive indexes.

[0156]

According to the surface light source device with a polarization function of the present invention, a light guide plate having a directional emission property, a polarization separation means having a reflection characteristic depending on a polarization component, and a polarization conversion element having a deflection prism surface are combined. The construction achieves a highly efficient recycling polarization process and enables the production of polarized illumination light with high energy utilization efficiency.

Further, according to the surface light source device with a polarization function according to the present invention, since clear directivity is preserved in the emitted light beam, it is possible to additionally use the prism sheet utilizing the prism action. It is possible to generate a polarized light beam that propagates in a desired direction.

Such characteristics are extremely advantageous when the surface light source device with polarization function of the present invention is used as a backlight of a liquid crystal display device, and the display quality of the liquid crystal display device is remarkably improved, and power consumption is reduced. It is sufficient to significantly improve the sex.

[Brief description of the drawings]

FIG. 1 is a sketch diagram illustrating a basic configuration of a conventional side light type surface light source device.

FIG. 2 is a sectional view showing a schematic structure of a surface light source device according to a first embodiment of the present invention.

FIG. 3 is a diagram illustrating a polarization function of a polarization separation plate 8.

FIG. 4 is a graph in which the horizontal axis represents the incident angle to the BK-7 plate and the vertical axis represents the transmittance of the P and S polarization components for each single transmission.

FIG. 5A is a diagram illustrating a typical structure and arrangement of a light emitting direction correction element, and FIG. 5B is a diagram showing a modified arrangement.

FIG. 6 is a diagram for explaining the function of a polarization separation plate using a multilayer film in the same format as FIG.

FIG. 7: Ti is formed on an optical glass plate BK-7 having a thickness of 1 mm.
6 is a graph showing the results of measuring the polarization separation function of a polarization separation plate formed by vacuum deposition of a multilayer film of Ox, SiO2 and ZrO2 while changing the incident angle from 55 degrees to 70 degrees.

FIG. 8 is a diagram showing an alignment direction of prism grooves when the polarization conversion element 10 is viewed from the light guide plate 1 side.

FIG. 9 is a diagram in which a cross section of the polarization conversion element 10 is locally extracted and drawn, and the optical path of the representative light beam L1 and the outline of the polarization conversion process are also shown in perspective.

FIG. 10 is a diagram for explaining the polarization conversion process in more detail.
A part (unit unit) of a prism groove for polarization conversion formed on the back surface of the polarization conversion element 10 is enlarged and drawn in perspective.
The deflection states of the optical paths 121 to 123 in FIG. 9 are also shown.

FIG. 11 is a cross-sectional view showing a schematic structure of a surface light source device according to a second embodiment in the same format as FIG.

FIG. 12 shows the orientation of the prism grooves and the optical axis when the polarization conversion element 50 is viewed from the light guide plate 1 side in the same format as in FIG.

FIG. 13 is a diagram in which the cross section of the polarization conversion element 50 is locally extracted and drawn, and the optical path of the representative light beam L1 and the outline of the polarization conversion process are also shown together in perspective.

FIG. 14 is an element exploded perspective view showing a basic arrangement when the surface light source device with a polarization function according to the present invention is used as a backlight of a liquid crystal display device.

FIG. 15 is a curve showing the conditions for keeping the effective scattering irradiation parameter E constant, where the horizontal axis is the correlation distance a and the vertical axis is the mean square τ of the dielectric constant fluctuation, and E = 50 [cm ⁻¹ ] and E = 100
The drawing is for the case of [cm ^-1 ].

FIG. 16 is a graph illustrating that the intensity of the forward scattering property of the light scattering guide changes with the correlation distance a.

FIG. 17 is a graph showing the intensity of light emitted from the light emitting surface 5 for each polarization component of the light guide plate 1 used in the first and second embodiments.

[Explanation of symbols]

1 light guide plate 2 light incident surface 3 reflector (silver foil) 4 prism sheet 4a, 4b prism surface of prism sheet 4 4'a, 4'b prism surface of prism sheet 4 '4e flat surface of prism sheet 4 4'c prism Flat surface of sheet 4'5 Light emitting surface 6 Back surface of light guide plate 7 End portion of light guide plate 8 Polarization separation plate 8a Light incident surface of polarization separation plate 8b Light emission surface of polarization separation plate 10,50 Polarization conversion element 11 Polarization Input / output surface of the conversion element 10 12 Back surface of the polarization conversion element 52 Quarter wave plate 53 Reflector plate (reflection layer) 101, 102, 111, 112 Prism surface 501, 502 of the polarization conversion element 10 of the polarization conversion element 50 Prism surface 121,122,123 Optical path in polarization conversion element 10 521,522,523 Optical path in polarization conversion element 50 AR1 to AR3 Air layer L Light source element (cold cathode) ; Fluorescent lamp) central point P the light guide plate 1 R reflector (silver foil)

Claims (9)

[Claims] 1. A light guide plate having a wedge-shaped cross section and having a directional emission property, a light supply means arranged in the vicinity of a thick end face of the light guide plate, and a light guide plate extending along a light emitting surface of the light guide plate. A light-transmitting polarized light separating means having a reflection characteristic depending on a polarization component, and arranged so as to extend along the surface of the light guide plate opposite to the light emitting surface, and from the back surface of the light guide plate. Including a polarization conversion element that makes the output light of the input light incident from the input / output surface to convert the S polarization component to the P polarization component and then outputs the light from the input / output surface toward the back surface of the light guide plate. On the output surface, a large number of deflection prism grooves for converting the propagation direction of the light emitted from the back surface of the light guide plate into the thickness direction of the polarization conversion element are parallel to the extending direction of the light entrance surface of the light guide plate. Formed surface with polarization function using polarization conversion element Source apparatus. 2. A light guide plate having a wedge-shaped cross section and having a directional emission property, a light supply means arranged in the vicinity of a thick end face of the light guide plate, and so as to extend along a light emitting surface of the light guide plate. A light-transmitting polarized light separating means having a reflection characteristic depending on a polarization component, and arranged so as to extend along the surface of the light guide plate opposite to the light emitting surface, and from the back surface of the light guide plate. Including a polarization conversion element that makes the output light of the input light incident from the input / output surface to convert the S polarization component to the P polarization component and then outputs the light from the input / output surface toward the back surface of the light guide plate. On the output surface, a large number of deflection prism grooves for converting the propagation direction of the light emitted from the back surface of the light guide plate into the thickness direction of the polarization conversion element are parallel to the extending direction of the light entrance surface of the light guide plate. S polarization is formed on the back side of the polarization conversion element. A plurality of polarization conversion prism grooves for converting the light into P polarization components are formed in a direction inclined by 45 degrees with respect to the extending direction of the light incident surface of the light guide plate. Surface light source device. 3. A light guide plate having a wedge-shaped cross section and having a directional emission property, a light supply means arranged in the vicinity of a thick end face of the light guide plate, and a light guide plate extending along a light emitting surface of the light guide plate. A light-transmitting polarized light separating means having a reflection characteristic depending on a polarization component, and arranged so as to extend along the surface of the light guide plate opposite to the light emitting surface, and from the back surface of the light guide plate. Of the output light of which is incident from the input / output surface to convert the S-polarized component into the P-polarized component, and then output from the input / output surface toward the back surface of the light guide plate, and the back surface side of the polarization conversion element. A plurality of deflection prism grooves for converting the propagation direction of the light emitted from the back surface of the light guide plate into the thickness direction of the polarization conversion element on the input / output surface of the polarization conversion element. Is formed parallel to the extending direction of the light incident surface of the light guide plate. On the back side of the polarization conversion element, a large number of polarization conversion prism grooves for converting an S polarization component into a P polarization component are provided at a direction inclined by 45 degrees with respect to the extending direction of the light incident surface of the light guide plate. A surface light source device with a polarization function that uses a polarization conversion element. 4. The light guide plate is composed of a light scattering light guide,
The value of the effective scattering irradiation parameter E [cm ^-1 ] of the light-scattering light guide is in the range of 0.5 [cm ^-1 ] ≤ E ≤ 50 [cm ^-1 ] and the refractive index that gives the light-scattering ability. Correlation function γ of inhomogeneous structure
The correlation distance a when (r) is approximated by γ (r) = exp [−r / a] (where r is the distance between two points in the light scattering guide).
The surface light source device using the polarization conversion element according to claim 1, wherein the value of [μm] is in the range of 0.06 [μm] ≦ a ≦ 35 [μm]. . 5. The light guide plate comprises a light scattering light guide,
The value of the effective scattering irradiation parameter E [cm ^-1 ] of the light-scattering light guide is within the range of 2.77 [cm ^-1 ] ≤ E ≤ 9.24 [cm ^-1 ] and the light-scattering ability is given. The correlation function γ (r) of the refractive index nonuniform structure is γ (r) = exp [−r / a] (however,
The value of the correlation distance a [μm] when approximated by r is the distance between two points in the light scattering guide) is 0.06 [μm] ≦ a ≦ 7 [μ
m] in a range, the surface light source device using the polarization conversion element according to any one of claims 1 to 3. 6. A light guide plate having a wedge-shaped cross section and having a directional emission property, a light supply unit arranged in the vicinity of a thick end face of the light guide plate, and a light guide plate extending along a light emitting surface of the light guide plate. A light-transmitting polarized light separating means having a reflection characteristic depending on a polarization component, and arranged so as to extend along the surface of the light guide plate opposite to the light emitting surface, and from the back surface of the light guide plate. Including a polarization conversion element that makes the output light of the input light incident from the input / output surface to convert the S polarization component to the P polarization component and then outputs the light from the input / output surface toward the back surface of the light guide plate. On the output surface, a large number of deflection prism grooves for converting the propagation direction of the light emitted from the back surface of the light guide plate into the thickness direction of the polarization conversion element are parallel to the extending direction of the light entrance surface of the light guide plate. The polarization conversion element has an S polarization on the back surface side. A quarter-wave plate oriented so that its polarization axis is oriented in a direction inclined by 45 degrees with respect to the extending direction of the light incident surface of the light guide plate in order to convert the component into a P-polarized component; A surface light source device with a polarization function using a polarization conversion element, which is provided with a reflector arranged outside a one-wave plate. 7. The light guide plate comprises a light-scattering light guide,
The value of the effective scattering irradiation parameter E [cm ^-1 ] of the light-scattering light guide is in the range of 0.5 [cm ^-1 ] ≤ E ≤ 50 [cm ^-1 ] and the refractive index that gives the light-scattering ability. Correlation function γ of inhomogeneous structure
The correlation distance a when (r) is approximated by γ (r) = exp [−r / a] (where r is the distance between two points in the light scattering guide).
The surface light source device using a polarization conversion element according to claim 6, wherein the value of [μm] is in the range of 0.06 [μm] ≦ a ≦ 35 [μm]. 8. The light guide plate comprises a light-scattering light guide,
The value of the effective scattering irradiation parameter E [cm ^-1 ] of the light-scattering light guide is within the range of 2.77 [cm ^-1 ] ≤ E ≤ 9.24 [cm ^-1 ] and the light-scattering ability is given. The correlation function γ (r) of the refractive index nonuniform structure is γ (r) = exp [−r / a] (however,
The value of the correlation distance a [μm] when approximated by r is the distance between two points in the light scattering guide) is 0.06 [μm] ≦ a ≦ 7 [μ
A surface light source device using the polarization conversion element according to claim 6, which is in the range of m]. 9. The polarization conversion according to claim 1, wherein a prism sheet for correcting the preferential propagation direction of the illumination light is arranged outside the polarized light separating means. A surface light source device with a polarization function using an element.