JPH1138357A - Color separating optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a color separating optical system having a good color sepn. characteristic at a low cost without impairing the image forming performance of a lens by using dichroic mirrors of a thin type. SOLUTION: This optical system 6 color separates the light emitted from a subject and past a projecting lens 7 and projects the light to an image plane. In such a case, the dichroic mirrors 8, 9 are disposed between the projecting lens 7 and the image plane to be projected and the following conditional equation range is satisfied: θ<=20 deg. when 0.3<=t (mm)<=1, or θ<=30 deg. when 0.1<=t (mm)<=0.3, or θ<=50 deg. when t (mm)<=0.1, where (t) is the plate thickness of the dichroic mirrors 8, 9, and θ is the angle formed by the normal of the dichroic mirrors 8, 9 and the main rays.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a color separation optical system of a color image input device.

[0002]

2. Description of the Related Art Conventionally, two types of constructions have been used as color image pickup optical systems. One is C
When an image is formed on a CD or the like, the image is formed without passing through a color separation optical system. For example, as shown in FIG. 9, when light from an object 21 such as a document is imaged on an image plane 23 via a lens 22, the light is projected on the image plane 23 in the sub-scanning direction indicated by an arrow A. Reduction type line C where each RGB sensor is lined up
It has a configuration in which a CD sensor is arranged. Or Figure 1
As shown in FIG. 10A, when light from a subject 21 such as a document is imaged on an image plane 23 via a lens 22 in the same manner as in FIG. Are arranged adjacent to each other and a line CCD sensor arranged on an array is used.

The other is to use a color separation optical system. In the conventional color separation optical system, color separation by a prism is general, and in a system using a dichroic mirror, illumination light is Mainly for color separation. Further, as for a configuration for preventing the deterioration of the imaging performance due to the color separation element, some proposals regarding correction of chromatic aberration of a lens have been conventionally proposed.

On the other hand, US Pat. No. 5,305,146
As described in Japanese Patent Application Laid-Open Publication No. H10-260, there is an image capturing apparatus using a color separation optical system using a dichroic mirror. Here, in each system of RGB, a dichroic film is deposited on a glass plate as a dichroic mirror, and in order to compensate for a difference in transmission thickness of a different plate glass in each of RGB,
The astigmatic difference is compensated for by using a lens having power in the sub-scanning direction on the CCD sensor image of the dichroic mirror.

[0005]

However, in the configuration shown in FIG. 9, when reading the object to be imaged 21, the R, G, and B sensors are separated in the sub-scanning direction. Since a delay occurs in the reading timing of the sensor, an MTF (modulation transfer function) deteriorates or a color shift occurs due to uneven speed at the time of scanning, for example, uneven speed of a scanner of a copier or uneven speed of a document transport at the time of panning. May occur. Further, a memory for delay is required. In the configuration as shown in FIG. 10, the size of one pixel increases, which is disadvantageous in terms of resolution.

Further, in the configuration described in the above-mentioned US Pat. No. 5,305,146, compared with the type shown in FIGS. 9 and 10, MTF deterioration and color misregistration are small and high resolution is obtained. Although it is advantageous in that a memory for delay is not required, it has the following problems. That is, since the amount of astigmatism originally generated in the flat plate is large, even if this aberration is corrected by the correcting lens, a large aberration remains at the absolute value level.

Also, the difference in astigmatic difference between the angle of view in the main direction and the on-axis direction may be large, and the correction cannot be made over the entire angle of view. For this reason, it is conceivable to use a lens having a free-form surface and an aspherical surface as a lens for correction, but this increases the cost. Furthermore, the CCD sensor of the correction lens itself,
Relative positioning with respect to the dichroic mirror needs to be performed with high accuracy, and the holding adjustment becomes troublesome, or the cost of the holding mechanism increases. In addition, the correction lens itself leads to an increase in cost.

In view of the above problems, the present invention uses a thin dichroic mirror, which has not been provided in the conventional color separation optical system, so that the imaging performance of the lens is not impaired and the color separation characteristics are good. It is an object to provide a low-cost color separation optical system.

[0009]

In order to achieve the above object, the present invention provides a color separation optical system that separates the color of light emitted from an object and passed through a projection lens and projects the color on an image plane.
A dichroic mirror is provided between the projection lens and the image plane to be projected, and satisfies the following conditional expression range.

When 0.3 ≦ t (mm) ≦ 1, θ ≦ 20 °, or when 0.1 ≦ t (mm) <0.3, θ ≦ 30 °, or t (mm) <0. When 1, the angle θ ≦ 50 ° where t: the thickness of the dichroic mirror θ: the angle between the normal line of the dichroic mirror and the principal ray.

Further, the light from the subject reflected by the dichroic mirror is projected by being reflected by a reflecting mirror such that the principal ray thereof is substantially perpendicular to the optical axis of the projection lens. Further, the dichroic mirror is configured so that the periphery of one surface thereof abuts on the holder and the periphery of the other surface is pressed by an elastic member so as to be sandwiched between the holder and the holder.

Alternatively, in a color separation optical system that separates light that has passed through a projection lens from a subject and projects it on an image plane, a dichroic mirror is provided between the projection lens and the image plane to be projected. The thickness of the dichroic mirror determines the allowable range of the angle between the normal to the dichroic mirror and the principal ray. The subject in the present claim refers to an object to be imaged such as a manuscript described in the present embodiment.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an example of a schematic configuration of an apparatus using the present invention. As shown in the figure,
This apparatus is a reduction-type reading optical system that relatively scans a document 1 to be imaged by a scanner 2 in an arrow direction to read an image, and a color separation optical system 6 is disposed behind a lens 7. I have. The scanner 2 includes a light source 3, a reflecting mirror 4, a mirror 5, and the like.

FIG. 2 shows an embodiment of the color separation optical system of the present invention. In the figure, reference numeral 6 denotes the entire color separation optical system, 7 denotes a lens, 8, 9 are dichroic mirrors, 10, 11, and 12 are linear sensors, 13 is a transmission type filter, and 14 and 15 are reflection mirrors. , 16 are lens optical axes. As shown in the figure, the color light transmitted through the lens 7 and reflected by the dichroic mirrors 8 and 9 is reflected at a shallow angle with respect to the lens optical axis 16 as it is.
This is inconvenient in arranging a light receiving element such as a CCD.

Therefore, in the present invention, the reflection mirror 1 shown in FIG.
These light beams are turned back by 4 and 15, respectively, so that the light receiving elements such as CCDs, that is, the light receiving surfaces of the linear sensors 10 and 11 are directed perpendicularly to the main scanning surface of the lens 7. By doing so, the focus of the light receiving element,
The directions of the adjustment axes for skew and resist can be made the same, and adjustment can be facilitated. The light transmitted through the two dichroic mirrors is received by the linear sensor 12 on the lens optical axis 16 via the filter 13.

The color separation optical system of the present invention is composed of a plurality of dichroic mirrors (8, 9) having a structure in which a dichroic film is formed on a transparent thin plate substrate, and the light that has passed (reflected or transmitted) through these is converted by a CCD or the like. By receiving light, RG
It can be decomposed into signals of three colors B. Further, the surface normal of each dichroic mirror is set at a slight angle with respect to the optical axis of the lens 7 and the thickness is set to 1.0 mm or less. Thus, the amount of axial astigmatism that occurs when an oblique flat plate is inserted into the optical path can be made sufficiently small to a level that can be ignored in optical performance.

For reference, FIG. 3 shows MTF data when the incident angle of the light beam on the dichroic mirror is 30 ° and the plate thickness is 1.0 mm. In this example, the distance H between the meridional image plane and the sagittal image plane is 100 μm or more, which indicates that the imaging performance is deteriorated and the depth of focus is reduced. On the other hand, as a design example satisfying the conditions of the present invention, FIG.
The MTF data when mm is shown. In this example, the positions of the meridional image plane and the sagittal image plane match well, and it can be seen that there is almost no performance degradation.

FIG. 5 is a graph showing the relationship between the incident angle and the astigmatic difference. As shown in the figure, it can be seen that the astigmatic difference increases with the acceleration as the incident angle increases, and the tendency increases as the plate thickness increases. FIG.
Is a graph showing the relationship between plate thickness and astigmatic difference. As shown in the figure, the astigmatism difference increases proportionally as the plate thickness increases, and the tendency increases as the incident angle increases.

From the above, it can be seen that the angle of incidence of the light beam on the dichroic mirror needs to be set within the following conditional expression range. 0.3 ≦ t (mm) ≦ 1, θ ≦ 20 °, or 0.1 ≦ t (mm) <0.3, θ ≦ 30 °, or t (mm) <0.1 , Θ ≦ 50 ° where t is the plate thickness, and θ is the incident angle, that is, the angle between the normal to the dichroic mirror and the principal ray.

This is similar to the case where the plate thickness is reduced,
The objective is to suppress the axial astigmatism, and when the incident angle is greater than this angle, the range in which the required MTF can be guaranteed for the depth of focus of a generally used lens becomes extremely narrow, This is because it lacks practicality. When both the incident angle and the plate thickness in the above conditional expressions exceed the conditional ranges, the amount of astigmatism generated becomes too large, and a correction optical system is required.

Further, usually, the depth of focus at which the MTF of the lens can be maintained at a required level is about 100 μm.
Since two or more dichroic mirrors are generally used, the permissible amount of astigmatic difference per mirror is up to about 50 μm. Also, t (mm) of the above conditional expression <0.1
Is a condition assuming a film-like dichroic mirror, but considering the flatness of the dichroic surface,
At the moment there is no suitable material. This is a part that requires future technology development. FIG. 7 shows an example of the characteristic of the spectral reflectance for each color separation of RGB in the color separation dichroic mirror.

Next, a method for holding an optical element suitable for the above-described optical system will be described with reference to FIG. Since a spectroscopic element such as a dichroic mirror has a small substrate thickness, the surface of the element is likely to be distorted depending on the manner of holding, and there is a high possibility that the imaging performance will deteriorate. For this reason, it is necessary to avoid applying stress to the element as much as possible, or to apply the stress uniformly to the entire element. If the back surface of the element is in point contact and is held by a leaf spring or the like, stress is generated at the leaf spring contact portion or the point contact portion, and cracking or distortion occurs. Therefore, the holding method according to the present invention uses a holder 31 for simultaneously or individually positioning a plurality of optical elements such as dichroic mirrors 8 and 9 shown in FIG. It is configured to be applied.

Further, the contact portion has a structure in which the entire periphery of the light-impermeable portion of the element is uniformly received, and the stress distortion due to the elastic force of the element itself is minimized. Then, the elastic member 32 is used to apply the element, and by uniformly applying pressure to the entire circumference of the element by this elastic deformation, it is possible to prevent the occurrence of local distortion. Then, as shown in FIG. 3A, the above-described pressure is generated by pressing the elastic member 32 against the mounting plate 33 fixed to the holder 31 with the screw 34. Note that the mounting plate and the screws are omitted in FIG.

[0024]

As described above, according to the present invention,
By using a thin dichroic mirror that was not available in the conventional color separation optical system, it is possible to provide a color separation optical system with good color separation characteristics at low cost without deteriorating the imaging performance of the lens. Can be.

In particular, according to the first aspect, even if a dichroic mirror is arranged on the image plane side of the reduction type reading optical system and color separation is performed, there is almost no deterioration in imaging performance due to on-axis astigmatism, and there is little deterioration. A color separation optical system can be configured with a space. Further, the generation of the polarization component due to the incident angle of the light beam on the interference film can be extremely reduced to a negligible level. Further, since an expensive free-form surface or aspherical lens does not need to be used, the cost is reduced.

According to the second aspect of the present invention, it is possible to arrange a sensor such as a CCD on a horizontal plane or a vertical plane, so that the position can be easily adjusted. Further, according to the third aspect, it is possible to hold a thin spectral element such as a dichroic mirror while preventing stress distortion and cracking.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a schematic configuration of an apparatus using the present invention.

FIG. 2 is a diagram showing one embodiment of a color separation optical system of the present invention.

FIG. 3 is a diagram showing MTF data when a condition of the present invention is not satisfied.

FIG. 4 is a diagram showing MTF data when a condition of the present invention is satisfied.

FIG. 5 is a graph showing a relationship between an incident angle and an astigmatic difference.

FIG. 6 is a graph showing a relationship between a plate thickness and an astigmatic difference.

FIG. 7 is a view showing an example of spectral reflectance characteristics for each color separation of RGB.

FIG. 8 is a diagram illustrating a method for holding a thin optical element.

FIG. 9 is a diagram illustrating an example of a color image capturing optical system that does not pass through a color separation optical system.

FIG. 10 is a diagram illustrating another example of a color image capturing optical system that does not pass through a color separation optical system.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Original 2 Scanner 3 Light source 4 Reflector 5 Mirror 6 Color separation optical system 7 Lens 8,9 Dichroic mirror 10,11,12 Linear sensor 13 Filter 14,15 Reflection mirror 16 Lens optical axis 31 Holder 32 Elastic member 33 Mounting plate 34 screw

Claims (4)

[Claims] 1. A color separation optical system that separates light emitted from a subject and passed through a projection lens and projects the light on an image plane, wherein a dichroic mirror is provided between the projection lens and the image plane to be projected. A color separation optical system characterized by satisfying the following conditional expression range: when 0.3 ≦ t (mm) ≦ 1, θ ≦ 20 ° or 0.1 ≦ t (mm) <0.3 Θ ≦ 30 ° or θ ≦ 50 ° when t (mm) <0.1, where t: the thickness of the dichroic mirror θ: the angle between the normal line of the dichroic mirror and the principal ray. 2. The light from the subject reflected by the dichroic mirror is reflected and projected by a reflection mirror such that a principal ray thereof is substantially perpendicular to an optical axis of the projection lens. The color separation optical system according to claim 1. 3. The dichroic mirror is fixed in such a manner that the periphery of one surface thereof is brought into contact with a holder and the periphery of the other surface is pressed by an elastic member so as to be sandwiched between the holder and the holder. The color separation optical system according to claim 1 or 2, wherein 4. A color separation optical system that separates light that has passed through a projection lens from a subject and projects the light on an image plane, wherein a dichroic mirror is provided between the projection lens and the image plane to be projected. A color separation optical system, wherein an allowable range of an angle between a normal line of the dichroic mirror and a principal ray is determined by a thickness of the dichroic mirror.