JP7562805B2 - Information display device - Google Patents

Description

The present invention relates to an information display device that projects an image onto the windshield or combiner of an automobile, train, airplane, etc. (hereinafter, generally referred to as "vehicle"), and to a projection optical system that allows the image to be observed as a virtual image through the windshield, and an information display device that uses the same.

A so-called head-up display (HUD) device that projects image light onto the windshield or combiner of an automobile to form a virtual image and display traffic information such as route information and congestion information, and automobile information such as remaining fuel and coolant temperature, is already known from the following Patent Document 1.

In this type of information display device, it is desirable to expand the area in which the driver can view the virtual image, but it is also an important performance factor that the virtual image has high resolution and is highly visible.

A head-up display device necessarily requires a windshield or combiner as the final reflective surface that provides a virtual image to the driver, and the inventors realized that in order to obtain high visibility and good resolution performance, it is important to improve the double image of the virtual image caused by double reflections that occur at the final reflective surface, the windshield or combiner.

On the other hand, a device has already been proposed in which the main body including the combiner is attached near the roof (sun visor) of the vehicle, as disclosed in the following non-patent document 1:

JP 2015-194707 A

PIONEER R&D (Vol.22, 2013)

The principle of virtual image generation by a concave mirror to realize a head-up display device according to conventional technology is that, as shown in Figure 33, a virtual image can be obtained by the concave mirror 1' by placing an object point AB inside the focal point F (focal length f) with respect to point O on the optical axis of the concave mirror 1'. For convenience of explanation, Figure 33 regards the concave mirror 1' as a convex lens with the same positive refractive power, and shows the relationship between the object point, the convex lens (for convenience of explanation, it is represented as a concave mirror in Figure 33), and the virtual image that is generated.

In the prior art, in order to expand the range in which the virtual image generated by the concave mirror 1' can be seen, it is sufficient to move the object point AB closer to the focal point F and make the concave mirror larger relative to the object dimension AB; however, in order to obtain the desired magnification, the radius of curvature of the concave mirror becomes small, making it difficult to achieve both. As a result, the mirror size becomes small, and although the effective magnification is large, only a virtual image with a small viewable range is obtained. For this reason, in order to simultaneously satisfy (1) the desired virtual image size and (2) the required virtual image magnification M = b/a, it is necessary to match the dimensions of the concave mirror to the viewing range and determine the virtual image magnification in consideration of the image display device.

For this reason, in the prior art, in order to obtain a large virtual image in the desired viewing range, it was necessary to increase the distance from the concave mirror 1' to the virtual image, i.e., to increase the distance between the final reflecting surface, the windshield or combiner (not shown), and the concave mirror, and at the same time increase the size of the concave mirror, as shown in FIG. 33, for example. However, no consideration was given to the double image of the virtual image caused by double reflection at the windshield or combiner.

For example, the example of a head-up display device disclosed in the above-mentioned Patent Document 1, which is a conventional technology, includes a device for displaying an image and a projection optical system for projecting the image displayed on the display device. The projection optical system has a first mirror and a second mirror in the optical path from the display device to the viewer, and the relationship between the angle of incidence in the image long axis direction on the first mirror and the angle of incidence in the image short axis direction on the first mirror, the distance between the image display surface of the display device and the first mirror, and the horizontal width of the virtual image viewed by the viewer satisfies predetermined conditions, thereby realizing compactness. However, there is no mention of any means for reducing the double image caused by reflection on both sides of the windshield (the driver's side and the two outer sides).

On the other hand, in a device that is mounted near the roof (sun visor) of a car as disclosed in Non-Patent Document 1, the occurrence of double images is reduced by forming an anti-reflection film on the reflective surface that does not face the driver. However, there are still safety issues, such as the possibility of the driver being injured if the HUD device becomes detached in the event of a collision.

As a result, it is believed that the method described in Patent Document 1 above, in which the windshield is used as a reflective surface, will become mainstream in the future, and so we have devised a technical means for reducing the reflection of the imaging light that creates a virtual image reflected on both sides of the windshield by using an ingenious technique in the projection optical system.

The present invention proposes a technical means for reducing the double image of a virtual image that occurs when the windshield is used as a reflective surface, using optical system innovations, as described in detail below. It also aims to provide an information display device that can form a highly visible virtual image in which the distortion and aberration of the virtual image seen by the driver are reduced to a practically acceptable level.

As an example of the present invention made to achieve the above object, an information display device that displays virtual image information on the windshield of a vehicle includes an image generation unit that generates an image of the image information, and an optical system that receives a light beam from the image generation unit and displays a virtual image that is an enlarged image of the image generated by the image generation unit, the optical system including a concave mirror and an optical element, the optical element being disposed between the image generation unit and the concave mirror and configured to correct aberration of the virtual image obtained corresponding to the driver's viewpoint position by the shape of the concave mirror and the shape of the optical element, the optical system including an optical element with negative refractive power, and the optical element with negative refractive power being configured such that image light from the image generation unit is incident on the optical element and is emitted toward the concave mirror.

In this way, according to the present invention, an information display device is realized in which the divergence angle of light entering the optical system of the image light from the image display device is controlled so as to reduce the double image that occurs in the virtual image described above and to obtain an image with improved visibility.

The present invention makes it possible to provide an information display device that forms a virtual image with improved visibility by miniaturizing the device, correcting distortion and aberration in the virtual image observed by the driver, and controlling the divergence angle of the light source beam entering the virtual image optical system to reduce double images that appear in the virtual image.

FIG. 2 is a schematic diagram showing a configuration of peripheral devices of the information display device according to the embodiment. FIG. 1 is a top view of an automobile equipped with an information display device. FIG. 2 is a diagram illustrating a difference in the radius of curvature of a windshield. 1 is a schematic diagram showing an information display device, a windshield, and a driver's viewpoint position. 1 is a schematic diagram showing an example of a virtual image optical system of an information display device; 3 is a ray diagram of the entire virtual image optical system of the information display device according to the embodiment. FIG. 4 is a ray diagram of a part of a virtual image optical system of the information display device according to the embodiment. FIG. 1 is a schematic diagram illustrating the principle of a virtual image type optical system. FIG. 1 is a schematic diagram illustrating the principle of double image generation. FIG. 1 is a schematic diagram illustrating the principle of the present invention. FIG. 13 is a diagram showing resolution performance evaluation points within an eye box according to an embodiment. FIG. 11 is a spot diagram showing the results of back-tracing using a green ray (a ray is projected from a virtual image to evaluate the imaging state at the image source), illustrating the results of the resolution performance evaluation in the example. FIG. 13 is a diagram showing distortion performance as viewed from the center of the eye box in the embodiment. 1 shows lens data according to an example. 1 shows lens data according to an example. FIG. 2 is a configuration diagram showing the arrangement of a video display device and a light source device. FIG. 2 is a schematic diagram showing the configuration of a light source device. 2 is a schematic diagram for explaining the emission state of light beams from an image display device and a light source device. FIG. 4 is a characteristic diagram illustrating the distribution of light emitted from a light source device. FIG. 3 is a schematic diagram showing the shape of a light guide of the light source device. FIG. 3 is a schematic diagram showing a cross-sectional shape of a light guide of the light source device. FIG. FIG. 1 is a conceptual diagram showing a method for evaluating the characteristics of a liquid crystal panel. 4 is a characteristic diagram showing transmittance characteristics in the left-right direction of the screen of a liquid crystal panel. FIG. 10 is a characteristic diagram showing the angular characteristics of luminance in the left-right direction of the screen when white is displayed on the liquid crystal panel. 10 is a characteristic diagram showing angular characteristics of backlight luminance of a liquid crystal panel in the left-right direction. 10 is a characteristic diagram showing the angular characteristics of the backlight luminance of a liquid crystal panel in the vertical direction. 10 is a characteristic diagram showing angular characteristics of contrast of a liquid crystal panel in the left-right direction. 4 is a characteristic diagram showing the transmittance characteristics in the vertical direction of a liquid crystal panel. FIG. 10 is a characteristic diagram showing the angular characteristics of brightness of a liquid crystal panel in the vertical direction. 10 is a characteristic diagram showing the angular characteristics of black display luminance of a liquid crystal panel in the left-right direction. 10 is a characteristic diagram showing the angular characteristics of contrast of a liquid crystal panel in the vertical direction. 10 is a characteristic diagram showing the angular characteristics of black display luminance of a liquid crystal panel in the vertical direction. FIG. 1 is a schematic diagram for explaining the principle of a virtual image optical system according to the prior art. 1 is a schematic diagram for explaining the change in reflectance of glass depending on the angle of incidence of S-polarized light and P-polarized light; FIG. 11 is a schematic diagram for explaining a specific method for reducing the Petzval sum of a virtual image optical system. FIG. 11 is a schematic diagram for explaining a specific method for reducing the Petzval sum of a virtual image optical system.

The following describes in detail the embodiments of the present invention with reference to the drawings. Note that the present invention is not limited to the following description, and various changes and modifications can be made by those skilled in the art within the scope of the technical ideas disclosed in this specification. In addition, in all drawings used to explain the present invention, parts having the same functions are given the same reference numerals, and repeated explanations may be omitted.

<Embodiments of Information Display Device>
FIG. 1 is a schematic diagram showing the configuration of peripheral devices of an information display device according to one embodiment of the present invention. In this embodiment, an information display device 100 that projects an image onto the windshield of an automobile will be described as an example.

This information display device 100 is a device (so-called HUD (Headup Display)) that displays various information reflected from a projection target 6 (in this embodiment, the inner surface of the windshield) as a virtual image VI (Virtual Image) to form a virtual image V1 in front of the vehicle in the driver's line of sight 8. The projection target 6 may be any member onto which information is projected, and may be not only the windshield described above, but also a combiner. In other words, the information display device 100 of this embodiment may be any device that forms a virtual image in front of the vehicle in the driver's line of sight 8 and allows the driver to view it, and the information displayed as a virtual image may naturally include, for example, vehicle information and foreground information captured by a camera (not shown) such as a surveillance camera or an around viewer.

In addition, the information display device 100 includes an image display device 4 that projects image light to display information, and a correction lens group 2 between the image display device 4 and the concave mirror 1 to correct distortion and aberration that occurs when the image displayed on the image display device 4 is converted into a virtual image by the concave mirror 1.

The information display device 100 is equipped with a control device 40 that controls the image display device 4 and the backlight 5. The optical components including the image display device 4 and the backlight 5 are a virtual image optical system described below, and include a concave mirror 1 that reflects light. The light reflected by this optical component is reflected by the projection member 6 and directed toward the driver's line of sight 8 (EyeBox: described in detail later).

Examples of the image display device 4 include a backlit LCD (Liquid Crystal Display) and a self-illuminating VFD (Vacuum Fluorescent Display).

On the other hand, instead of using the image display device 4 described above, an image can be displayed on a screen using a projection device, and then the image can be made into a virtual image using the concave mirror 1 described above, and reflected by the windshield 6, which is the object to be projected, toward the driver's viewpoint 8.

Such a screen may be constructed, for example, from a microlens array in which microlenses are arranged two-dimensionally.

To reduce distortion of the virtual image, the shape of the concave mirror 1 should be such that the upper part shown in FIG. 1 (the area where light is reflected below the windshield 6, which is relatively close to the driver's viewpoint) has a relatively small radius of curvature so that the magnification is large, while the lower part (the area where light is reflected above the windshield 6, which is relatively far from the driver's viewpoint) has a relatively large radius of curvature so that the magnification is small. In addition, even better correction can be achieved by tilting the image display device 4 with respect to the optical axis of the concave mirror to correct the difference in virtual image magnification described above and reduce the distortion itself that occurs.

On the other hand, as shown in Figs. 2 and 3, the windshield 6 of a passenger vehicle has a different radius of curvature Rv in the vertical direction of the body and a different radius of curvature Rh in the horizontal direction, and generally, the relationship is Rh>Rv. Therefore, when the windshield 6 is taken as a reflecting surface, it becomes a toroidal surface of a concave mirror. For this reason, in the information display device 100 of this embodiment, the shape of the concave mirror 1 is set to have different average radii of curvature in the horizontal and vertical directions so as to correct the virtual image magnification due to the shape of the windshield 6, that is, to correct the difference in the radii of curvature in the vertical and horizontal directions of the windshield 6. At this time, the shape of the concave mirror 1 is a function of the distance r from the optical axis in a spherical or aspherical shape symmetrical with respect to the optical axis (a shape expressed by the formula 2 described later), and since the horizontal and vertical cross-sectional shapes at distant locations cannot be controlled individually, it is preferable to correct it as a function of the coordinates (x, y) of the surface from the optical axis of the mirror surface as a free-form surface expressed by the formula 1 described later.

Returning to FIG. 1, a lens element 2, for example, is placed between the image display device 4 and the concave mirror 1 as a transmissive optical component, and the direction of light emitted to the concave mirror 1 is controlled to correct distortion in accordance with the shape of the concave mirror 1, while at the same time achieving aberration correction of the virtual image, including astigmatism caused by the difference between the horizontal and vertical radii of curvature of the front glass 6 mentioned above.

In addition, in order to further improve the aberration correction ability, the optical element 2 described above may be a plurality of lenses. Alternatively, a curved mirror may be placed instead of the lens element to control the incident position of the light beam on the concave mirror 1 while folding back the optical path, thereby reducing distortion. As described above, it goes without saying that even if an optical element optimally designed to improve the aberration correction ability is provided between the concave mirror 1 and the image display device 4, this does not deviate from the technical idea or scope of the present invention. Furthermore, by changing the thickness of the optical element 2 described above in the optical axis direction, in addition to the original aberration correction, the optical distance between the concave mirror 1 and the image display device 4 can be changed, and the display position of the virtual image can be changed continuously from a distant to a close position.

The image display device 4 may also be tilted relative to the optical axis normal of the concave mirror 1 to correct the difference in magnification of the virtual image in the vertical direction.

On the other hand, it is known that a factor that reduces the image quality of the information display device 100 is that the image light emitted from the image display device 4 toward the concave mirror 1 is reflected on the surface of the optical element 2 arranged along the way, returns to the image display device 4, and is reflected again and superimposed on the original image light, thereby reducing the image quality. For this reason, in this embodiment, not only is an anti-reflection film formed on the surface of the optical element 2 to suppress reflection, but it is also preferable to design the lens surface shape of either or both of the image light entrance surface and exit surface of the optical element 2 with constraints on the surface shape so that the above-mentioned reflected light is not concentrated on a portion of the image display device 4 (for example, a shape with a concave surface facing the image display device 4).

Next, in the image display device 4, in addition to the first polarizing plate arranged close to the liquid crystal panel to absorb the reflected light from the optical element 2 described above, if a second polarizing plate is arranged separately from the liquid crystal panel, the deterioration of image quality can be reduced. Also, the backlight 5 of the liquid crystal panel is controlled so that the direction of incidence of light incident on the liquid crystal panel 4 is efficiently incident on the entrance pupil of the concave mirror 1. At this time, if the divergence angle of the light beam incident on the liquid crystal panel is reduced, not only can the image light be efficiently directed to the driver's eye point, but also, as shown in Figures 27 and 31, it is possible to obtain an image with high contrast and good visibility. The contrast performance against the divergence angle of the image is more noticeable in the horizontal direction, and excellent characteristics can be obtained if it is within ±20 degrees. In order to further improve the contrast performance, it is recommended to use a light beam within ±10 degrees.

On the other hand, it is preferable to use a solid-state light source with a long product life as the light source, and moreover, it is preferable to use an LED (Light Emitting Diode) that has little change in light output due to fluctuations in the ambient temperature, and to perform polarization conversion using a PBS (Polarizing Beam Splitter) equipped with optical means for reducing the divergence angle of the light.

Polarizing plates are arranged on the backlight 5 side (light entrance surface) and the optical element 2 side (light exit surface) of the liquid crystal panel, which increases the contrast ratio of the image light. If an iodine-based polarizing plate with a high degree of polarization is used for the polarizing plate on the backlight 5 side (light entrance surface), a high contrast ratio can be obtained. On the other hand, by using a dye-based polarizing plate on the optical element 2 side (light exit surface), high reliability can be obtained even when external light is incident or the ambient temperature is high.

When a liquid crystal panel is used as the image display device 4, a problem occurs in which certain polarized waves are blocked and the image cannot be seen, particularly when the driver is wearing polarized sunglasses. To prevent this, it is preferable to place a λ/4 plate on the optical element side of the polarizing plate placed on the optical element 2 side of the liquid crystal panel, thereby converting image light aligned in a specific polarization direction into circularly polarized light.

The control device 40 obtains various information from the navigation system 61, such as the speed limit and number of lanes of the road corresponding to the current position of the vehicle, and the planned route of travel of the vehicle set in the navigation system 61, as foreground information (i.e., information displayed in front of the vehicle using the virtual image).

The driving assistance ECU 62 is a control device that realizes driving assistance control by controlling the drive system and control system according to obstacles detected as a result of monitoring by the surroundings monitoring device 63. Examples of driving assistance control include well-known technologies such as cruise control, adaptive cruise control, pre-crash safety, and lane keeping assist.

The surroundings monitoring device 63 is a device that monitors the situation around the vehicle. Examples include a camera that detects objects around the vehicle based on images captured around the vehicle, and a detection device that detects objects around the vehicle based on the results of transmitting and receiving detection waves.

The control device 40 acquires information from the driving assistance ECU 62 as foreground information (e.g., distance to the preceding vehicle, direction of the preceding vehicle, location of obstacles and signs, etc.). In addition, an ignition (IG) signal and vehicle status information are input to the control device 40. Of these pieces of information, the vehicle status information is information acquired as vehicle information, and includes, for example, warning information indicating a predetermined abnormal state, such as the remaining amount of fuel in the internal combustion engine and the temperature of the cooling water. It also includes the operation result of the turn signal, the traveling speed of the vehicle, and even shift position information. The control device 40 described above is activated when an ignition signal is input. This concludes the description of the entire information display device system of the embodiment of the present application.

First Embodiment of Virtual Image Optical System
Next, the virtual image optical system and the image display device according to this embodiment will be described in further detail below.

As described above, Fig. 2 is a top view of a car equipped with the information display device 100 of this embodiment, and in front of the driver's seat of the car body 101, there is a windshield as a projection target member 6. The inclination angle of the windshield with respect to the car body differs depending on the type of car. Furthermore, the inventors investigated the radius of curvature of the windshield in order to realize an optimal virtual image optical system. As a result, as shown in Fig. 3, the windshield has a different radius of curvature Rh in the horizontal direction parallel to the ground surface of the car and a radius of curvature Rv in the vertical direction perpendicular to the horizontal axis, and it was found that there is generally the following relationship between Rh and Rv.
Rh>Rv

It was also found that the difference in the radius of curvature, i.e., Rh relative to Rv, is often in the range of 1.5 to 2.5 times.

Next, the inventors investigated the inclination angle of commercially available windshields. The results showed that, although it differed depending on the vehicle type, it was 20 to 30 degrees for minicars and one-box types, 30 to 40 degrees for sedan types, and 40 degrees or more for sports cars. Therefore, in this embodiment, the virtual image optical system was designed taking into consideration the difference between the horizontal radius of curvature Rh of the windshield parallel to the vehicle's contact surface and the vertical radius of curvature Rv perpendicular to the horizontal axis, as well as the inclination angle of the windshield.

More specifically, since the horizontal radius of curvature Rh and the vertical radius of curvature Rv of the windshield, which is the object to be projected, are significantly different, good aberration correction is achieved by providing an optical element 2 in the virtual image optical system that is axially asymmetric with respect to the horizontal axis of the windshield and the axis perpendicular to this axis with respect to the optical axis (Z axis).

Next, the inventors investigated miniaturization of the information display device 100. The conditions for the investigation were a horizontal FOV of 7 degrees and a vertical FOV of 2.6 degrees, and a virtual image distance of 2 m. At the beginning of the investigation, the basic components were a concave mirror 1 (simply shown as a flat mirror in FIG. 5 below) that generates a virtual image, an image display device 4, and a backlight 5, and a single optical path folding mirror was placed between the image display device 4 and the concave mirror 1. Simulations were performed using the arrangement of each component and the distance from the image display device 4 to the concave mirror 1 as parameters to minimize the volume of the information display device 100.

As a result, when the image light from the image display device 4 was arranged so that it did not interfere with each component, the volume was 3.6 liters. After that, in order to further reduce the size, we investigated a direct method that does not use the optical path folding mirror.

The configuration of the virtual image optical system of this embodiment will be described with reference to Figure 4. This Figure 4 is an overall configuration diagram showing the basic configuration for studying miniaturization in the virtual image optical system of this embodiment shown in Figure 1. For simplicity of explanation, the optical elements for correcting aberration and distortion are omitted, and the vertical cross-sectional shape is shown, similar to the window glass 6 shown in Figure 1. The image display device 4 is assumed to be a liquid crystal panel, and the basic configuration is one in which a backlight 5 is arranged, and the image display device 4 is placed at a position where the displayed image is converted into a virtual image by the concave mirror 1.

At this time, as shown in FIG. 5, the design constraint is that the image light R2 generated from the image at the center of the screen of the image display device 4, the image light R1 from the top end, and the image light R3 from the bottom end are each positioned so that they do not interfere with the image display device 4 and block the light when reflected by the concave mirror 1.

The inventors took into consideration the above-mentioned design constraints, and at the same time, set the FOV horizontal: 7 degrees, vertical: 2.6 degrees, and further set the virtual image distance to 2 m, and calculated the volume of the information display device 100 using the distance Z between the concave mirror 1 and the image display device 4 (liquid crystal panel and backlight 5) as a parameter. When the distance Z is 100 mm, the vertical dimension of the concave mirror 1 can be made the smallest. When the distance Z is set to 75 mm, the angle α2 between the horizontal plane and the concave mirror 1 becomes large, and the vertical dimension of the concave mirror 1 also becomes large. When the distance Z is further shortened to 50 mm or less, the angle α3 between the horizontal plane and the concave mirror 1 becomes large, and the vertical dimension of the concave mirror 1 also becomes larger.

As described above, when simulating the relationship between the set height and set depth of the image display device 4 using distance Z as a parameter, the set depth can be reduced by making distance Z smaller, but on the other hand, the set height increases. Similarly, in terms of the relationship between distance Z and set volume L, compared to the spatial volume from the image display device 4 to the concave mirror (referred to as the optical path volume), the change in the set volume (including the LCD drive circuit, light source drive circuit, and backlight volume) changes at a distance Z of 60 mm.

From the above, it was found that in order to miniaturize the information display device 100, it is necessary to realize a virtual image optical system with a short distance Z in which the image displayed on the image display device 4 is directly magnified by the concave mirror 1, and that the vertical center of the screen of the image display unit of the image display device 4 needs to be located below the center of the concave mirror 1.

On the other hand, with this arrangement, the distance between the image display device 4 and the upper end of the concave mirror 1 (corresponding to light ray R1) is long, and the distance between the image display device 4 and the lower end of the concave mirror 1 (corresponding to light ray R3) is short. Therefore, it is advisable to arrange the image display device 4 so that the distance between the image display device 4 and the concave mirror 1 is as uniform as possible.

In the virtual image optical system of this embodiment (see Figures 6 and 7), distortion correction of the virtual image and aberration correction by optical elements that correct aberrations generated by the virtual image are performed between the image display device 4 and the concave mirror 1, which will be explained later using Figure 8. That is, by placing the image display device 4 (object point) inside the focal point F (focal length f) with respect to point O on the optical axis of the concave mirror 1, a virtual image can be obtained by the concave mirror 1. For convenience of explanation, Figure 8 regards the concave mirror 1 as a convex lens with the same positive refractive power, and shows the relationship between the object point, the convex lens (for convenience of explanation, it is represented as a concave mirror in Figure 8), and the virtual image generated.

In this embodiment, an optical element 2 is disposed to reduce distortion and aberration occurring in the concave mirror 1. This optical element may be a transmissive optical lens or a concave mirror, but when the image light from the image display device 4 is:
(1) When a telecentric light beam is incident on a reflecting surface, the refractive power of the lens or concave mirror 1 is approximately zero;
(2) When the image light from the image display device 4 diverges and enters the optical element, the optical element has a positive refractive power;
(3) When the image light from the image display device 4 is condensed and enters the optical element, the optical element has a negative refractive power;
In this way, the direction (angle and position) of the light beam incident on the concave mirror is controlled, and distortion aberration of the virtual image that occurs is corrected. Furthermore, in the case of a transmissive optical lens, the interaction between the entrance surface on the image display device 4 side and the exit surface on the concave mirror 1 side corrects aberration related to the imaging performance that occurs in the virtual image. Although one optical element has been described in this embodiment, multiple transmissive optical elements may be used, or a combination of a reflective optical element (mirror) and a transmissive optical element may be used.

At this time, as described above, the size of the virtual image seen by the driver differs between the upper and lower ends of the virtual image due to the inclination of the windshield, that is, the distance a between the image display device 4 and the concave mirror 1 and the distance b between the concave mirror 1 and the virtual image. As a result, a double virtual image occurs with the windshield or combiner as the reflecting surface. Therefore, in this embodiment, a technology has been developed to reduce the double virtual image that occurs in such cases by using optical system innovations, and the details of this technology are described below.

<Mechanism of virtual and double image generation>
As a result of various investigations, the inventors have developed a technique for reducing double images caused by virtual images, which will be described in detail below, based on the findings described below.

As shown in Figure 9, the virtual image reflected by the top of the windshield and viewed by the driver is caused by the inclination of the windshield and the light rays that generate the virtual image enter the windshield at an angle. If the thickness of the windshield is t, then the reflection position a of the normal light reflected by the reflective surface closest to the driver (hereafter referred to as reflective surface 1) and the reflection position b of the back-reflected light reflected by the reflective surface farther from the driver (hereafter referred to as reflective surface 2) are shifted vertically upward by a distance L, forming two virtual images.

The normal virtual image created by normal light and the second virtual image created by back-reflected light have almost the same brightness because the reflectance of light incident on the windshield from the air is 4% and the reflectance at the interface between the windshield and the air is 4%. For this reason, reducing the brightness of the virtual image created by back-reflected light is essential to achieving good resolution performance for virtual images.

On the other hand, the virtual image that is reflected at the bottom of the windshield and viewed by the driver is similar to the reflection at the top shown in Figure 9, as the windshield is tilted and the light rays that generate the virtual image enter the windshield at an angle, but the other back-reflected light is shifted above the regular reflected light, forming two virtual images.

Furthermore, in the left-right direction of the screen, the back-reflected light is shifted away from the point where the optical axis of the concave mirror intersects with the windshield compared to the regular reflected light, forming two virtual images.

As mentioned above, if the refractive index of the windshield is 1.5, the relationship between the angle of incidence of the image light on the windshield and the reflectance is as shown in Figure 34. In the case of perpendicular incidence, the reflectance is about 4% for both S-polarized and P-polarized light, but when the angle of incidence exceeds 25 degrees, the reflectance of S-polarized light increases.

For this reason, when using an LCD as an image display device, the reflectivity of the windshield differs depending on which polarization is used for the image output light, and the brightness of the virtual image visible to the driver may change depending on the angle of incidence on the reflective surface.

Furthermore, if the area in which the driver can see the virtual image is expanded without changing the distance between the windshield and the concave mirror, the angle at which the image light enters the windshield increases, resulting in double images appearing above, below, left and right of the screen, hindering the sense of focus of the virtual image.

The inventors therefore discovered that by tilting the image display device 4 with respect to the optical axis of the concave mirror 1 as shown in FIG. 8, the image magnification M' = b'/a' at the upper end of the virtual image can be made to approximately match the image magnification M = b/a at the lower end of the virtual image, thereby reducing the distortion aberration that occurs.

Furthermore, the average radius of curvature of the vertical cross-sectional shape of the optical element 2 is set to different values from the average radius of curvature of the horizontal cross-sectional shape, thereby correcting the distortion aberration and the aberration that reduces the imaging performance of the virtual image caused by the optical path difference resulting from the difference between the vertical radius of curvature Rv and the horizontal radius of curvature Rh of the front glass described above.

As described above, in the information display device 100 that obtains a virtual image by directly reflecting image light onto the windshield 6, the most important thing in ensuring the imaging performance of the virtual image is to correct the aberration caused by the optical path difference resulting from the difference between the vertical radius of curvature Rv and the horizontal radius of curvature Rh of the windshield 6.

For this reason, the inventors have reduced the degradation of virtual image formation performance caused by the above-mentioned difference in the radius of curvature of the windshield by using a free-form surface shape (see equation 1 below) that allows the shape of the surface to be defined as a function of absolute coordinates (x, y) from the optical axis, as opposed to the aspheric surface shape (see equation 2 below) that has been used in conventional optical design and defines the shape of the lens surface or mirror surface as a function of the distance r from the optical axis.

The aspheric shape that defines the shape of a lens surface or mirror surface as a function of the distance r from the optical axis is expressed by the following formula 2.

The refractive index of an automobile windshield is usually n=1.5, and the reflectance per surface is 5%. As described above, the information display device reflects a virtual image on the windshield to focus the image in the driver's eyebox. For this reason, as shown in FIG. 9, the image light is separated into regular reflected light reflected by reflective surface 1 on the inside of the windshield and back-reflected light reflected by reflective surface 2 exposed to the outside air, and is recognized as a double image by the driver's eyes. The direction in which this double image occurs differs depending on the vertical and horizontal directions of the windshield, and when the information display device 100 is placed below the windshield, the double image generated by the back-reflected light appears above the image generated by the regular reflected light.

Similarly, even if the information display device 100 is placed above the windshield, the double image caused by the backside reflected light appears above the image caused by the regular reflected light.

On the other hand, double images in the horizontal direction on the windshield occur on the outside (away from the driver) in the periphery of the windshield because the radius of curvature is smaller at the periphery than at the center of the windshield.
(1) An anti-reflective coating is applied to the surface of the windshield exposed to the outside air to reduce light reflected from the back side.
(2) Furthermore, the inventors have devised a method for reducing the appearance of double images in a projection optical system that generates a virtual image by applying the measures described below.

Next, an embodiment for reducing the above-mentioned double images will be described in detail with reference to FIG. 10. Note that, for simplicity, a real image projection optical system will be used for the explanation.

Figure 10 shows the relationship between object point P1 and image point P0 in a real image projection optical system. Imaging performance is evaluated by equally dividing the entrance pupil and directing light rays from the virtual image side (the original image point) toward the panel surface (the original object point), which is the object point, and evaluating the imaging performance on the panel surface.

The inside of the driver's eyebox was divided as shown in Figure 11, and ray tracing was performed at equal intervals to evaluate the imaging performance on the panel surface corresponding to each evaluation point. The results based on the lens data shown in Figures 14 and 15 for this embodiment show that, as shown in Figures 12 and 13, the magnitude of aberration generated at the center and periphery of the screen is different, so the spot shape differs depending on the position on the screen, and the spot does not become concentric with respect to the chief ray passing through the center of the entrance pupil (the point where it intersects with the optical axis), resulting in blurring (aberration).

Therefore, in order to reduce aberrations that occur outside the chief ray, it is advisable to design the lens so that light rays passing through the upper peripheral part of the pupil are directed inward from the chief ray. Specifically, it is advisable to arrange an optical element having a shape such that the refractive power in the optical path through which light rays passing through the upper peripheral part above the position of the light ray through which the chief ray passes is relatively stronger than the refractive power in the optical path through which the chief ray passes.

This will be explained in the case of reverse tracing, in which light rays are sent from an image point to an object point, as shown in Figure 10. In this method, the object point is the virtual image side and the image point is the panel surface, the entrance pupil of the virtual image optical system is equally divided, light rays are sent, and the imaging performance at the panel surface is evaluated. Even in the virtual image optical system, the relative refractive power of the virtual image optical system differs between the meridional ray shown by the solid line and the sagittal ray shown by the dashed line.

For this reason, if the focus performance is optimal with sagittal rays, the meridional rays will be focused in front of the panel surface, and aberration will occur on the panel surface (shown in gray (mesh) in the figure). For this reason, of the double images that occur on the windshield described above, when the information display device 100 is placed below the windshield, the double image that occurs due to back-reflected light will occur above the image due to regular reflected light. Therefore, in order to reduce this, it is advisable to make the relative refractive power of the virtual image optical system through which the light rays that pass above the center of the entrance pupil pass smaller than the relative refractive power of the virtual image optical system through which the light rays that pass at the center and below the center of the other entrance pupil pass, so that aberration will occur below the chief ray.

On the other hand, the radius of curvature of a double image in a direction horizontal to the windshield is smaller at the periphery than at the center of the windshield, so in the periphery, the double image appears to the outside (away from the driver). Therefore, since the double image caused by back-reflected light appears outside the image caused by regular reflected light, in order to reduce this, it is advisable to make the relative refractive power of the virtual image optical system through which light rays passing outside the center of the entrance pupil pass smaller than the relative refractive power of the virtual image optical system through which light rays passing at the center of the other entrance pupil and inside the center pass, so that aberration occurs inside the chief ray.

Instead of the above-mentioned lens design, as shown in FIG. 35, the panel surface 4A, which is the image display device 4 constituting the real image projection optical system, can be curved to fit the curved surface of the windshield to obtain the same effect as above. More specifically, since the vertical radius of curvature of the windshield is smaller than the horizontal radius of curvature, if the windshield is replaced with a concave mirror, the optical power in the vertical direction is large. Therefore, the vertical radius of curvature of the panel is made smaller than the horizontal radius of curvature, and the optical Petzval sum is reduced in the entire system, thereby reducing the curvature of the field. This can be achieved by configuring the panel surface 4A to be curved convexly toward the light source 5A. Note that, as mentioned above, the windshield has different radii of curvature in the vertical and horizontal directions, so it would be preferable to appropriately set the curvature of the panel surface 4A to correspond to the different radii of curvature of the windshield.

In order to more efficiently capture light into the virtual image optical system described above, it is advisable to match the radius of curvature of panel surface 4B with the radius of curvature of light source 5B, as shown in FIG. 36.

Next, the configuration according to the embodiment of the present invention for reducing the above-mentioned double images and forming a highly visible virtual image will be described in detail with reference to Figs. 16 to 32. Fig. 16 is an enlarged view of the main parts of the liquid crystal panel and backlight light source 5 as the image display device 4 of the virtual image optical system according to the above-mentioned embodiment. An image is displayed on the liquid crystal panel display surface 11 by modulating the light from the backlight with a video signal input from the flexible substrate 10 of the liquid crystal panel, and a virtual image is generated from the displayed image by the virtual image optical system (in the embodiment, a free-form concave mirror and a free-form optical element), and the image information is conveyed to the driver.

In the above configuration, the light source element of the backlight light source 5 is a relatively inexpensive and reliable LED light source used as a solid light source. Since the LED is a surface-emitting type that is used to achieve high output, the light utilization efficiency is improved using the technical ingenuity described below. The light-emitting efficiency of the LED with respect to the input power varies depending on the light-emitting color, but is about 20 to 30%, and most of the remainder is converted into heat. For this reason, the frame on which the LED is attached is provided with heat-dissipating fins 13 made of a material with high thermal conductivity (for example, a metal material such as aluminum) to dissipate heat to the outside, which has the effect of improving the light-emitting efficiency of the LED itself.

In particular, LEDs currently on the market that emit red light have a significant drop in light emission efficiency when the junction temperature increases, and at the same time the chromaticity of the image changes, so it is preferable to give priority to lowering the LED temperature and increase the area of the corresponding heat dissipation fins to improve cooling efficiency. In the example shown in Figure 17, a light guide 18 is used to efficiently guide the diffused light from the LED to the liquid crystal panel 4, but it is preferable to combine it as a backlight light source that is entirely covered by, for example, an exterior member 16 to prevent dust and the like from adhering.

In addition, FIG. 17 shows an enlarged view of the main parts of the light source unit, including the LEDs as the light source, the light guide, and the diffusion plate. As is clear from FIG. 17, the openings 21a, 22a, 23a, and 24a that take in the divergent light rays from the LEDs of the light funnels 21, 22, 23, and 24 are made flat, and a medium is inserted between them and the LEDs to optically connect them, or they are made convex to have a focusing effect, thereby making the divergent light from the light source as parallel as possible and reducing the angle of incidence of the light incident on the interface of the light funnel. As a result, the divergence angle can be further reduced after passing through the light funnel, making it easier to control the light source light that is reflected by the light guide 18 and directed toward the liquid crystal panel.

Furthermore, in order to improve the efficiency of use of the divergent light from the LED, a PBS (Polarizing Beam Splitter) is used at the joint 25 between the light funnels 21-24 and the light guide 18 to perform polarization conversion and convert the light into the desired polarization direction, thereby improving the efficiency of the light incident on the LCD.

As mentioned above, when the polarization direction of the light from the light source is aligned, it is even better to use a material with low birefringence for the light guide 18, so that when the direction of polarization rotates and passes through the liquid crystal panel, problems such as coloring when displaying black do not occur.

As described above, the light flux from the LED with a reduced divergence angle is controlled by the light guide, reflected by the total reflection surface provided on the inclined surface of the light guide 18, diffused by the diffusion member 14 arranged between the opposing surface and the liquid crystal panel, and then enters the liquid crystal panel 4 as an image display device. In this embodiment, as described above, the diffusion member 14 is arranged between the light guide 18 and the liquid crystal panel 4, but the same effect can be obtained by providing the end surface of the light guide 18 with a diffusion effect, for example, by providing a fine uneven shape.

Next, the configuration of the light guide 18 described above and the effects obtained thereby will be described with reference to FIG. 20. FIG. 20 is an external view showing the light guide 18 of this embodiment. The light beam, whose divergence angle has been reduced by the light funnels 21 to 24 shown in FIG. 17, enters the light incident surface 18a of the light guide 18. At this time, the divergence angle in the vertical direction (the up and down direction in FIG. 21) is controlled by the effect of the shape of the incident surface (the cross-sectional shape is shown in FIG. 21), and the light propagates efficiently within the light guide 18.

Figure 21 is an enlarged cross-sectional view of the main part of the light guide, in which the light source light, whose divergence angle has been reduced by the light funnels 21-24, passes through the joint 25, enters from the entrance surface 18a as described above, and is totally reflected by the prism 18 provided on the opposing surface toward the opposing surface 17. The total reflection prism 18 is formed by dividing its shape into a stepped shape in the vicinity of the entrance surface 18a (enlarged view of part B) and at the end (enlarged view of part A) according to the divergence angle of the light beam incident on each surface, thereby controlling the angle of the total reflection surface. On the other hand, the division dimensions of the total reflection surface described above are used as variables to control the arrival position and energy amount of the divided light beam after reflection so that the light beam incident on the liquid crystal panel 4, which is the image display device, has a uniform light amount distribution within the exit surface of the liquid crystal panel 4.

Figure 18 shows the results of a simulation of the state in which light emitted from the backlight passes through the liquid crystal panel in the information display device 100 of this embodiment. Figure 18(a) is a diagram showing the state of light emission as viewed from the longitudinal direction of the liquid crystal panel, and Figure 18(b) is a diagram showing the state of light emission as viewed from the lateral direction of the liquid crystal panel. In this embodiment, in order to widen the horizontal angle of the FOV beyond the design, the horizontal diffusion angle is made larger than the vertical direction, and the design is such that the brightness of the virtual images viewed by the left and right eyes does not change drastically even if the driver moves the position of their eyes by shaking their head, etc.

In addition, by reducing the divergence angle of the backlight in the vertical direction, the divergence angle of the image displayed on the liquid crystal panel in the vertical direction of the screen is reduced, thereby suppressing the occurrence of double images. Figure 19 shows the luminance distribution on the exit surface of the liquid crystal panel 4 when using a backlight in which the light exit direction and intensity are controlled using a light guide 18, as in this embodiment. As is clear from Figure 19, in addition to the luminance distribution in the vertical direction (short side direction) of the screen, the slope of the luminance decrease outside the effective range in the vertical direction (long side direction) of the screen can be reduced.

As shown in Figures 23 and 28, the light emitted from the liquid crystal panel used as the image display device in the information display device 100 of this embodiment shows a predetermined transmittance in the range of ±50° when the viewing angles in the left-right and up-down directions are parameters (see Figure 22). Better transmittance characteristics can be obtained by setting the viewing angle range within ±40°. As a result, as shown in Figures 24 and 29, the brightness of the screen varies greatly depending on the direction (viewing angle) from which the screen is viewed in the left-right and up-down directions of the display screen. This is due to the angle characteristics of the backlight brightness shown in Figures 25 and 26.

For this reason, the inventors obtained high brightness by narrowing the viewing angle characteristics of the backlight to a small range by controlling the angle of the total reflection surface of the light guide 18 and the divergence angle of the light source light from the LEDs by the light funnels 21-24 so that the light emitted from the liquid crystal panel 4 to be taken into the virtual image optical system is as perpendicular to the screen as possible. Specifically, as shown in Figures 24 and 29, in order to obtain a high-brightness image, light in the range of ±30° in the left and right viewing angles is used, and considering the contrast performance shown in Figures 27 and 31, by narrowing it to ±20° or less, a virtual image using a source image with good image quality can be obtained.

As mentioned above, the contrast performance that determines the image quality of an image display device is determined by how much the luminance when black is displayed (referred to as black display luminance in Figures 30 and 32), which is the basis for determining image quality, can be reduced. For this reason, it is preferable to use an iodine-based polarizing plate with a high degree of polarization between the liquid crystal panel 4 and the backlight.

On the other hand, by using a dye-based polarizer as the polarizer provided on the optical element 2 side (light exit surface), high reliability can be achieved even when external light is incident or the environmental temperature is high.

When performing color display on the liquid crystal panel 4, a color filter is provided corresponding to each pixel. Therefore, when the light source color of the backlight is white, the color filter absorbs a lot of light, resulting in a large loss. Therefore, the inventors have used multiple LEDs as shown in FIG. 17 above:
(1) Adding a green LED, which contributes more to brightness than using multiple white LEDs.
(2) Add a red or blue LED to the white LED to improve the glossiness of the image.
(3) By arranging red, blue, and green LEDs separately and adding a green LED, which contributes greatly to brightness, and driving the LEDs separately, the color reproduction range is expanded, the glossy colors are improved, and the brightness is also improved.
(4) By implementing the above (3), the transmittance of each color filter with respect to the peak luminance of the red, blue, and green LEDs is increased, thereby improving the overall brightness.
(5) Furthermore, as a second embodiment of the backlight, a PBS is placed between the light funnel and the light guide to align the light to a specific polarization, thereby reducing damage to the polarizing plate on the entrance side of the liquid crystal panel. It goes without saying that the polarization direction of the polarizing plate placed on the entrance side of the liquid crystal panel should be the direction in which the polarized waves aligned in a specific direction pass after passing through the PBS (Polarizing Beam Splitter).

As described above, in the image display device 4 according to the embodiment of the present invention, a λ/4 plate can be provided on the exit surface of the liquid crystal display panel to circularly polarize the exiting light. As a result, the driver can monitor a clear virtual image even when wearing polarized sunglasses.

Furthermore, by forming the reflective film of the reflective mirror used in the virtual image optical system from a metal multilayer film, the angle dependency of the reflectance is small and the reflectance does not change depending on the polarization direction (P waves or S waves), making it possible to maintain uniform chromaticity and brightness on the screen.

Furthermore, by providing an optical component that is an ultraviolet reflective film or a combination of an ultraviolet reflective film and an infrared reflective film between the virtual image optical system and the windshield, the liquid crystal display panel and polarizing plate can be protected from temperature rise and damage even when external light (sunlight) is incident, so there is no loss of reliability for the information display device.

The virtual image optical system is optimally designed, taking into account the difference between the radius of curvature in the horizontal direction of the vehicle and the radius of curvature in the vertical direction of the windshield, which was the projection target in the conventional technology, and a concave mirror 1 with its concave surface facing the windshield 6 is placed between the windshield and the image display device or intermediate image display unit, thereby enlarging the image of the image display device 4 and reflecting it on the windshield 6. At this time, an optical element is placed between the concave mirror 1 and the image display device 4, and the image light beam that forms the enlarged image (virtual image) of the image that is imaged corresponding to the driver's viewpoint position passes through the optical element placed between the image displays, correcting the distortion and aberration generated by the concave mirror 1. Therefore, compared to the conventional virtual image optical system with only a concave mirror, a virtual image with significantly reduced distortion and aberration can be obtained.

Furthermore, in the configuration of this embodiment shown in FIG. 1, the virtual image obtained by reflection on the upper part of the windshield 6 (upper part in the vertical direction of the vehicle body) needs to be imaged farther away. Therefore, in order to image well the image light beam emitted from the upper part of the image display device on which the corresponding image is displayed, the focal length f1 of the optical element arranged between the concave mirror 1 and the image display device 4 described above is short, and conversely, the virtual image obtained by reflection on the lower part of the windshield 6 (lower part in the vertical direction of the vehicle body) needs to be imaged closer. Therefore, in order to image well the image light beam emitted from the lower part of the image display device on which the corresponding image is displayed, the composite focal length f2 of the multiple optical elements arranged between the concave mirror 1 and the image display device 4 described above should be set relatively long.

In addition, in this embodiment, in order to correct the screen distortion of the virtual image observed by the driver due to the difference between the radius of curvature of the windshield 6 in the horizontal direction (parallel to the ground) and the vertical direction (perpendicular to the horizontal direction of the windshield), optical elements with different axial symmetries with respect to the optical axis are arranged in the virtual image optical system, thereby achieving the correction of the above-mentioned distortion.

Above, we have described a surface light source device suitable for use in an electronic device equipped with an image display device according to various embodiments of the present invention. However, the present invention is not limited to only the above-mentioned embodiments, and includes various modified examples. For example, the above-mentioned embodiments are detailed descriptions of the entire system in order to explain the present invention in an easy-to-understand manner, and are not necessarily limited to those having all of the configurations described. In addition, it is possible to replace part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. In addition, it is possible to add, delete, or replace part of the configuration of each embodiment with other configurations.

100...information display device, 101...automobile, 1...concave mirror, 2...optical element, 4...image display device, 4A, 4B...liquid crystal display panel, 5A, 5B...backlight light source, 6...projected member (windshield), 7...housing, V1...virtual image, 8...eye box (observer's eye), 9...light source unit, R1...upper limit image light, R2...central image light, R3...lower limit image light, 10...flexible substrate, 11...image display surface, 12...frame, 13...fin, 14...diffusing member, 16...exterior member, 17...exit surface, 18...light guide, 20...light funnel unit, 21-24...light funnel, 36...emitted light from liquid crystal panel.

Claims (6)

An information display device that displays virtual image information on a windshield of a vehicle,
an image generating unit that generates an image of the image information;
an optical system for receiving the light beam from the image generating unit and displaying a virtual image obtained by enlarging the image generated by the image generating unit;
The optical system includes a concave mirror and an optical element,
the optical element is disposed between the image generating unit and the concave mirror, and is configured to correct an aberration of a virtual image obtained corresponding to a viewpoint position of a driver by a shape of the concave mirror and a shape of the optical element;
the optical system includes an optical element having a negative refractive power, and the optical element having a negative refractive power is configured to collect image light from the image generator, cause the image light to be incident on the optical element, and emit the image light toward the concave mirror,
An optical member that absorbs or reflects infrared rays of external light is provided between the optical system and the windshield.
Information display device. 2. The information display device according to claim 1,
The shape of the concave mirror is such that it has different average radii of curvature in the horizontal and vertical directions so as to compensate for the difference between the radii of curvature of the windshield in the vertical and horizontal directions.
Information display device. 2. The information display device according to claim 1,
the image generating unit is provided at an angle with respect to the optical axis of the concave mirror so that an image magnification at an upper end of a virtual image and an image magnification at a lower end of the virtual image are equal to each other;
Information display device. 2. The information display device according to claim 1,
The image generating unit includes a light source having a radius of curvature that matches the radius of curvature of a light exit surface of the image generating unit .
Information display device. The information display device according to claim 1 ,
The image generating unit includes a screen plate, displays an image on the screen plate, reflects image light from the screen plate on the windshield, and causes the reflected light to enter the eyes of the driver of the vehicle.
Information display device. The information display device according to claim 5,
The screen plate includes a microlens array in which microlenses are arranged two-dimensionally.
Information display device.

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