Image display device and head-mounted display

Abstract

To provide a technique that facilitates achieving higher resolution in a lamination system light field image display device.SOLUTION: In a light field system image display device 1 in which two or more displays are laminated while sandwiching a medium layer 30, only one of the displays is a display for performing color display, and a display closest to a front is a display for performing monochromatic display.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an image display device, and more particularly to an image display device using light field technology. The present invention also relates to a head-mounted display equipped with the image display device. [Background technology]

[0002] In recent years, light field technology, which controls the light rays emitted from an image display and incident on the user's eyes, has been attracting attention. Humans can see various objects when light rays reflected from the object enter the pupil. Light field technology artificially reproduces these reflected light rays on an image display, resulting in a sense of depth similar to what humans see in nature.

[0003] There are several types of image display devices that use light field technology, and representative examples include a microlens array type and a stacked type. In the microlens array method, an array of lenses with diameters on the order of micrometers is attached to the surface of the display panel, and the microlens array splits the light emitted from the display into bundles of rays in specific directions and directs them toward the viewer's eyes.

[0004] The other stacked display method, as described in Patent Document 1, has a structure in which two display panels are stacked with a medium layer sandwiched between them. The first display panel, located farther from the viewer, is equipped with a backlight or the like and has the function of emitting light. The second display panel, located closer to the viewer, does not have the function of emitting light and is illuminated by the light emitted from the first display. In the stacked method, a ray of light emitted from a pixel on a first display panel passes through a pixel on a second display panel and enters the viewer's eye, and the ray of light is controlled by the combination of pixels through which it passes.

[0005] In the microlens array method, one microlens covers multiple pixels, so in principle a decrease in display resolution is unavoidable. On the other hand, the stacked display method requires careful alignment of the first and second display panels, but has the advantage that it is not so difficult to increase the resolution. The latest displays have been reported to have more than 900 pixels per inch. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Special Publication No. 2020-521174 Summary of the Invention [Problem to be solved by the invention]

[0007] The inventors have found that when pixel size is reduced in the stacked method, there is an optical limit to the display resolution. The inventors have solved this problem and completed the present invention.

[0008] An object of the present invention is to provide a technique that makes it easier to achieve higher resolution in a stacked light field image display device. [Means for solving the problem]

[0009] A first aspect of the present invention is a light field type image display device in which two or more displays are stacked with a medium layer sandwiched between them. In this image display device, only one of the displays is a color display, and the display closest to the front is a monochrome display. This image display device is configured so that all pixels of the monochrome display are capable of transmitting light rays incident from the color display. The thickness of the medium layer is set to a value that allows light rays from multiple pixels of a color display to be incident on one pixel of a monochrome display and that does not cause the diffraction limit of the multiple incident light rays. In this image display device, the display farthest from the front may have a light-emitting function.

[0010] A second aspect of the present invention is a head-mounted display including the image display device according to the first aspect and an eyepiece. [Effects of the Invention]

[0011] The present invention contributes to further improving the resolution of stacked light field image display devices. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a schematic cross-sectional view showing an image display device according to a first embodiment of the present invention. [Figure 2] FIG. 10 is a schematic cross-sectional view showing a conventional image display device. [Figure 3] FIG. 2 is a diagram showing ray paths in the image display device according to the first embodiment. [Figure 4] FIG. 4 is a schematic cross-sectional view showing an image display device according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] A first embodiment of the present invention will be described below with reference to FIGS. 1 is a diagram schematically illustrating an image display device 1 according to this embodiment. The image display device 1 is a light field type image display device, and includes two liquid crystal displays: a first display 10 and a second display 20. The image display device has the second display 20 facing the front, and the user views the displayed image from the front side.

[0014] The first display 10 has a backlight 11, a liquid crystal layer 12, and a color filter 13 arranged in this order, and has a structure generally similar to that of known color liquid crystal displays. In this embodiment, the pixel 15 of the first display 10 includes a first sub-pixel 15a in which a red filter R of the color filter 13 is arranged, a second sub-pixel 15b in which a green filter G of the color filter 13 is arranged, and a third sub-pixel 15c in which a blue filter B of the color filter 13 is arranged.

[0015] The second display 20 includes only a liquid crystal layer 22, and does not include a backlight or a color filter. In other words, the second display 20 has a structure in which the backlight and color filter are removed from the first display 10. Although the second display 20 does not include a color filter, for the sake of convenience, the subpixels 25a, 25b, 25c, etc. of the second display 20 will be referred to as subpixels in the following description. In the second display 20, each of the subpixels 25a, 25b, 25c, etc. has the same aperture size as each of the subpixels of the first display 10. 1 shows a schematic cross-sectional view, and in this embodiment, the first display 10 and the second display 20 have rectangular or square shapes of the same dimensions when viewed from the front. The first display 10 and the second display 20 are arranged so that their four sides coincide with each other when viewed from the front of the image display device 1 as seen from the second display 20 side.

[0016] The first display 10 and the second display 20 are disposed with a medium layer 30 sandwiched between them. A typical example of the medium layer is an air layer, but layers made of argon (refractive index 1.000281), carbon dioxide (refractive index 1.000449), helium (refractive index 1.000036), hydrogen (refractive index 1.000140), nitrogen (refractive index 1.000297), oxygen (refractive index 1.000276), or mixtures thereof, which have refractive indices similar to those of air (refractive index 1.0002926), can also be used. Furthermore, a vacuum layer (refractive index 1.0000) can also be used and is included in the medium layer of the present invention.

[0017] The path of light rays in the image display device 1 will be described. First, light rays are emitted from the backlight 11 of the first display 10. The emitted light rays pass through the liquid crystal layer 12 and the color filter 13, and further through the medium layer 30, and then travel toward the second display 20.

[0018] 2 shows a conventional image display device 100. Like the image display device 1, the image display device 100 is a stacked light field display, but both of the two displays 110 and 120 are provided with color filters 13. In the image display device 100, light rays from the display 110 pass through sub-pixels in which color filters of the same color are arranged. For example, as shown in Fig. 2, light rays that pass through a green filter G and exit the display 110 pass through a sub-pixel in the display 120 in which a green filter G is arranged, enter the viewer's eye, and are perceived as green.

[0019] To improve the display resolution of a stacked light field display, the pixel size of the two displays can be reduced. For example, if the aperture size of the sub-pixels that make up each pixel is reduced to one-third, the display resolution will theoretically triple.

[0020] However, the inventors' investigations revealed that there is an optical limit to such resolution improvement. This is due to interference of light incident on adjacent sub-pixels on the display 120 closest to the viewer. In other words, it was found that the diffraction limit caused by light interference causes image blurring, resulting in degradation of image quality.

[0021] The diffraction limit will be explained below using the simplified example of circular apertures. Suppose two circular apertures of diameter D are located a certain distance apart from each other's circle centers. When these two apertures are illuminated with incoherent light, the diffracted image at the apertures can be expressed as the superposition (interference) of light passing through the two apertures. When the diffracted images from the two apertures interfere with each other, the diffraction limit is the distance resolution at which the two images can be distinguished as two points. There are various definitions of the diffraction limit, but the most well-known is the Rayleigh diffraction limit, which is expressed in equation 1 below.

[0022]

number

[0023] In Equation 1, λ is the wavelength of light, and 550 nm is generally used, to which the human eye has good sensitivity. L is the rear distance from the aperture. When the aperture shape is circular, the coefficient is 1.22, but when it is rectangular, the coefficient is 1, so the diffraction limit δ of a rectangular aperture is expressed by Equation 2 below.

[0024]

number

[0025] If the distance between the diffraction images is δ or more, the diffraction images from the two apertures can be separated and identified. Conversely, if it is smaller than δ, the diffraction images from the two apertures will appear as a single point, resulting in degraded image quality. The diameter s of the Airy disk of the diffraction image at the rear distance L is expressed by the following equation 3:

[0026]

number

[0027] The inventors focused on this phenomenon and designed a configuration in which no color filter is provided on the second display 20, onto which light emitted from the first display 10 is incident. As a result, as shown in Fig. 3, light that passes through the second subpixel 15b and enters the second display 20 can pass not only through the subpixel 25b located at the position corresponding to the green filter G, but also through the subpixels located at the positions corresponding to the subpixels 25a, 25c, etc.

[0028] As a result, the amount of light that passes through the second subpixel 15b of the first display 10 and enters the second display 20 and passes through the subpixel of the second display 20 is three times that of the conventional image display device 100 shown in Figure 2. In other words, if the aperture size of the subpixel of the first display 10 and the aperture size of the subpixel of the second display 20 are the same, the display resolution of the image display device 1 is, in principle, three times that of the image display device 100.

[0029] As described above, the image display device 1 according to this embodiment has the advantage that it is easy to improve the resolution while suppressing degradation of image display quality due to the diffraction limit. In the image display device 1, a monochrome image is displayed on the second display 20, but since colored light is incident from the first display 10 behind it, a color image is visible to the viewer.

[0030] A second embodiment of the present invention will be described with reference to Fig. 4. In the following description, components common to those already described will be assigned the same reference numerals and redundant description will be omitted.

[0031] FIG. 4 is a schematic cross-sectional view showing an image display device 200 according to this embodiment. The image display device 200 is a three-layer stacked light field display having a first display 210, a second display 20, and a third display 230 between them. The first display 210 includes a backlight 11 similar to the first embodiment, but does not include a color filter. The third display 230 includes a liquid crystal layer 232 and a color filter 233. That is, the first display 210 has a structure in which the color filter is removed from the third display 230 and a backlight is attached, and the second display 20 has a structure in which the color filter is removed from the third display 230. As in the first embodiment, the first display 210 and the second display 20 do not include a color filter, but for convenience of explanation, they will be referred to as subpixels.

[0032] In the image display device 200, light rays from the first display 210 are incident on each subpixel of the third display 230. Furthermore, light rays that have passed through each subpixel of the third display 230 are incident on each subpixel of the second display 20. 4, as in the first embodiment, the number of light rays emitted from the third display 230 that can pass through the second display 20 is three times that of the light rays emitted from the third display 230, which makes it easy to improve the display resolution while suppressing the diffraction limit. Furthermore, by providing the third display 230, it is possible to display images with greater depth than in the first embodiment.

[0033] In this embodiment, the display with the color filter need not be the display closest to the viewer, and therefore the display with the color filter is not limited to the third display 230 described above, and may be the first display 210. Also, four or more displays may be provided, provided that only one display has a color filter and it is not the closest to the viewer.

[0034] The image display device according to the present invention will be further described using examples and comparative examples. The technical scope of the present invention is not limited solely by the specific content of the examples.

[0035] Example 1 Example 1 is an example corresponding to the first embodiment. Two IPS (In-Plane Switching) liquid crystal display panels of the same size and rectangular shape when viewed from the front were prepared. One display was equipped with a backlight and a color filter to serve as the first display, and the other was used as the second display. The display resolution is 480 pixels per inch (480 ppi) and the pixel aperture size is 52.9 μm.

[0036] The first display and the second display were completely overlapped in front view with a gap of 7 mm between them and fixed in this state to obtain the image display device according to Example 1. This image display device has an air layer with a thickness of 7 mm as a medium layer.

[0037] An eyepiece was attached to the image display device according to Example 1 to fabricate a head-mounted display (HMD) according to Example 1.

[0038] The image display quality according to Example 1 was evaluated by visual observation using an HMD to determine whether or not there was blurred display.

[0039] In Example 1, the diameter of the Airy disk at the position of the second display is 145.5 μm according to the above-mentioned formula 3. Because the light emitted from the first display can pass through the second display in three times the quantity, the pixel aperture size at this time is equivalent to 52.9 μm × 3 = 158.7 μm, which theoretically shows that the HMD of Example 1 can avoid the diffraction limit. When visually inspecting the HMD according to Example 1, no blurring of the display was observed, and the display was good.

[0040] Example 2 Example 2 is an example corresponding to the second embodiment. Three IPS-type liquid crystal display panels similar to those in Example 1 were prepared. One of the panels was equipped with a backlight to serve as the first display, the other was equipped with a color filter to serve as the third display, and the remaining panel was used as the second display. The first display, the third display, and the second display were completely overlapped in a front view with a gap of 7 mm between each other and fixed in this state to obtain an image display device according to Example 2. This image display device has an air layer with a thickness of 7 mm as a medium layer.

[0041] As in Example 1, an HMD according to Example 2 was fabricated and the image display quality was evaluated. In Example 2, the diameter of the Airy disk at the position of the second display is 145.5 μm, similar to Example 1. The pixel aperture size of the second display is also equivalent to 158.7 μm, similar to Example 1, and therefore it has been theoretically shown that the diffraction limit can also be avoided in the HMD of Example 2. No blurring of the display was observed visually in the HMD according to Example 2, which was favorable. Furthermore, an image display with a greater sense of depth than Example 1 was possible.

[0042] (Comparative Example 1) An image display device and an HMD according to Comparative Example 1 were produced in the same manner as in Example 1, except that a color filter was attached to the second display. In Comparative Example 1, the diameter of the Airy disk at the position of the second display is 145.5 μm, the same as in Example 1. On the other hand, the aperture size of the pixel of the second display remains at 52.9 μm due to the inclusion of a color filter, and it has been theoretically shown that the HMD of Comparative Example 1 cannot avoid the diffraction limit. In fact, visual evaluation of the image quality revealed blurred display and was not good.

[0043] (Comparative Example 2) An image display device and an HMD according to Comparative Example 2 were fabricated in the same manner as in Example 2, except that color filters were attached to the first display and the second display. In Comparative Example 2, the diameter of the Airy disk at the position of the second display is 145.5 μm, the same as in Example 2. On the other hand, the aperture size of the pixel of the second display remains at 52.9 μm due to the inclusion of a color filter, and it has been theoretically shown that the HMD of Comparative Example 2 cannot avoid the diffraction limit. In fact, visual evaluation of the image quality revealed blurred display and was not good.

[0044] Although the embodiments and examples of the present invention have been described above, the specific configurations are not limited to these embodiments, and modifications and combinations of the configurations may be made without departing from the spirit of the present invention.

[0045] For example, in the present invention, the mechanism by which the first display emits light is not limited to the backlight described above. In other words, when a self-luminous display is used as the first display, various known structures can be used, such as an organic light-emitting diode (OLED) display, or a display equipped with a mini LED or micro LED in which the sub-pixels themselves emit light.

[0046] Furthermore, when the first display is configured to display color, it may be configured without a color filter by including, for example, red, green, and blue mini LEDs or micro LEDs.

[0047] Furthermore, the image display device itself may not have a light-emitting function, and may be configured to be able to display images by using a separate light source disposed behind it.

[0048] In the image display device according to the present invention, if the aperture size of the subpixel of the second display that displays monochrome is too small, the diffraction limit may occur. As shown in the examples, this can be prevented by calculating the diameter of the Airy disk and determining the aperture size of the subpixel of the second display based on this. [Explanation of symbols]

[0049] 1,200 Image display device 10, 210 First Display 20 Second Display 30 Medium Layer 230 Third Display

Claims

1. A light field type image display device in which two or more displays are stacked with a medium layer sandwiched therebetween, Only one of the displays is a color display, the display closest to the front is a monochrome display, all pixels of the monochrome display are configured to be able to transmit light rays incident from the color display, the thickness of the medium layer is set to a value such that light rays from a plurality of pixels of the color display are incident on one pixel of the monochrome display and the incident light rays do not reach the diffraction limit; Image display device.

2. The display furthest from the front has a light-emitting function. The image display device according to claim 1 .

3. The image display device according to claim 1 or 2; An eyepiece and Equipped with Head-mounted display.

Images