Image display device and head-mounted display

Abstract

To provide a technology for avoiding deterioration in quality of a display image due to a diffraction phenomenon in a laminated light field image display device.SOLUTION: Disclosed is a light field type image display device 1 in which two or more displays are laminated with a medium layer 30 sandwiched between each display. In at least one set of displays arranged adjacent to each other, an aperture size of pixels of the display closer to the front face is larger than the aperture size of the pixels of the display farther from the front face. The aperture size of the pixels in all displays increases monotonically from the side far from the front face toward the side close to the front face.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 reduction 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 reducing the pixel size in a stacked display system causes degradation in the quality of the displayed image due to the phenomenon of light diffraction. The inventors have solved this problem and completed the present invention.

[0008] An object of the present invention is to provide a technique for avoiding degradation of the quality of a displayed image due to diffraction phenomena 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 the displays. In at least one pair of adjacently arranged displays, the pixel aperture size of the display closer to the front is larger than the pixel aperture size of the display farther from the front, and the pixel aperture sizes of all the displays monotonically increase from the side farther from the front to the side closer to the front. The aperture size of the pixel of the display closer to the front is larger than the diameter of the Airy disk formed by the aperture of the pixel of the display farther from the front. .

[0010] A second aspect of the present invention is a head-mounted display comprising an image display device according to the first aspect and an eyepiece lens arranged on the front side of the image display device with its light incident surface facing the image display device. [Effects of the Invention]

[0011] The present invention contributes to avoiding degradation of the quality of the displayed image in a stacked light field image display device. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a schematic cross-sectional view showing an image display device according to one embodiment of the present invention. [Figure 2] FIG. 10 is a schematic cross-sectional view showing a conventional image display device. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, one embodiment of the present invention will be described 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 a liquid crystal layer 22 and a color filter 23, but does not include a backlight. In this embodiment, the pixel 25 of the second display 20 includes a first sub-pixel 25a in which a red filter R of the color filter 23 is disposed, a second sub-pixel 25b in which a green filter G of the color filter 23 is disposed, and a third sub-pixel 25c in which a blue filter B of the color filter 23 is disposed.

[0016] The aperture size of the pixel 25 of the second display 20 is larger than the aperture size of the pixel 15 of the first display 10. That is, the aperture size of the first subpixel 25a in the color filter 23 of the second display 20 is larger than the aperture size of the first subpixel 15a in the color filter 13 of the first display 10. Similarly, the aperture size of the second subpixel 25b is larger than the aperture size of the second subpixel 15b, and the aperture size of the third subpixel 25c is larger than the aperture size of the third subpixel 15c.

[0017] 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.

[0018] The first display 10 and the second display 20 are disposed with the medium layer 30 sandwiched between them. That is, the first display 10 and the second display 20 are disposed generally parallel to each other and spaced apart by a distance corresponding to the thickness of the medium layer 30. A typical example of a 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.

[0019] 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.

[0020] 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 the sub-pixel sizes of the two displays 110 and 120 are the same. 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.

[0021] 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.

[0022] However, the inventors' investigations revealed that there is an optical limit to such an improvement in resolution. This is due to interference between light incident on adjacent 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 a deterioration in the quality of the displayed image.

[0023] The diffraction limit will be explained below using the simplified example of circular apertures. Consider the case where two circular apertures of diameter D are located a certain distance apart from each other's circle centres. 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.

[0024]

number

[0025] 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.

[0026]

number

[0027] 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, degrading the quality of the displayed image. The diameter s of the Airy disk of the diffraction image at the rear distance L is expressed by the following equation 3:

[0028]

number

[0029] The inventors focused on this phenomenon and succeeded in avoiding the diffraction limit by making the aperture size of the pixel 25 in the second display 20 larger than the diameter of the Airy disk, which can be calculated using the distance between the two displays, which is roughly determined by the medium layer, and the aperture size of the pixel 15 in the first display 10. Specifically, the diffraction limit at adjacent pixels in the second display is avoided by making the aperture size of each subpixel in pixel 25 of the second display 20, into which light rays emitted from pixel 15 of the first display 10 are incident, larger than the aperture size of each subpixel in pixel 15 of the first display 10. This allows light rays that pass through pixel 15 and enter pixel 25 to pass through pixel 25 without optical obstruction, and furthermore, it is possible to avoid the diffraction limit caused by adjacent pixels in the second display, resulting in the display of a high-quality image without blur.

[0030] As described above, the image display device 1 according to this embodiment can suppress deterioration of image display quality due to the diffraction limit. Furthermore, by making the pixel aperture size of the second display located in the foreground the largest, the range of diameter values ​​of the Airy disk that does not cause the diffraction limit can be widened, which has the advantage of increasing the degree of freedom in the distance between the displays and making it easier to configure the display device.

[0031] In this embodiment, to simplify the explanation, the number of stacked displays is two, but this is merely one aspect of the present invention, and other configurations are also possible. For example, in the case of a three-layer stacked light field display structure, in addition to a first display and a second display, a third display may be arranged to be located closest to the front of the user, and the pixel aperture size may be the smallest in the first display, the second smallest in the second display, and the third display the largest, such that the pixel aperture size of the display closest to the user is the largest and the pixel aperture sizes become smaller as the display becomes farther from the user.

[0032] Alternatively, the pixel aperture size in the first display may be the smallest, and the pixel aperture size in the second display and the pixel aperture size in the third display may be equal and second smallest, so that the pixel aperture sizes in all displays other than the display farthest from the user are equal and larger than the pixel aperture size in the display farthest from the user. In this way, in the display device according to this embodiment, it is sufficient that the pixel aperture size monotonically increases from the innermost display, which is farthest from the front, to the innermost display, and it is effective even if there are multiple displays with the same aperture size.

[0033] 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.

[0034] (Example) 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 equipped with only a color filter to serve as the second display. The display resolution of the first display is 200 pixels per inch (200 ppi), and the aperture size of each sub-pixel is 42.3 μm. The display resolution of the second display is 100 pixels per inch (100 ppi), and the aperture size of each sub-pixel is 84.7 μm.

[0035] The first and second displays were completely overlapped in front view with a gap of 7 mm between them, and fixed in this state to obtain an image display device according to the example. This image display device has an air layer with a thickness of 7 mm as a medium layer.

[0036] An eyepiece was attached to the image display device according to the example, and a head-mounted display (HMD) according to the example was fabricated.

[0037] The image display quality according to the example was evaluated by visual observation using an HMD to check whether or not there was blur in the display.

[0038] In the example, the diameter of the Airy disk at the position of the second display is 78.0 μm according to Equation 3 above, while the aperture size of the pixel at the second display is 84.7 μm. From the above, it has been theoretically shown that in the HMD of the embodiment, the pixel aperture size of the second display and the diameter of the Airy disk satisfy the above conditions, and therefore it is possible to avoid impairment of image display quality due to diffraction. In fact, when visually inspecting the HMD image according to the example, no blurring of the display was observed, and the image was good.

[0039] (Comparative Example 1) An image display device and an HMD according to Comparative Example 1 were fabricated in the same manner as in Example, except that the display resolution of the second display was 200 pixels per inch (200 ppi). In Comparative Example 1, the diameter of the Airy disk at the position of the second display is 78.0 μm, the same as in Example 1. On the other hand, the aperture size of each subpixel of the second display is 42.3 μm, the same as that of the first display, and smaller than the diameter of the Airy disk. From the above, it is theoretically shown that the HMD according to Comparative Example 1 cannot avoid the diffraction limit. In fact, visual evaluation of the quality of the HMD image revealed blurred display and was not good.

[0040] (Comparative Example 2) An image display device and an HMD according to Comparative Example 2 were fabricated in the same manner as in Example, except that the distance between the first display and the second display was set to 3.5 mm. In Comparative Example 2, the diameter of the Airy disk at the position of the second display is 90.9 μm, while the aperture size of the pixel in the second display is 84.7 μm, which is smaller than the diameter of the Airy disk. From the above, it is theoretically shown that the HMD according to Comparative Example 2 cannot avoid the diffraction limit. In fact, visual evaluation of the quality of the HMD image revealed blurred display and was not good.

[0041] 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.

[0042] For example, in the present invention, the means for providing a light-emitting function to the display is not limited to the backlight described above. That is, when a self-emitting display is used as the display, various known structures can be employed, such as an organic light-emitting diode (OLED) display or a display including mini LEDs, micro LEDs, etc., in which the sub-pixels themselves emit light. In this case, the first display may be configured without a color filter by including, for example, red, green, and blue mini LEDs, micro LEDs, etc.

[0043] 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.

[0044] The display device according to this embodiment can display in color as a whole as long as one of the multiple displays is configured to be capable of displaying in color. The display capable of displaying in color may be located anywhere.

[0045] In a monochrome display, each aperture functions as an independent pixel, and sub-pixels do not exist. Even when the display device according to this embodiment is configured to include a monochrome display, the same effect can be achieved by setting the size relationship between each aperture and the sub-pixel aperture of the display equipped with a color filter as described above.

[0046] In the image display device according to the present invention, as shown in the examples, the diameter of the Airy disk is calculated, and the aperture size of the subpixel of the second display and the distance between the first display and the second display are determined based on this, so that the aperture size of each display constituting the display device can be freely set and the effect can be achieved. [Explanation of symbols]

[0047] 1 Image display device 10 Primary Display 11 Backlight 15a, 25a: First sub-pixel (pixel) 15b, 25b Second sub-pixel (pixel) 15c, 25c Third sub-pixel (pixel) 20 Second Display 30 Medium Layer

Claims

1. A light field type image display device in which two or more displays are stacked with a medium layer sandwiched between the displays, In at least one pair of the displays arranged adjacent to each other, the pixel aperture size of the display closer to the front is larger than the pixel aperture size of the display farther from the front, The aperture size of all the pixels of the display monotonically increases from the side farther from the front surface to the side closer to the front surface, the aperture size of the pixel of the display closer to the front is larger than the diameter of an Airy disk formed by the apertures of the pixel of the display farther from the front; Image display device.

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

3. An image display device according to claim 1 or 2, an eyepiece disposed on the front side of the image display device with a light incident surface facing the image display device; Equipped with Head-mounted display.

Images