A large-viewing-angle color holographic 3D display system based on liquid crystal grating

Abstract

The application provides a large-viewing-angle color holographic 3D display system based on a liquid crystal grating, which comprises a red light source, a green light source, a blue light source, a shutter I, a shutter II, a shutter III, a half-transmission half-reflection mirror I, a half-transmission half-reflection mirror II, a reflecting mirror, a signal controller, a beam expander, a lens I, a spatial light modulator, a half-transmission half-reflection mirror III, a lens II, an aperture, a liquid crystal grating and a lens III. The red light source, the green light source, the blue light source and the three shutters are used for generating red light, green light and blue light which are emitted in time sequence. The half-transmission half-reflection mirror I, the half-transmission half-reflection mirror II and the reflecting mirror are used for superimposing the optical axes of the red light, the green light and the blue light. The signal controller is used for controlling the on-off states of the three shutters, simultaneously generating holographic images of a 3D object in red, green and blue three color channels which are superimposed with a blazed grating, and loading the holographic images in time sequence to the spatial light modulator. The liquid crystal grating is located at the back focal plane of the lens II and at the front focal plane of the lens III, the holographic diffraction light fields of the red, green and blue three color channels are second-order diffracted in different liquid crystal layer regions of the liquid crystal grating, and second-order diffraction images with consistent intervals are generated. After passing through the lens III, a large-viewing-angle color holographic reconstruction image is received by a CCD.

Description

I. TECHNICAL FIELD

[0001] The present application relates to holographic display technology, and more particularly, to a large-viewing-angle color holographic 3D display system based on liquid crystal grating. II. BACKGROUND

[0002] Holographic display technology can completely record and reconstruct the wavefront information of a 3D object, and is one of the most promising naked-eye 3D display technologies, which has attracted widespread attention. However, the pixel size of the commonly used spatial light modulator on the market is in the micron level, and the maximum viewing angle of the monochromatic holographic 3D display achieved internationally is about 60°. In addition, in the process of realizing color holographic display, the holograms of the red, green and blue color channels all need to be calculated and displayed separately. For example, researchers at Bilkent University used red, green and blue lasers to irradiate three spatial light modulators loaded with red, green and blue holograms, respectively, to achieve color holographic display effect. However, the system structure required by this technology is usually complex, the cost is also high, and the spatial alignment accuracy of the red, green and blue holographic reconstruction images is very high. Researchers at the National Institute of Information and Communications Technology in Japan built a color holographic 3D display system using time division multiplexing method. Researchers at Tokyo University of Agriculture and Technology built a color holographic display system using a single high refresh rate digital micromirror device. In this system, a fiber-coupled laser diode emits red, green and blue lasers at different angles and times to irradiate the digital micromirror device, and then the high-order diffracted light is used for holographic reconstruction. Although the existing methods can realize color holographic display based on a single or three spatial light modulators, they all have the problem of limited display viewing angle. III. SUMMARY

[0003] The present application provides a large-viewing-angle color holographic 3D display system based on liquid crystal grating. As shown in FIG. 1, the system includes a light source 1, a liquid crystal grating 2, a spatial light modulator 3, a lens 4 and a screen 5. Figure 1As shown, the system comprises a red light source, a green light source, a blue light source, a shutter I, a shutter II, a shutter III, a half-mirror I, a half-mirror II, a mirror, a signal controller, a beam expander, a lens I, a spatial light modulator, a half-mirror III, a lens II, an aperture, a liquid crystal grating and a lens III. The red light source, the green light source, the blue light source and the three shutters are used to generate red light, green light and blue light which are emitted in time sequence. The half-mirror I, the half-mirror II and the mirror are used to coincide the optical axes of the red light, the green light and the blue light. The red light, the green light and the blue light which have coincided the optical axes are reflected by the half-mirror III to the spatial light modulator after collimation and expansion by the beam expander and the lens I. The signal controller is used to control the on-off states of the three shutters, to generate holograms of the red, green and blue color channels of a 3D object which have superimposed the blazed grating, and to load them in time sequence to the spatial light modulator. The loading sequence of the holograms of the red, green and blue color channels is consistent with the emission sequence of the red light, the green light and the blue light. The holographic diffraction light field of the red, green and blue color channels which has been modulated by the spatial light modulator is incident to different liquid crystal layer regions of the liquid crystal grating after passing through the lens II and the aperture. The liquid crystal grating is located at the back focal plane of the lens II and at the front focal plane of the lens III, and the second-order diffraction of the diffracted image is controlled by applying a voltage to the liquid crystal grating. The holographic diffraction light field of the red, green and blue color channels is second-order diffracted in different liquid crystal layer regions of the liquid crystal grating, and second-order diffracted images which have consistent intervals are generated. After passing through the lens III, a color holographic reconstruction image with a large viewing angle is received by the CCD.

[0004] As shown in the accompanying drawings, Figure 2 The liquid crystal grating comprises an upper substrate, a liquid crystal layer, a pixel electrode, a common electrode and a lower substrate, wherein the liquid crystal layer is divided into three regions, region I, region II and region III. The widths of the common electrodes of the three regions are w r , w g and w b , respectively. In each region, the width of the pixel electrode is the same as the width of the common electrode. The pitches of the liquid crystal gratings of region I, region II and region III are d r , d g and d b , respectively. A driving voltage is applied to the pixel electrodes of the three regions, and the voltage is equal in size, and the common electrodes of all regions are grounded. Through such a design, the periods of the liquid crystal molecules of region I, region II and region III of the liquid crystal grating when the voltage is applied are different from each other, and therefore the pitches of the different regions are also different from each other. The pitch of region I is the largest, the pitch of region II is the second, and the pitch of region III is the smallest. In addition, the adjacent electrodes between different regions are all set to common electrodes, so as to eliminate the edge electric field of the adjacent regions and improve the control accuracy of the different regions of the liquid crystal grating.

[0005] The system proposed in this invention achieves large-view color holographic 3D display through the following method: First, information from the red, green, and blue color channels of the 3D object is extracted. Then, a hologram for each color channel is generated using a signal controller, and a corresponding blazed grating is superimposed on the hologram. The resolution of both the hologram and the blazed grating is the same as the resolution of the spatial light modulator, a×b, where a is the horizontal resolution and b is the vertical resolution. The parameters of the blazed grating satisfy the following formula:

[0006] 2d sinγ=λ (1)

[0007] d = np (2)

[0008] Where d is the pitch of the blazed grating, γ is the blazed angle of the blazed grating, λ is the wavelength, p is the pixel spacing of the spatial light modulator, and n is the number of pixels in a single period of the blazed grating. The blazed angle γ is expressed by formula (3):

[0009]

[0010] in It is the phase change amount in each cycle of the blazed grating. When a blazed grating is superimposed on a hologram, and collimated incident light is perpendicularly incident on the hologram, the holographic diffraction field is deflected to a certain extent, with a deflection angle θ = 2γ.

[0011] As attached Figure 3 As shown, the red channel hologram is superimposed on blazed grating I, the green channel hologram on blazed grating II, and the blue channel hologram on blazed grating III, respectively, and then loaded onto the spatial light modulator in time sequence T1, T2, and T3. At time T1, the signal controller opens shutter I and closes shutters II and III. At this time, the spatial light modulator is illuminated by collimated and expanded red light, and the red channel hologram and blazed grating I are loaded onto the spatial light modulator. At time T2, the signal controller opens shutter II and closes shutters I and III. At this time, the spatial light modulator is illuminated by collimated and expanded green light, and the green channel hologram and blazed grating II are loaded onto the spatial light modulator. At time T3, the signal controller opens shutter III and closes shutters I and II. At this time, the spatial light modulator is illuminated by collimated and expanded blue light, and the blue channel hologram and blazed grating III are loaded onto the spatial light modulator. When red, green, and blue light illuminate the holograms and corresponding blazed gratings of the three color channels in time sequence T1, T2, and T3 respectively, the diffracted light fields of the three colors are seen simultaneously due to the persistence of vision effect of the human eye.

[0012] After the holographic diffraction light field is modulated by the liquid crystal grating, it undergoes secondary diffraction, producing M second-order diffraction images. The number of second-order diffraction images M satisfies the following formula:

[0013]

[0014] wherein δ is the incident angle of the light beam entering the liquid crystal grating, D is the pitch of the liquid crystal grating, and M≧4 is ensured by adjusting the wavelength λ, the pitch D of the liquid crystal grating and the incident angle δ.

[0015] The wavelengths of the red light, the green light and the blue light are respectively denoted as λ r , λ g and λ b . r g b The holographic diffracted light field of the red, green and blue color channels respectively irradiates different liquid crystal layer regions of the liquid crystal grating after passing through the lens II. As shown in the accompanying drawings, at T1, the deflection angle of the holographic diffracted light field of the red channel is θ r , the holographic diffracted light field of the red channel converges to a point on the focal plane of the lens II after passing through the lens II with a focal length f, and M red second-order diffraction images are generated after modulation by the liquid crystal grating region I. At this time, the interval L r between two adjacent red second-order diffraction images and the distance H r from the converging point on the focal plane of the lens II to the optical axis are respectively: Figure 4

[0016]

[0017] H r = f tanθ r (6)

[0018] At T2, the deflection angle of the holographic diffracted light field of the green channel is θ g , the holographic diffracted light field of the green channel converges to a point on the focal plane of the lens II after passing through the lens II with a focal length f, and M green second-order diffraction images are generated after modulation by the liquid crystal grating region II. At this time, the interval L g between two adjacent green second-order diffraction images and the distance H g from the converging point on the focal plane of the lens II to the optical axis are respectively:

[0019]

[0020] H g = f tanθ g (8)

[0021] At T3, the deflection angle of the holographic diffracted light field of the blue channel is θ b , the holographic diffracted light field of the blue channel converges to a point on the focal plane of the lens II after passing through the lens II with a focal length f, and M blue second-order diffraction images are generated after modulation by the liquid crystal grating region III. At this time, the interval L b between two adjacent blue second-order diffraction images and the distance H b from the converging point on the focal plane of the lens II to the optical axis are respectively:respectively:

[0022]

[0023] H b = f tan theta b (10)

[0024] Adjusting the deflection angle theta of the holographic diffraction light field of red, green and blue color channels r , theta g and theta b , when H r -H g =L r , H g -H b =L g , H r -H b =2L b , M-2 color second-order diffraction images are received on the CCD, at this time the viewing angle of the color holographic reconstruction image is (M-2) beta, beta is the color holographic 3D display viewing angle when the liquid crystal grating is not used, and the following formula is satisfied:

[0025]

[0026] Preferably, the hologram generation method of the 3D object in the system is a Fresnel diffraction-based computer hologram generation method, including a lookup table method, a new lookup table method, a wavefront phase reconstruction method and an angular spectrum algorithm. Four, brief description of the drawings

[0027] Fig. 1 is a structural schematic diagram of a large-viewing-angle color holographic 3D display system based on a liquid crystal grating according to the present application. Figure 1 Fig. 2 is a structural schematic diagram of a liquid crystal grating according to the present application.

[0028] Figure 2 Fig. 3 is a schematic diagram of a hologram loading process according to the present application.

[0029] Fig. 4 is a schematic diagram of a large-viewing-angle color holographic display principle according to the present application. Figure 3 (a) is a holographic diffraction light field distribution schematic diagram of a red channel; (b) is a holographic diffraction light field distribution schematic diagram of a green channel; (c) is a holographic diffraction light field distribution schematic diagram of a blue channel; and (d) is a color holographic diffraction reconstruction schematic diagram.

[0030] Figure 4 Figure 4 Figure 4 Figure 4 Figure 4

[0031] The figure numbers in the above-mentioned figures are as follows: ​​​​​​​

[0032] (1) red light source, (2) green light source, (3) blue light source, (4) shutter I, (5) shutter II, (6) shutter III, (7) half-mirror I, (8) half-mirror II, (9) mirror, (10) beam expander, (11) lens I, (12) signal controller, (13) spatial light modulator, (14) half-mirror III, (15) lens II, (16) diaphragm, (17) liquid crystal grating, (18) lens III, (19) CCD.

[0033] It should be understood that the above-mentioned figures are only schematic and not drawn to scale. V. DETAILED DESCRIPTION

[0034] The following detailed description of an embodiment of a large-viewing-angle color holographic 3D display system based on a liquid crystal grating is provided to further describe the present application. It is necessary to point out that the following embodiment is only used to further illustrate the present application and cannot be understood as a limitation on the protection scope of the present application. Those skilled in the art can make some non-essential improvements and adjustments to the present application according to the above description of the present application, which still falls within the protection scope of the present application.

[0035] The system embodiment of the present application is as follows: in the experimental system, the wavelength of the red light source is 672 nm, the wavelength of the green light source is 533 nm, the wavelength of the blue light source is 475 nm, the spatial light modulator is a reflective pure-phase spatial light modulator, the pixel pitch of the spatial light modulator is 3.74 μm, the resolution is 3840x2160, the refresh rate is 180 Hz, the phase modulation capability is 2π, and the focal length of lens I, lens II and lens III is all 30 cm. The signal controller synchronously controls the on-off state of the three shutters, the generation and loading of the hologram on the spatial light modulator. The color object "lotus" is used as the recorded object, the resolution is 1500x900, and the reconstruction distance is 15 cm. The signal controller is used to generate the holograms of the red, green and blue channels of the "lotus" and the corresponding blazed gratings and to time-sequentially load them onto the spatial light modulator. The resolution of the holograms and the blazed gratings is all 3840x2160. The phase change of a single period of the blazed gratings is all 2π, the number of pixels of a single period of the blazed grating I is 3, the number of pixels of a single period of the blazed grating II is 4, and the number of pixels of a single period of the blazed grating III is 14.5.

[0036] The three-channel holographic diffracted light field generated by the spatial light modulator is incident into the corresponding regions of the liquid crystal grating after passing through the lens II. In the liquid crystal grating, the width of the three regions is 1.6 cm. The pitch of region I is 12.6 μm, the pitch of region II is 10 μm, and the pitch of region III is 8.9 μm. When a voltage is applied on the pixel electrode, a spatially non-uniform gradient electric field distribution is formed between the pixel electrode and the common electrode, which induces the liquid crystal molecules to form a parabolic phase distribution, and produces a light splitting effect similar to a phase-type grating. Under the condition of applying a voltage, since the pitch of each region of the liquid crystal grating is different, the three regions form three liquid crystal gratings with different periods, which diffract the holographic diffracted light field of the red, green and blue channels to the second order, so that the interval between the second-order diffracted images of each color channel is 1.6 cm. The incident angle of the control beam into the liquid crystal grating is controlled, so that the holographic diffracted light field of the red, green and blue channels generates nine second-order diffracted images with equal intervals after being modulated by the liquid crystal grating, thereby obtaining seven completely overlapped color second-order diffracted images, realizing large-viewing-angle color holographic 3D display. At this time, the viewing angle of the color holographic reconstruction image is 50.96°, while the viewing angle of the color holographic reconstruction image without using the liquid crystal grating is 7.28°.

Claims

1. A large view angle color holographic 3D display system based on liquid crystal grating, characterized in that, The system comprises a red light source, a green light source, a blue light source, a shutter I, a shutter II, a shutter III, a half-transmission half-reflection mirror I, a half-transmission half-reflection mirror II, a mirror, a signal controller, a beam expander, a lens I, a spatial light modulator, a half-transmission half-reflection mirror III, a lens II, an aperture, a liquid crystal grating and a lens III; wherein the red light source, the green light source, the blue light source and the three shutters are used to generate red light, green light and blue light which are emitted in time sequence, the half-transmission half-reflection mirror I, the half-transmission half-reflection mirror II and the mirror are used to coincide the optical axes of the red light, the green light and the blue light, the red light, the green light and the blue light which coincide in the optical axes are reflected by the half-transmission half-reflection mirror III to the spatial light modulator after collimation and expansion by the beam expander and the lens I; the signal controller is used to control the on-off state of the three shutters, to generate simultaneously holograms of the red, green and blue color channels of a 3D object which are superimposed with a blazed grating, and to load them in time sequence to the spatial light modulator, the loading sequence of the holograms of the red, green and blue color channels is consistent with the emission sequence of the red light, the green light and the blue light, the holographic diffraction light field of the red, green and blue color channels which is modulated by the spatial light modulator is incident to different liquid crystal layer regions of the liquid crystal grating after passing through the lens II and the aperture, the liquid crystal grating is located at the back focal plane of the lens II and at the front focal plane of the lens III, the second-order diffraction of the diffraction image is generated by applying a voltage to the liquid crystal grating, the holographic diffraction light field of the red, green and blue color channels is second-order diffracted in different liquid crystal layer regions of the liquid crystal grating, second-order diffraction images which are completely consistent in interval are generated, and a color holographic reconstruction image with a large viewing angle is received by the CCD after passing through the lens III; The liquid crystal grating comprises an upper substrate, a liquid crystal layer, a pixel electrode, a common electrode and a lower substrate, wherein the liquid crystal layer is divided into three regions, region I, region II and region III, the common electrode widths of the three regions are w r , w g and w b , the width of the pixel electrode is the same as the width of the common electrode in each region, the liquid crystal grating pitches of region I, region II and region III are d r , d g and d b respectively; a driving voltage is applied to the pixel electrodes of the three regions, and the voltage sizes are equal, the common electrodes of all regions are grounded, through such a design, the liquid crystal molecule arrangement periods of region I, region II and region III of the liquid crystal grating are different when the voltage is applied, so the pitches of different regions are also different, the pitch of region I is the largest, the pitch of region II is the second, and the pitch of region III is the smallest; the adjacent electrodes between different regions are all set as common electrodes, so as to eliminate the edge electric field of adjacent regions and improve the control precision of different regions of the liquid crystal grating; The system realizes large-viewing-angle color holographic 3D display by the following method: first, the information of the red, green and blue color channels of a 3D object is extracted, then the hologram of each color channel is generated by using the signal controller, and the corresponding blazed grating is superimposed on the hologram, the resolution of the hologram and the blazed grating is the same as that of the spatial light modulator, both are a x b, wherein a is the lateral resolution and b is the longitudinal resolution; the parameters of the blazed grating satisfy the following formula: 2dsinγ=λ d=np Wherein, d is the pitch of the blazed grating, γ is the blaze angle of the blazed grating, λ is the wavelength, p is the pixel interval of the spatial light modulator, n is the number of pixels of a single period of the blazed grating, and the blaze angle γ is expressed as: wherein is the amount of phase change per period of the blazed grating, The blazed grating is superimposed on the hologram, and when the collimated incident light vertically irradiates the hologram, the holographic diffraction light field produces a certain deflection, and the deflection angle θ = 2γ. The red channel hologram and the blazed grating I, the green channel hologram and the blazed grating II, and the blue channel hologram and the blazed grating III are superimposed respectively, and then loaded onto the spatial light modulator in time sequence T1, T2, T3; at T1 moment, the shutter I is opened, and the shutters II and III are closed by the signal controller; at this time, the spatial light modulator is irradiated by the collimated and expanded red light, and the red channel hologram and the blazed grating I are loaded onto the spatial light modulator; at T2 moment, the shutter II is opened, and the shutters I and III are closed by the signal controller; at this time, the spatial light modulator is irradiated by the collimated and expanded green light, and the green channel hologram and the blazed grating II are loaded onto the spatial light modulator; at T3 moment, the shutter III is opened, and the shutters I and II are closed by the signal controller; at this time, the spatial light modulator is irradiated by the collimated and expanded blue light, and the blue channel hologram and the blazed grating III are loaded onto the spatial light modulator; when the red, green and blue lights irradiate the holograms and the corresponding blazed gratings of the red, green and blue color channels respectively in time sequence T1, T2, T3, three kinds of color diffraction light fields are simultaneously seen according to the visual persistence effect of the human eye. After the holographic diffraction light field is modulated by the liquid crystal grating, M second-order diffraction images are generated, and the number M of the second-order diffraction images satisfies the following formula: Wherein δ is the incident angle of the light beam entering the liquid crystal grating, by adjusting the wavelength λ, the pitch D of the liquid crystal grating and the incident angle δ, M≧4 is ensured; the wavelengths of red light, green light and blue light are respectively denoted as λ r , λ g , λ b , the holographic diffraction light field of the red, green and blue color channels respectively irradiates different liquid crystal layer regions of the liquid crystal grating after passing through the lens II, at T1 moment, the deflection angle of the holographic diffraction light field of the red channel is θ r , the holographic diffraction light field of the red channel converges to a point on the focal plane of the lens II after passing through the lens II with the focal length f, and then the modulation of the liquid crystal grating region I produces M red second-order diffraction images, at this time, the interval L r between two adjacent red second-order diffraction images and the distance H r from the converging point on the focal plane of the lens II to the optical axis are respectively: H r = ftan θ r At T2, the deflection angle of the holographic diffractive light field of the green channel is θ g The holographic diffractive light field of the green channel is converged into a point on the focal plane of the lens II with a focal length f, and after modulation by the liquid crystal grating area II, M green second-order diffractive images are generated, at this time, the interval L g between the adjacent two green second-order diffractive images and the distance H g from the converging point on the focal plane of the lens II to the optical axis are respectively: H g = ftan θ g At T3, the deflection angle of the holographic diffracted light field of the blue channel is θ b , the holographic diffracted light field of the blue channel passes through the lens II with a focal length f and converges to a point on the focal plane of the lens II, and after modulation by the liquid crystal grating area III, M blue second-order diffracted images are generated, and at this time, the interval between two adjacent blue second-order diffracted images is L b and the distance H from the converging point on the focal plane of the lens II to the optical axis b are respectively: H b = ftan θ b Adjusting the deflection angle θ of a holographic diffracted light field of red, green, blue color channels r , θ g , and θ b , when H r -H g = L r , H g -H b = L g , H r -H b = 2L b , M-2 color second order diffraction images are received on the CCD, and the viewing angle of the color holographic reconstruction image is (M-2)β, wherein: β is the color holographic 3D display viewing angle without using the liquid crystal grating.

2. The large-viewing-angle color holographic 3D display system based on liquid crystal grating of claim 1, wherein, The hologram generation methods of the 3D objects in the system are all based on the Fresnel diffraction calculation hologram generation method, including the lookup table method, the new lookup table method, the wavefront phase reconstruction method and the angular spectrum algorithm.

Images