Patent ID: 11860385
Assignee: BEIHANG UNIVERSITY
Field: Optics (Instruments)
Classification: CPC G | IPC G

Claim 1:
2. A large-viewing-angle holographic true 3D display method, based on a tunable liquid crystal grating, wherein a system comprises a laser, a filter, a beam expander, a semi-transparent semi-reflective mirror, a spatial light modulator, a lens I, a diaphragm, the tunable liquid crystal grating, a polaroid, a signal controller, a lens II and a receiving screen; the laser, the filter and the beam expander are configured for generating collimated incident light; the semi-transparent semi-reflective mirror is positioned between the beam expander and the spatial light modulator, the collimated incident light irradiates the spatial light modulator after passing through the semi-transparent semi-reflective mirror, and the spatial light modulator is loaded with a hologram of a 3D object, a diffracted light passes through the lens I after being reflected by the spatial light modulator and the semi-transparent semi-reflective mirror, the diaphragm is positioned behind the lens I and configured for eliminating a high-order diffracted light in the holographic true 3D display, wherein parameters of the lens I are same to parameters of the lens II; the tunable liquid crystal grating is located on a back focal plane of the lens I and on a front focal plane of the lens II, the tunable liquid crystal grating is driven by a voltage, wherein the voltage is applied to control a diffraction image to generate a secondary diffraction so as to enlarge a viewing angle and a size of the holographic true 3D display system, the signal controller is configured for synchronously controlling the voltage of the tunable liquid crystal grating and generation and loading of the hologram, wherein after a secondary diffraction image passes through the polaroid and the lens II, a viewer sees a holographic reconstructed image on the receiving screen;
wherein the tunable liquid crystal grating comprises an upper glass substrate, an upper electrode, a liquid crystal layer, a lower electrode, an earth electrode and a lower glass substrate, wherein the upper electrode is applied with a direct current and the lower electrode with an alternating current, wherein when the electrodes are applied with a voltage, an uneven electric field distribution is generated between the upper electrode and the lower electrode, inducing liquid crystal molecules to generate a central symmetric distribution of gradient refractive index, thereby causing deflection of light rays entering the liquid crystal layer to form a grating effect, and a period of the grating is controlled by controlling the direct current applied to the upper electrode, thus achieving a function of adjusting the period of the liquid crystal grating;
the system realizes holographic true 3D display with a large viewing angle by following method: using the signal controller to generate the hologram of the 3D object, with a resolution of the hologram being same as that of the spatial light modulator, both being a×b, wherein a indicates a horizontal resolution, b indicates a vertical resolution, and a size of the diaphragm is adjusted so as to ensure that only diffraction image of one order passes through the tunable liquid crystal grating; in an initial state, no voltage is applied to the tunable liquid crystal grating, where the viewing angle of the system is θ0; when the voltage is applied to the tunable liquid crystal grating, the diffraction image is subjected to a secondary diffraction, by adjusting the voltage, after passing through the tunable liquid crystal grating, so that M secondary diffraction images are generated, and when a planar light wave vertically irradiates the tunable liquid crystal grating, a complex amplitude {tilde over (E)} on a grating surface meets formula:, E
      ~
    
    ⁡
    
      (
      x
      )
    
  
  =
  
    {
    
      
        
          
            1
            +
            
              B
              ⁢
              
                
              
              ⁢
              cos
              ⁢
              
                
                  2
                  ⁢
                  π
                
                d
              
              ⁢
              x
            
          
        
        
          
            x
            ≤
            
              
              
                Nd
                2
              
              
            
          
        
      
      
        
          0
        
        
          
            x
            ≥
            
              
              
                Nd
                2
              
              
            
          
        
      
    
  

wherein a symmetric center of the tunable liquid crystal grating is taken as an origin, x indicates a transverse displacement from any point on the grating surface to the origin, d indicates a grating pitch, N indicates a grating period number, B indicates a peak transmittance of the tunable liquid crystal grating and changes with voltage change; a diffraction order M of the tunable liquid crystal grating is represented by:, M
  =
  
    
      d
      ⁢
      
        
      
      ⁢
      sin
      ⁢
      
        
      
      ⁢
      θ
    
    λ
  

wherein θ indicates an incident angle of the diffracted light to the tunable liquid crystal grating, 60°≤θ≤90°, λ indicates a wavelength of the incident light, wherein when the diffracted light is vertically incident on the tunable liquid crystal grating, θ=90; a polarization state of the polaroid is adjusted, so that the secondary diffraction image is uniformly displayed on the receiving screen in intensity after passing through the polaroid and the lens II, and large-viewing-angle holographic true 3D display effect can be obtained on the receiving screen, wherein the viewing angle of the holographic display system is M×θ0;
the system realizes large-size holographic true 3D display by the following method: a resolution of the spatial light modulator is a×b, a horizontal resolution of the recorded 3D object is m, the signal controller is used to expand the horizontal resolution of the 3D object to n and generate the hologram of the 3D object, wherein n=m+a and the hologram has a resolution of 2a×b, the hologram is equally divided into two sub-holograms, namely sub-hologram I and sub-hologram II, each with a resolution of a×b; at a moment T1 the sub-hologram I is loaded to the spatial light modulator, and no voltage is applied to the tunable liquid crystal grating, at this time since there is no voltage applied to the tunable liquid crystal grating, its function is equivalent to that of transparent glass, and after the diffracted light of the sub-hologram I passes through the tunable liquid crystal grating, the polaroid and the lens II, the reconstructed image is displayed on the receiving screen; at the moment T2 the sub-hologram II is loaded to the spatial light modulator by using the signal controller, meanwhile the tunable liquid crystal grating is applied with a voltage to generate a zero-order primary maximum and ±1 order secondary maximum on the spectral plane, wherein a distance L between the zero-order primary maximum and the ±1 order secondary maximum meets formula:, L
  =
  
    
      λ
      ⁢
      
        
      
      ⁢
      f
    
    d
  

wherein f indicates a focal length of the lens II, wherein since the tunable liquid crystal grating is positioned in the back focal plane of the lens I and in the front focal plane of the lens II, the lens I and lens II thereby form a 4f system, the spectral plane of the tunable liquid crystal grating is located in the back focal plane of the 4f system, the transverse displacement and the polarization state of the polaroid are adjusted to filter out the zero-order primary maximum and the −1 order secondary maximum out and only +1 order secondary maximum is taken into consideration, i.e., the diffraction image of the sub-hologram II has a transverse translation of L after passing through the tunable liquid crystal grating, with translation amount set as L, wherein L equalizes a transverse size of the spatial light modulator:

L=ap, wherein p indicates a pixel pitch of the spatial light modulator, at which time the reconstructed image of the sub-hologram II can be seen at the +1 order secondary maximum; when switching time of T1 and T2 is fast enough, a seamless splicing effect, in space, of the sub-hologram I and sub-hologram II is realized according to a visual persistence effect of human eyes, thereby achieving a purpose of enlarging a display size, and in a case that a reconstruction distance is not changed, the size of the holographic true 3D display is enlarged to be k times of an original size, and k meets the formula:, k
  =
  
    
      a
      +
      m
    
    m
  

wherein 0<m≤a.