Hologram optical element and surface light source device using the hologram optical element

Abstract

A hologram optical element having a thin form and a high degree of light transmittance, moreover that provides superior handling ease, as well as a surface light source device employing this hologram optical element. The angle at which light can be bent in this hologram optical element, has low wavelength dependency, and the hologram optical element enables prevention of spectral separation in white light incident from an oblique direction which is bent to a vertical direction and emitted. A transmitting diffraction grating, when light of wavelengths λ1, λ2 and λ3 within the ranges 0.46≦λ1≦0.50 μm (blue light), 0.53≦λ2≦0.57 μm (green light), 0.60≦λ3≦0.64 μm (red light) is incident at angle θi, the maximum diffraction angle for diffractive efficiency of each wavelength is within the range from −5 degrees to +5 degrees.

1 . A surface light source device comprising a hologram optical element and a surface light source, the hologram optical element having properties that the angle at which light can be bent has a low degree of wavelength dependency, spectral separation in white light incident thereto from an oblique direction is prevented, and the light is bent to a vertical direction and emitted, and the hologram optical element is arranged above the light emitting surface of the surface light source. 
   
   
       2 . The surface light source device according to  claim 1 , said hologram optical element being a transmitting diffraction grating wherein, when light of wavelengths λ 1 , λ 2  and λ 3  within the range 0.46≦λ 1 ≦0.50 μm, 0.53≦λ 2 ≦0.57 μm, 0.60≦λ 3 ≦0.64 μm is incident at angle θi, the maximum diffraction angle for diffractive efficiency of each wavelength is within the range from −5 degrees to +5 degrees. 
   
   
       3 . The surface light source device according to  claim 1 , that employs a diffuser in addition to the hologram optical element. 
   
   
       4 . The surface light source device according to  claim 3 , wherein the diffuser is a hologram diffuser that diffuses incident light within a determined range of angles in space. 
   
   
       5 . The light source device according to  claim 4 , wherein the hologram diffuser is formed as an integrated body with the light emitting surface of a light guide plate.