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 λ 1, λ2 and λ 3 within the ranges 0.46≦λ 1 ≦0.50 μm (blue light), 0.53≦λ 2 ≦0.57 μm (green light), 0.60≦λ 3 ≦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 hologram optical element that is a transmitting diffraction grating, wherein 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 wherein the cross-sectional form of the grating is a sawtooth form in which the lengths of the two sides (edges) intersecting at the teeth ends differs by 10% or more, and the interior angle is equal to or below 60°. 
   
   
       2 . The hologram optical element according to  claim 1 , 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 hologram optical element according to  claim 1 , in which, when light of three wavelengths λ 1 , λ 2  and λ 3  that are within the range 0.46≦λ 1 ≦0.50 μm, 0.53≦λ 2 ≦0.57 μm, and 0.60≦λ 3 ≦0.64 μm is incident at angle θi, the maximum order of diffraction for diffractive efficiency of each wavelength is (m+m 0 ), m, (m−m 0 ) (provided that m 0 =1, 2, . . . ), wherein m is within the range that fulfills expression (1) and expression (2) following, and average period d fulfills expression (3):
     m×{λ 2×(1−sin δ/sin θ i )−λ1 }≦m 0×λ1 ≦m×{λ 2×(1+sin δ/sin θ i )−λ1}  (1)       m×{λ 3−λ2×(1+sin δ/sin θ i )}≦ m 0×λ3 ≦m×{λ 3−λ2×(1−sin δ/sin θ i )}  (2)   (Where δ is within the range, 0≦δ≦5 (degrees)) and
     d=m×λ 2/sin θ i   (3) 
   
   
   
       4 . The hologram optical element according to  claim 1 , wherein the cross-sectional form of the grating of the hologram optical element approximates a stair like form of N levels (N=4, 5, 6, 7, 8, . . . ). 
   
   
       5 . The hologram optical element according to  claim 4 , formed of material having a refractive index n, where the average depth h of the grating grooves is, h=Δ×d/(n−1) (0.4≦α≦1.0, d being the average period of the diffraction grating). 
   
   
       6 . The hologram optical element according to  claim 1 , formed of material having a refractive index n, where the average depth h of the grating grooves is, h=α×d/(n−1) (0.4≦α≦1.0, d being the average period of the diffraction grating).