Patent ID: 6894743

Claim:
An optical modulation element, comprising: a liquid crystal layer having a helix pitch p and having an average refractive index n, wherein said liquid crystal layer is held between a pair of transparent substrates having opposing transparent electrodes; a white light source for sending white light onto a surface of one of said transparent substrates of said liquid crystal layer in an oblique direction, said white light comprising a plurality of wavelengths including a wavelength λ, wherein said white light is incident on said liquid crystal layer at an angle θ; and at least one flat mirror arranged outside the other one of said transparent substrates to reflect the incident light transmitted through said liquid crystal layer toward said light source, wherein a light beam having said wavelength λ is selectively reflected from the liquid crystal layer, and wherein said selected wavelength λ is determined by selecting at least one of the incident angle θ and the helix pitch p, in accordance with the relationship p·cos θ=λ/n.