Patent ID: 10133077

Abstract:
In a luminous flux diameter expanding element, a light guide body includes an incident-side diffraction grating (first incident-side diffraction grating) provided on a surface thereof and an exit-side diffraction grating (first exit-side diffraction grating) provided on another surface thereof. The incident-side diffraction grating and the exit-side diffraction grating have the same grating direction and the same grating period. The diffraction angle of a positive first-order diffracted light and the diffraction angle of a negative first-order diffracted light of a beam incident on the incident-side diffraction grating are greater than or equal to a critical angle determined by the refractive index of the light guide body. The positive first-order diffracted light and the negative first-order diffracted light of the beam diffracted by the incident-side diffraction grating propagate in opposite directions along a y-direction within the light guide body and are emitted from the exit-side diffraction grating.