Patent ID: 8325166

Claim:
An optical device comprising: a light guide plate receiving, for each of N types, where N is an integer of 1 or more, of wavelength bands, a plurality of parallel light beams with different incident angles each corresponding to view angles within a predetermined view angle range, each of the parallel light beams traveling in parallel, and the light guide plate guiding the received parallel light beams according to principle of total inner reflection; a first volume hologram grating of reflection type having a diffraction configuration which includes N types of interference fringes each corresponding to the N types of wavelength bands, and diffracting and reflecting the parallel light beams which have entered the light guide plate, so as to be reflected inside the light guide plate according to the principle of total inner reflection; and a second volume hologram grating of reflection type having a diffraction configuration which includes N types of interference fringes each corresponding to the N types of wavelength bands, and diffracting and reflecting the parallel light beams which have propagated inside the light guide plate according to the principle of total inner reflection, so as to be emitted from the light guide plate as they are in parallel, respectively, and wherein a diffraction efficiency of the second volume hologram grating is constant throughout the second volume hologram grating, wherein the optical device is configured, for a wavelength band selected from the N types of wavelength bands, to satisfy a relationship of ‘P>L’, where ‘L’ represents a central diffraction wavelength in the first and second volume hologram gratings, the central diffraction wavelength being defined for a parallel light beam corresponding to a central view angle, and ‘P’ represents a peak wavelength of the parallel light beams which is to enter the light guide plate.