| 摘要 |
PROBLEM TO BE SOLVED: To obtain an inexpensive optical sheet having sufficient optical functions and durability by making the center part and a peripheral part different in grating periods and/or grating directions of a blazed diffraction grating. SOLUTION: Since the angle of diffraction of a diffraction grating differs by wavelength (wavelength dispersion), light approximating white, if required, is realized by changing the period of the diffraction grating in a relatively narrow area. That is, diffracted light beams different by wavelengths are mixed to avoid the effects of wavelength dispersion, thus obtaining diffracted light approximating white. With respect to setting of the narrow area, it is sufficient that it be changed within a range smaller than 0.1 to 5μm, when illuminating light is thrown with the axis deviated (the center optical axis of illuminating light is deviated from that of display light), and that if it is changed within a range smaller than 2 to 20μm, when illuminating light is irradiated coaxially (when the center optical axis of illuminating light coincides with that of display light).
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