Method of manufacturing optical element and projection device

A method of manufacturing an optical element used in a projection device, the projection device having a light source configured to emit light conforming to a non-uniform light intensity distribution function, the method comprising:

• multiplying the non-uniform light intensity distribution function by a diffraction intensity and angle function of at least one first grating to obtain a product function;
• determining whether the product function is substantially equal to 1 in a predetermined range of angle or wavelength such that the light, when compensated by the at least one first grating, conforms to a uniform light intensity distribution function;
• if the product function is not substantially equal to 1 in the predetermined range of angle or wavelength, adjusting the diffraction intensity and angle function by adjusting at least one of at least one surface period width and at least one slant angle of the at least one first grating;
• if the product function is substantially equal to 1 in the predetermined range of angle or wavelength, determining at least one pair of incident angles respectively of a reference beam and a signal beam according to the diffraction intensity and angle function;
• and
• recording at least one piece of holographic material with the reference beam and the signal beam respectively at the at least one pair of incident angles, so as to manufacture at least one holographic element with the at least one first grating therein.