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Micro-projector has found wide application in the field of mobile phone, pico-projector et al. However, because of the
depth of the field of the projection lens, precise focus mechanical design is needed for the optical engineer to get fine and
clear image.
Wavefront coding technique is a novel technique which is introduced by Dowski and Cathey in 1995. By adding an
aspheric phase mask at the pupil of the optical system, wavefront coding technique can make the imaging performance of
the optical system insensitive to the defocus, and can also make the system insensitive to the defocus related aberrations,
such as spherical aberration, chromatic aberration, field curvature, astigmatism and so on.
In this paper, a projection lens with long depth of field for the CF-LCOS projector is developed to satisfy the new
requirement. We first design the projection lens with the traditional optical design software to let it project the image to
the average projection distance. For example, if the lens is designed with in the range of 0.65-2.0m, then about 1.0m is
selected as the preliminary projection distance. After the lens (with a 3mm plane plate at the stop surface) is optimized
with the CODE V in this distance, the plane plate is replaced with cubic phase plate. Then the lens is optimized again
(the phase plate parameter varied for optimization) to get the final lens. In the new optimization, the resolution should be
3 times of the real resolution according to the LCOS resolution. This new optimized lens has the same MTF or PSF for
different fields between 0.65 and 2m projection distances, this ensures the high resolution with no focus operation.
Because of the MTF satisfies the LCOS requirement between the distance of 0.65 to 2m, no resolution is damaged and
also no focus operation is need in this effective projection distance , thus no mechanical focus system is needed.
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