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To meet the requirements of the current projection display system. This paper present an ultra-short throw catadioptric projection lens with a freeform mirror. This catadioptric structure consists of two parts. The first part is a set of rotationally symmetrical refractive lenses and the second part is a freeform mirror. In our design, Texas Instruments's Digital Light Processing (DLP) is chosen as the image source. A 0.65inch DMD is employed as the spatial light modulator. The two parts of system are firstly designed respectively, and then they are combined for final design. And a three-dimensional direct design method is firstly employed to design the freeform mirror. The final designed projection lens composes of nine lenses and a XY polynomial mirror, of which the F number is 3.5, the focal length is 6.05mm, and the screen size is 60inch. The designed system achieves projected the display screen with a projection distance of 440mm, which the throw ratio is 0.29. Results show that the MTF of all the FOVs are over 30% at 66lp/mm. The maximum distortion is less than 4%. Due to the high optical performance and compact structure, this designed projection lens is suitable for household and educational applications.
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