Abstract
An off-axis flat-concave dual-mirror (FCDM) configuration for an ultrashort focal length projection lens based on the digital light processor (DLP) upright projection-type projector is proposed in this paper. This projection lens is composed of two lens groups: the FCDM system and the relay optical system. The flat mirror and the contour of the concave mirror are determined according to the relationship between the chief ray from the exit pupil of each field of view (FOV) and the corresponding assigned position on the screen. A curved image is formed between exit pupil and concave mirror, aiming to correct field curvature. The second lens group consists of a set of conventional refractive lenses which convert a curved object onto the image panel. Based on this method, a projection lens with an ultrashort throw ratio (TR) of 0.34, including a full FOV 139.8 $^{\circ}$ and distortion of less than 1%, is designed. Additionally, the colorimetric performance, uniformity, optical efficiency and size of the designed projector are analyzed. Results show that the proposed projector system with high optical performance is compact and suitable for household or educational applications.
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