Distortion correction for an overhead projector system.

摘要

An overhead projector (27) is provided which is able to project a uniformly focused image (36) of a transparency onto a vertical surface (32) at an oblique angle above the horizontal plane without introducing keystone distortion by offsetting a projection lens (28) and its associated mirror (33) from the center of a transparency stage (30)'bf the projector (27) in the direction of projection and directing lightthrough the transparency and to the projection lens (28) at an angle which s equal to the oblique angle above the horizontal plane at which the image is projected. Light is directed to the projection lens (28) by offsetting the upper lens (40) of a two-element circular Fresnel condensing lens system (40, 44) in the direction of displacement of the projection lens (28).