Abstract: A transmissive overhead projector includes a Fresnel lens assembly which has an f-number of about 0.25 and is capable of dioptrically focusing light at an incident angle of at least 60.degree. with respect to a perpendicular to the Fresnel lens assembly. This refractive power of the Fresnel lens assembly allows the overhead projector light source to be positioned very close to the Fresnel lens assembly and consequently allows a significant reduction in the base height of the overhead projector.

Abstract: An illumination system for use in an overhead projector which includes a tungsten filament light source, a back concave reflector, and a plano-convex glass condenser. The position of the light source relative to the concave reflector is controlled to give a specific longitudinal separation of the light source filament and its magnified image which results in a radiation parallax which produces improved efficiency, illumination and light uniformity at the projected image of the overhead projector.

Abstract: A dual-magnification rear screen projector having a fixed projection lens and two auxiliary lenses which may be positioned before and after the projection lens in the light path to change the magnification of the system. The system is parfocal in that none of the lenses need be repositioned axially along the light path to focus the system at either magnification.

Abstract: An overhead projector is provided which is able to project a uniformly focused image of a transparency onto a vertical surface at an oblique angle above the horizontal plane without introducing keystone distortion by offsetting a projection lens and its associated mirror from the center of a transparency stage of the projector in the direction of projection and directing light through the transparency and to the projection lens at an angle which is equal to the oblique angle above the horizontal plane at which the image is projected. Light is directed to the projection lens by offsetting the upper lens of a two-element circular Fresnel condensing lens system in the direction of displacement of the projection lens.

Abstract: An overhead projector which projects a distortion-free image onto a vertical screen at an oblique angle above the horizontal plane. In a first embodiment of the invention, a projection lens and mirror are displaced from the center of a transparency stage in the direction of projection and a thin, linear, incrementally-grooved prismatic sheet is positioned parallel to the stage and between the stage and a lighting means to redirect light transmitted through the stage and the transparency to the projection lens. In a second embodiment of the invention, the grooved prism is formed as an integral part of the lighting means.

Abstract: A point focus radiation concentrator using two linear echelon surfaces crossed at approximately 90 degrees with the two linear echelon surfaces having a generally cylindrical shape with a radius of curvature approximately equal to the focal distance. The two echelon surfaces may be formed on separate sheets positioned in close proximity to each other or may be formed on opposite sides of a single sheet. The cross section of the curved concentrator surface may be circular or other aspherical shape, such as parabolic, ellipsoid and hyperbolic.

Abstract: A variable focal length overhead projector lens having four elements positioned in a projection head with two elements on either side of a folding mirror and having a single movable negative lens element closest to the screen being movable to vary the focal length and magnification at a fixed object-to-image distance to fill the screen.

Abstract: A high gain, front projection screen with controlled light distribution, comprising a translucent sheet containing light diffusing particles, having a matte-finished front surface and an incremental reflector rear surface.

Abstract: A small-area focus solar concentrator comprising a linear echelon refractor and a linear echelon reflector. The increments of the refractor are crossed at approximately 90.degree. to the increments of the reflector. The refractor and reflector cooperate to focus solar radiation incident on the front surface of the refractor to a small area focus in front of the refractor.The refractor-reflector structure permits relatively low cost, high power concentrators using refractor-reflector matrices. Also, the refractor-reflector structure can be used to focus normal or non-normal incident radiation outside the path of the radiation to reduce or eliminate blockage of the radiation by an absorber located at the focus.