Source enlargement for light projectors

The advent of the iodine cycle incandescent lamp with its single coil filament has necessitated new approaches to the design of light projection systems. Older systems generally used planar or biplanar filament lamps which form a square or rectangular image upon projection that could be conveniently spread to the desired field size by conventional spread type fluted lenses. The single coil filament, especially if it is mounted axially to the reflector axis, will project an intense circular spot in the illuminated field which defies spreading or smoothing by conventional means. Correction of this defect falls under the general classification of source enlarging and may be accomplished in either of two ways: (1) by an optical element surrounding the source to increase its apparent size; or (2) by small optical elements on the surface of the reflector which in effect become secondary light projectors with the desired beam divergence. This paper discusses the optical parameters that govern the design of these sytems.