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This paper discusses the optical and opto-mechanical design of a new laser head developed at Polaroid for printing Helios binary film for printing high quality medical hard copy images. The head is part of an external drum printer for 14' X 17' film. The pixel size is 84 X 84 μm, produced by four lasers, with the smallest printable spot 3 X 6 micrometer, to produce 4096 gray levels. Two pixels side-by-side are simultaneously printed. The head has eight independent 840 nm diode lasers manufactured by Polaroid. Each laser emits up to 1.1 W over an emission length of about 100 μm, with a particularly uniform nearfield irradiance. The lasers are microlensed to equalize the divergences in the two principal meridians. Each packaged laser is aligned in a field-replaceable illuminator whose output beam, focused at infinity, is bore-sighted in a mechanical cylinder. The illuminators are arranged roughly radially. Eight lenses image the laser nearfields on a multi-facet mirror produced by diamond machining. The mirror facets truncate the beams to give the desired pixel shapes and separations. A reducing afocal relay images the mirror onto the film. The final element is a molded aspheric lens, mounted in an actuator to maintain focus on the film. The focusing unit also comprises a triangulation-based focus sensor. The alignment procedures and fixtures were devised concurrently with the head for manufacturing simplicity. The main physical structure is a casting, into which reference surfaces are machined. All optical subassemblies are attached to this casting, with a mixture of optical alignment and self-location. Semi-kinematic cylinder-in-V methodology is utilized. The active alignment steps are done in a sequence that tends to reduce errors from previous steps.
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