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May, 1944 IMPROVEMENT IN ILLUMINATION EFFICIENCY 303 and a film-exposure aperture. This printer is typical of those requir- ing Type A illumination with both transverse and longitudinal uni- formity at the film aperture. The Bell and Howell Model D continuous printer of the later type is representative of those requiring Type B illumination. Essentially, the optical system consists of a lamp, a ground-glass diffuser close to an adjustable diaphragm which moves longitudinally, and the printing aperture. The sides and hub of a large sprocket used to drive the film form a narrow tunnel through which light must pass. Only transverse uniformity is required at the printing aperture, and only longitudinal uniformity at the exposure-control apertufV L, 16 DIOPTER SPHERICAL LENS IS DIOPTER SPHERKAL. LENS Ho DIOPTER SPHERICAL LENS ALL LENSES HAVE A DIAMETER OF 46mnt S - HEADLIGHT LAMP FIG. 4. In the following sections, various ways of modifying the illuminat- ing systems of these printers to provide a desired increase in exposure are described. Further examples would present few fundamental differences and, in any case, would call for treatment according to the principles which have been discussed. If an existing printer is equipped with some type of condenser or mirror system, the systems described here could not, in most cases, be added but could be used if replacement or modification were contemplated. Relay Condenser System Applied to the Duplex Printer.—A relay system which has been designed for use in a Duplex printer is shown in Fig. 4. In order to avoid altering an existing light-change