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298 KUNZ, GOLDBERG, AND IVES Vol 42, No. 5 tunnel, both transverse and longitudinal uniformity are provided at the printing aperture. It is evident, however, that such a system (Fig. 2a) is unsuitable for printers requiring Type B illumination. As stated before, the efficiency of illumination is very low. Unaided Extended Light Source (Type B Illumination).—In order to have Type B illumination without additional optical elements, the scheme shown in Fig. 26 can be used. The lamp has a uniform extended filament structure along one axis and must be posi- tioned so that the filament completely fills the exposure-control aper- ture, either longitudinally or transversely as required, when viewed from any position in the printing aperture. Then, as the diaphragm is moved across the exposure-control aperture, the effective length of the filament decreases, thus reducing the amount of light reaching the printing aperture in proportion. Uniformity, at least on the axis chosen, is insured at the exposure-control aperture, while both lon- gitudinal and transverse uniformity are obtained at the printing aperture except for possible effects of reflections and shadows formed by the lamp. Obviously, this illumination system can be used also in printers in which Type A illumination is acceptable. As in the case discussed before, the efficiency is usually below 1 per cent. Light Source with Diffuser.—The effect of shadows from blem- ishes in the glass lamp envelope can be minimized by inserting a diffuser, such as a sheet of ground glass, between the lamp filament and the printing aperture. The diffuser then becomes a secondary light source. If the Type B requirements must be satisfied, the dif- fuser is best located close to the adjustable diaphragm, as shown in Fig. 2c. The 3 systems discussed so far constitute the least efficient class. The concentrated filament-type source with a diffuser located at the tunnel entrance will be taken as a basis of reference for comparison with other methods to be discussed. Simple Lens with Diffused Secondary Light Source.—A very great increase in illumination could be obtained by forming an image of the light source in the printing aperture but this is not prac- ticable because of the nonuniformity of available light sources. A worth-while gain can be made, nevertheless, by the use of a modifica- tion of this scheme in which a diffuser placed closely enough to the lamp to be illuminated intensely is imaged in the printing aperture by a lens (Fig. 2d). The diffusion should be sufficient to hide the lamp filament pattern without causing an excessive light loss. A projec-