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May, 1944 IMPROVEMENT IN ILLUMINATION EFFICIENCY 299 tion lamp with as small an envelope diameter as possible should be used. Since the illumination intensity depends on the cone angle of the light beam falling on the printing aperture, the use of a lens of large diameter located close to that aperture is favorable. Opposed to this are limitations set by the relative apertures that are attainable in practice and the existence of the tunnel. For a given lens, the largest possible cone angle is attained if the diffuser and lens are placed at such distances that an image of the largest area of diffuser, having acceptable brightness uniformity, just fills the printing aper- ture. The focal length of the lens chosen in a given situation de- pends on the sizes and distances involved. If only the Type A illumination is necessary, the lens may be lo- cated in the tunnel, but other considerations may oppose this. For printers requiring Type B illumination, longitudinal and transverse uniformity at both the exposure-control and the printing apertures can be obtained if the lens is close to the former. The intensity can be increased by a factor of 3 to 5 times in a typical situation, as compared with the first 3 cases described. Spherical Mirror with a Concentrated Light Source. — A system which is capable of giving greatly increased efficiency is shown in Fig. 2e. It makes use of a spherical mirror to project an enlarged image of a concentrated-filament light source on a ground-glass dif- fuser. In the case of Type A illumination, the position of the diffuser assuring greatest efficiency is as close to the printing aperture as the uniformity requirements at that aperture permit. However, if the diffuser is located in a tunnel, no advantage can be gained beyond the position where a significant part of the light from the mirror begins to be cut off by the walls of the tunnel. The focal length of the mirror should be chosen in accordance with dimensions of the tunnel and lamp house as well as those of the lamp, so as to form an image of the desired size on the diffusing glass. The diameter of the mirror should be large because the brightness of the image formed on the ground glass depends on its relative aperture. It should be noted, however, that because of the light distribution characteristics of a ground glass diffuser, illumination reaching it at large angles of incidence falls mainly outside the printing aperture where it is lost. Conse- quently, the increase in illumination produced at the printing aper- ture by using larger relative apertures may be less than would other- wise be expected.