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May, 1944 IMPROVEMENT IN ILLUMINATION EFFICIENCY 307 m for the curvature of field of the objective lenses L 5 Le, thus producing a substantially flat image at the printing aperture. As it is desirable to keep the power of the cylindrical elements LiL 2 as low as possible, a lamp with a filament composed of com- paratively long coils should be selected. In the particular application discussed here, the power of the lamp is limited to 100 w, and the Cl3-T8 l /2 prefocus projection lamp is considered acceptable. A similar lamp with a CC13 filament could be substituted if necessary but, because of the shorter coil length of this type of filament, the vertical adjustment of the lamp would be critical. In addition, the coiled-coil lamps are less satisfactory for this use because of the tendency of adjacent coils to short circuit with consequent shifting of intensity. There are a number of suitable lamps of higher wattage and most of them could be used if a slight change of the cylindrical condenser LiL 2 was made to allow for a larger diameter lamp en- velope. If no restrictions regarding voltage or power are imposed, the 50-v, 200-w biplane lamp should prove particularly advantageous, because its image would fill the exposure-control aperture more com- pletely, and because its straight rugged coils are well suited to serve as a light source in a cylindrical lens system. All lenses may be made of spectacle crown glass, and felt-polished surfaces are entirely satisfactory. A variation of =•= 1 diopter in the power of LI and L 2 , and of =± l /8 diopter of all other lenses can be tolerated. The optical parts are mounted in a tube, as shown in Fig. 6, and the tube, in turn, is supported by a bracket carrying the new lamp house. In order to avoid a cumbersome construction, a small lamp house with forced air cooling is used. It is necessary to make an open- ing in the rear wall of the printer and in some printers to mill out part of the ledge shown by the broken lines in Fig. 6 to provide clear- ance for the lens tube. It is also advisable to mill flat a portion of the rear wall of the regular lamp house around the opening. The optical system may then be mounted on a plate, as illustrated in Fig. 6, and secured with pins and screws. With this construction, the position of lamp and lenses in the printer is so well determined that either the whole assembly or the lamp alone can be removed and put back whenever desired without needing readjustment or recalibration. As is usual with systems which form an image of a lens or diffuser surface at or near the printing aperture, precautions must be taken to