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300 KUNZ, GOLDBERG, AND IVES Vol 42, No. 5 For Type B illumination, the diffuser must be located close to the exposure-control aperture, as illustrated in Fig. 2e. The filament coils of a projection lamp are imaged by the mirror close to the ground-glass diffuser at such magnification that the height of the image just exceeds that of the wide-open aperture. With a vertical- coil lamp filament, only longitudinal uniformity exists at the expo- sure-control aperture, but both transverse and longitudinal uniformity are provided at the printing aperture if the diffusion is sufficient. The efficiency of illumination is increased by a factor of 20 or more with this system as compared with the first 3 cases. This large in- crease in illumination is attained because a considerably enlarged image of the filament is formed at a shorter distance from the film. The degree of improvement would be far less if the mirror were used as it is commonly with condensers to form an interlaced image of the filament coils at unit magnification in the filament plane. Single Lens with a Concentrated Light Source.—A system closely related to the one just described makes use of a single con- denser lens, as shown in Fig. 2/, instead of the mirror. The same considerations regarding the selection of the lens and the position of the various elements of the system, as described in the preceding sec- tion, apply here. The maximum efficiency of a system of this type is commonly about l /% of that attainable by the use of the mirror sys- tem because of lower relative aperture. Lens Relay Systems.—The systems for increasing the illumina- tion described so far make use of either extended light sources or diffusing elements. Diffusers are wasteful of light and can be avoided if a lens system is employed in such a way as to provide uniform il- lumination at the printing aperture. It is well known that the photometric properties of any optical sys- tem are fixed if the size and location of two sets of conjugated aper- tures, called aperture stops and field stops, respectively, are specified. In general, one of these sets is formed by all the cross sections of the optical path which are uniformly filled with light, while the other sys- tem consists of the light source and its images. Efficient light sources usually lack brightness uniformity. Type A Illumination. —If uniform illumination of only one plane in the printer is sufficient, a relay optical system like that shown in Fig. 3a is satisfactory. The condenser lens LI (field stop) forms an image of the light source on the objective lens L 2 (aperture stop). The latter, in turn, forms in the printing aperture an image of LI which