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May, 1944 IMPROVEMENT IN ILLUMINATION EFFICIENCY 301 is uniform in brightness. Except for losses by absorption and stray reflection of light in the optical system, the over-all effect of the lenses is equivalent to replacing the actual light source by an imaginary light source of the same brightness but larger and located at LZ. For best results, the filament image should therefore be large and as close to the printing aperture as is compatible with uniformity requirements imposed by the inverse square and cosine laws. The effect of dirt specks on the condenser surface LI can be minimized by imaging on the film not the surface itself but some other cross section of the light beam slightly removed from it; it must be recognized, however, that the lens diameter necessary to attain a given optical efficiency is smallest if the aperture and field stops are located at the objective and condenser lenses, respectively. Type B Illumination. —Since all effective stops in a system must be- long to either one of the 2 families, and since all apertures of each set EXPOSURE-CONTROL IkPHRAGM f~\ EXPOSURE-CONTROL o ^DIAPHRAGM Q HORIZONTAL MERIDIAN FIG. 3. are optically conjugated, the exposure-control aperture and the printing aperture must be parts of different systems of stops if Type B illumination is required. With tungsten-filament lamps, the relay condenser, as ordinarily used, provides uniform illumination in one system of stops only. The other set which is conjugate with the light source shares its nonuniformity. It has always been considered in- evitable that evenness of illumination in this other family of stops should be obtained only at the cost of reduction in efficiency by the use of special light sources or the insertion of diffusers. Lin the special case where the uniformity required is in one direction ly at each of the 2 printer apertures, and in mutually perpendicular directions, this loss in efficiency can be avoided by the addition of a cylindrical condenser to the simple relay system just discussed. Typical of this case is a continuous printer in which uniformity is re- quired only in the longitudinal direction at the exposure-control