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tends a slightly smaller angle at the condenser when the lamp bulb is touching the condenser housing than is subtended by a .3 inch source at a distance of an inch, or less, as with condensers No. 1 and No. 3, there is no gain in the use of this lamp, apart from the higher brilliancy obtainable for any given lamp life. On the other hand, it will be observed from the tables that d^ for condenser No. 2 is .137 inches, at which distance it would take a .39-inch source to subtend the same angle as the 900 watt lamp at 1.75 inches — or the 900-watt lamp should give, if close against the condenser, a value of C of about .065 — a little better than the prismatic.
If the same relation holds between the prismatic No. 3 and the plano-convex in the case of the Safety Standard film aperture — a very reasonable assumption — the best uniform illumination obtainable with the prismatic condenser and the 1.5 inch projection lens, is about .035 ; whereas condensers No. 1, No. 2, and No. 3, with .3 inch sources in available bulbs, will give perhaps .039, .036, and .036. The substitution of the 900-watt lamp in a 2.5-inch diameter bulb will, as before, give these same values of illumination for condensers No. 1 and No. 3, while for condenser No. 2 the new source, being the equivalent of a .39-inch source at 1.37 inches will increase the illumination to .037. The use of a larger projection lens, as recommended above, will increase the value very materially, as shown in curves e and d. These, of course, are but approximate figures, based on apparently justifiable assumptions — one of which is that the intrinsic brilliancy of the sources are the same, and that the average brilliancy over any considerable part of any source is the same as that of any other considerable part. Such a condition obtains in the monoplane filament lamp, for example, where any considerable area contains approximately the same amount of filament, dark spaces, and, when using a mirror behind the filament, filament image.
Thus far. all the measurements of illumination have been given in lumens delivered to the projection lens (in the case of the motion picture set-ups) per unit of brilliancy. To determine the actual number of lumens on the screen this must be multiplied by the specific brilliancy of the source and by the efficiency of the projection lens.
As before, it seems best, when comparing condensers, to regard the efficiencies of the projection lenses as an entirely separate problem, involving, as it does, for each type of lens a factor practically constant for every size and type of condenser used. It has been shown before this Society (^), that the loss in projection lenses consists largely of the reflection losses — approximately 4 per cent, at each surface between air and glass. Absorption losses may bring this to 10 per cent, for each single lens or cemented combination. Knowing, then, the number (n) of such combinations in any lens, its efficiency E = 9" is easily calculated.
The specific brilliancy of the incandescent filament source as ordinarily used in projection is
l = A i + A mi = i (A -hmA-)
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