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certainly lower than 65 per cent, or 70 per cent., because of reflection loss at the cylindrical bulb.
The calculation of I would be quite difficult, but its measurement for any given lamp is very easy. Of course the higher the temperature the greater the brilliancy, and since, in the majority of projection work, cost of power is not an important factor, the interest centers in the relation of brilliancy to the life of the lamp.
Table 3 shows the results of a rise of potential across (A) a 30volt, 5.5 ampere monoplane filament lamp having the filament within a square .3 inches on a side and mounted in a 1.25-inch bulb: (B) a 30volt 30-ampere monoplane filament lamp having the filament within a .5-inch square and mounted in a 2.5-inch bulb.
The volts, amperes, candle power and the temperature at 30 volts, were measured. The intrinsic brilliancy was calculated as indicated, and the temperature from lamp data. The life being inversely proportional to the rate of evaporation (^) could be calculated for any temperature, assuming a life of 100 hours at rated potential.
Figure 12 is a graph showing the relation between intrinsic brilliancy and life for the lamps as given above. The greater brilliancy for the same life due to the large diameter of wire in the 900-watt lamp will be noticed.
In conclusion, to illustrate the simplicity of the calculation from the data given, of lumens (Ls), there may be taken as an example an equipment consisting of the 900 watt motion picture lamp (B, Fig. 12) run at 30 amperes, a prismatic condenser, a standard aperture, and a 2,5 inch projection lens of 5.5 inch focal length. In this case, if the projection lens consists of four separate glasses.
Ls= IxExG -^ 20200x655x. 062=820
Considering the roughness of this approximation, this value agrees very well with that found in practice. (^)
(1) Trans. Soc. Mot. Pict. Eng. No. 10, May, 1920. (^) Trans. Soc. Mot. Pict. Eng. No. 9, Oct. 1919.
(^) The curves Pj — -' from which maximum possible illumination was previously determined, are given in Fig. 7 for comparison.
(4) "Absorption and Reflection Losses in Motion Picture Objectives." Kellner; Trans. Soc. Mot. Pict. Eng. Oct. 1920.
(5) "Vapor Pressure of Metallic Tungsten." Langmuir ; Phys. Rev. II, p. 329, Nov. 1913.
(6) "Condenser Lenses for Theatre Motion Picture Equipment." Egeler ; Trans. Soc. Mot. Pict. Eng. No. 12, May 1921.
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