British Kinematography (1950)

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42 BRITISH KINEMATOGRAPHY Vol. 16, NO. 2 heat in the radiation (which we wish to reduce as much as possible). In the following experiments the visible light has been measured with an accurately calibrated visual photometer, its units being lumens. The total radiation was measured by the rate of heating of a blackened metal block, and may be conveniently expressed in watts (1 watt=0.24 calories per second). Consequently the comparison, the luminous efficiency of the radiation, will be in lumens per watt, and we want this figure to be as high as possible. Table I gives the luminous efficiency of radiation under various conditions. TABLE 1 Luminous Efficiency of Radiation Source and Filter Lumens per watt Tungsten projector bulb, Class Al 3100°K, 500 watt, 1 10 volt. . . Tungsten bulb (3100CK) filtered through 2mm. of ON20 heatabsorbing glass Full Sunlight High-intensity arc, 290 amps., 16 mm. pos. H-I arc, filtered through 5 cms. water H-I arc, filtered through 5 cms. water and 2 mm. ON20 glass . . White light (5500CK) with no ultra-violet or infra-red (0'4 -0*7 u) Yellow-green light (Vf =0*55 u) to which the eye is most sensitive 26 105 80 85 155 190 220 620 In this table the figure for unfiltered tungsten light is experimentally determined, and due to the inaccuracies likely to arise is only approximate. This luminous efficiency of radiation must be distinguished from the overall luminous efficiency, lumens per electrical watt, which will be somewhat lower. The 4-times improvement to tungsten light at 3100°K obtained with 2 mm. of ON20 glass may be increased to about 6 times by using 3 mm. thickness, with slightly more greenish colour. The figure for the unfiltered arc is experimentally determined, and may be subject to some error. An attempt was made to observe only a small part of the flame above the crater. In practical terms, the luminous efficiency of the radiation from a mirror-arc projector system is only slightly higher than this, the glass of the mirror giving little filtering. The improvements to be obtained by filtering the arc are experimentally measured and are reasonably accurate, being also supported by a considerable number of indirect experiments. A 2-2-times improvement over the open arc can be obtained. When modifying a projector by adding a heat-absorbing cell the improvement will not be quite so great.8 The penultimate item is the theoretical figure for the maximum luminous efficiency which could be obtained for white light (from a black-body at the optimum temperature, 5500°K), if perfect filtering of the ultra-violet and infrared could be devised. The last item, which, of course, has no practical application to projectors, shows the absolute maximum obtainable by using light of the colour to which the eye is most sensitive.