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A number of 400-watt projection lamps were color matched with black bodies immersed in freezing platinum, rhodium, and iridium. From these lamps working standards of color temperature were prepared, interpolation between the three fundamental points being accomplished by making use of the systematic differ- ence between the color temperature of the lamp and the brightness temperature of the inside of a particular turn of the coiled tungsten filament.
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Wensel et al. (1934) studied this question.
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