Changes in color temperature of tungsten-filament lamps at constant voltage

Abstract

Seasoned lamps, operated at constant voltage, gradually decrease in color temperature because of evaporation of the filament. Since the current does not decrease enough to correspond to an appreciable change in filament temperature, this decrease in color temperature is ascribed in large part to the accumulation of the familiar brown film on the inside surface of the bulb. The size and shape of the bulb is, therefore, important; each type must be separately investigated. Results are given for five types. The effect of operating seasoned lamps is a gradual decrease in color temperature, linear with time, which, like the rate of evaporation of tungsten, was found for 400-watt projection lamps, to be nearly proportional to the thirtieth power of the temperature. The effect of seasoning new lamps for one hour at about rated voltage was a rise in color temperature from 3050° to about 3140°K, approximately half of which occurred in the first three minutes.