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Operating pressure of incandescent and tungsten-halogen lamps and influence of envelope temperature on life

The gas pressure rise in incandescent lamps, from cold to running, has been both measured and calculated. A two zone model has been used to represent the conditions in a lamp, taking account of the temperature dependence of the gas thermal conductivity to obtain the gradient and mean temperature within the Langmuir sheath. Agreement between measured and calculated results is within ±10% for linear tungsten-halogen lamps and within ±3% for general lighting service types. The influence of envelope temperature (which is related to ambient temperature) on filament life has also been theoretically studied, and assuming that the rate controlling process is entirely evaporation, reveals that an increase in envelope temperature is expected to reduce the life of narrow bore tubular lamps but increase the life of large bulb types.