Cooling-times of tungsten filament lamps
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During steady-state operation the hot coiled filaments in incandescent lamps provide luminous flux for illumination, but when it is switched off the temperature as well as the light output drops quite fast. The cooling-time of a lamp is the time required for the hot filament to cool down to ten per cent light output after the circuit is opened. The exercise of estimating luminous flux and cooling-times for typical 10, 100, 500 and 1000 W lamps have been undertaken for the first time for the benefit of students. This problem involves three disciplines electricity, optics and heat. The electricity provides power to heat the filament quickly, followed by optics which helps us in determining the light output and lastly the discipline heat is responsible for cooling the hot filament largely through Stefan-Boltzmann law. It is shown that the supposition of linear configurations for the filaments neither matches luminous flux nor the cooling-times and both fall short. HS Leff suggestion of introducing a shadow factor which reduces the exposed surface area, as it so happens in the coiled filaments, successfully explained the measured observations.
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