Lamp Life

Author: E.E. Kimberly

Incandescent lamps, except those used in series street-lighting circuits, are rated in watts at a certain rated voltage. They are also designed for a certain length of life, such as 500, 750, 1000, 2000, and 3000 hours.

The life of a lamp is dependent largely on the operating temperature of its filament. Its efficiency as a light producer also depends on its temperature. An economic balance between maximum life and maximum efficiency is attained by the manufacturer, so that the cost of the lumen-hours from the lamp will be a minimum. An incandescent lamp operated at low filament temperature will have a long life, but its lumen output per watt of power input will be low. An incandescent lamp operated at very high filament temperature will have a high lumen output per watt of input, but its life will be short. The rated voltage of a lamp is that voltage at which the best economic balance is obtained between efficiency and life. However, the inconvenience and cost of replacing a highly efficient 500-hour lamp frequently in an inaccessible location may be so great that a less efficient 2000-hour or 3000-hour lamp would be preferred because of less frequent replacements.

When the filament of an incandescent lamp is lighted, particles of filament are driven off. These particles collect on the inside of the bulb, and cause darkening and hence a reduction in efficiency of light production. This darkening may so reduce the efficiency that it would be more economical to replace the lamp, even though the filament has not burned out. In practice, however, this is seldom done. The voltage at a lamp should not depart more than 3 per cent from the rated voltage.

Frequent starting of fluorescent lamps may take more life from them than long hours of burning because electrode evaporation is much more violent in the starting period.