IES LM-45-15

This approved method describes the procedures to be followed and the precautions to be observed in performing uniform and reproducible measurements of the electrical and photometric characteristics of general service incandescent filament lamps under standard conditions.

Incandescent filament lamps produce radiant power as a result of electric current passing through a tungsten filament, which is surrounded by an inert atmosphere or vacuum within a glass or quartz envelope. Some lamps contain halogens that are employed to maintain a clean bulb wall. Such lamps may also employ bulb coatings that redirect infrared energy back to the filament for improved efficacy or to filter radiation for color control.

As long as the filament remains intact, current will flow, heating the filament to incandescence. Since the desired incandescence occurs at high filament temperatures, the surface of the tungsten filament is continually vaporized during lamp operation. As a result, the filament wire diameter is non-uniformly decreased along its length until at some point, the high current density causes excessive local heating and vaporization, which causes filament failures. The rate of evaporation is dependent on the local filament temperature, plus gas density and pressure.

Incandescent filament lamps are typically affected by variables such as operating cycle, conditions imposed by the fixture, orientation and vibration. In general, the test conditions should not diverge widely from conditions of service. Practical considerations require that any test conditions and programs be designed to give comparable results when used by various laboratories. The recommendations of this IES Approved Method have been made with these objectives in mind.