Dimming according to daylight – effect on lamp life

Eino Tetri and Viorel Gligor, Helsinki University of Technology, Lighting Laboratory, Finland

Keywords

daylight, dimming, electronic ballast, fluorescent lamp, lamp life test

Abstract

The aim of the research was to find out the effect of dimming on lamp life of fluorescent lamps when lighting is dimmed according to daylight. A control system based on daylight makes great demands on lamps and ballasts. If an artificial lighting system is adjusted to keep the lighting level constant, lamps have to endure both continuous burning on low light levels and rapid dimming from high light levels to low light levels. Lamp life was studied with a lamp life test. In addition of undimmed test groups, lamps were dimmed statically to a 1 %, 5 % or 15 % luminous flux level or dimmed dynamically up and down thus simulating dimming according to daylight. The dimming levels were chosen according to daylight measurements. Two dimming systems were measured for a period of one week. The energy savings made by using daylight and automatic lighting control, during one week, was 54 % with one system and 67 % with the other. The total lighting costs include, in addition to the energy costs, the lamp costs and the relamping costs. This study has eliminated the concern about the increase of costs through shortened lamp life. Results show that with electronic ballasts, lamps will reach the nominal lamp life even if the lamps are dimmed according to daylight. To conclude neither the lamp nor the electronic ballast are obstacles for wider use of daylight with artificial lighting.