Luminous Flux – Source efficiency vs. System Efficiency

Source efficiency is based on the amount of light produced by a lamp at room temperature. It is distinct from system efficiency that refers to the amount of usable light delivered to the target area. There are several causes why raw lumens produced by a light source are far higher than the actual light delivered on a surface.

Losses due to trapped light - Trapped light and reflection inefficiency are the first source of lower light output from traditional lamps.  As long as LEDs had not entered the scene all bulbs produced lights in a 360 degree sphere and the comparison was easy. LEDs changed all that with the directional nature of their light.

In a traditional bulb (incandescent, metal halide, HPS etc.) a considerable portion of the light output is directed upwards. This light must then be reflected down. The efficacy of the reflector in turn is determined by the quality of finish, operating conditions and ambient temperature. The quality of the reflector and, therefore, the amount of light reflected degrades over time. The actual amount of light coming out of a fixture is, thus, considerably lower.

An HPS lamp has a larger profile, a single source of light and a considerabel amount of light trapped in the relector.

An LED light by contrast has a small profile, several sources of light ensuring better light distribution. All the light is produced and directed downwards. There are no problems of reflector effciency, aging of reflector coating and consequent loss in light output.