Mar 2013

Lumens, Footcandles and Candlepower

These are commonly used and sometimes misunderstood lighting terms. I will try not get too technical in explaining these terms, but these are terms with which you should be somewhat familiar.

Lumen – The quantity of light produced by a given light source. All light sources are rated in lumens. You will become more familiar with lumens because all the new light bulb packaging will have lumen information printed on it. This is part of the Government’s requirement to make consumers more energy conscious. The amount of lumens is not determined by the size of the light source. We do not actually see lumens as such. What we see are lumens reflected off a surface.

One Footcandle

Footcandles – The measurement of how much light falls on a surface. One lumen falling on one square foot of a surface is one footcandle. A footcandle was originally determined by placing a candle a foot away from a surface and measuring the amount of light on that surface. Footcandles (FC) are the measurement used to determine the light required for different tasks. As with lumens we do not see footcandles until they are reflected off a surface.

Candlepower – The measurement of the intensity of light in a specific direction. A candela is a unit of candlepower. Candlepower is generally used to describe the light distribution of directional light bulbs like an MR16 or a PAR30. The amount of light on a surface is determined by dividing the candlepower of the light source by the distance squared (the inverse square law). The ratio is 4:1. Thus the further away the light source, the less light strikes the surface by a factor of 4.

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