Foot-Candle (fc)

The imperial measure of illuminance, defined as the light from one standard candle falling on a surface one foot away.

The definition is unusually easy to picture, which is rare in photometry. Put a standard candle one foot from a wall. The light landing on one square foot of that wall is one foot-candle.

The distinction it carries is the one most people get wrong. A lumen measures light leaving the source. A foot-candle measures light arriving at a surface. A powerful bulb in a large room produces plenty of lumens and very few foot-candles on the desk in the corner, which is why lighting a workspace is a question about geometry rather than wattage.

It remains in working use in the United States for specifying lighting levels in offices, warehouses and sports grounds. Britain has gone over to lux, its metric equivalent, where the same light is measured against a square metre instead. Since a square metre is a good deal larger, the same illumination yields a bigger number: one foot-candle is 10.76 lux, and multiplying by ten is close enough for most purposes.