Foot-candle

General Electric's "foot candle" advertising novelty, see below

A foot-candle (sometimes footcandle; abbreviated fc, lm/ft², or sometimes ft-c) is a non-SI unit of illuminance or light intensity widely used in photography, film, television, and the lighting industry.

The unit is defined as the amount of illumination the inside surface an imaginary 1-foot radius sphere would be receiving if there were a uniform point source of one candela in the exact center of the sphere. Alternatively, it can be defined as the illuminance on a 1-square foot surface of which there is a uniformly distributed flux of one lumen. This can be thought of as the amount of light that actually falls on a given surface. The foot-candle is equal to one lumen per square foot.

The SI derived unit of illuminance is the lux. One footcandle is equal to 10.76 lux, although in the lighting industry, typically this is approximated as 1 footcandle being equal to 10 lux.

In the lighting industry, footcandles are a common unit of measurement used to calculate adequate lighting levels of workspaces in buildings or outdoor spaces.

In the motion picture cinematography field, incident light meters are used to measure the number of footcandles present, which are used to calculate the intensity of motion picture lights, allowing cinematographers to set up proper lighting-contrast ratios when filming.

Since light intensity is the primary factor in the photosynthesis of plants, horticulturalists often measure and discuss optimum intensity for various plants in foot-candles. Full, unobstructed sunlight has an intensity of approximately 10,000 fc. An overcast day will produce an intensity of around 1,000 fc. The intensity of light near a window can range from 100 to 5,000 fc, depending on the orientation of the window, time of year and latitude.

Foot-candles can be easily measured and calculated with the use of a (manual) camera equipped with a built-in light meter. With the film speed set to ASA 25 and the shutter speed set to 1/60th of a second, focus on a sheet of white paper placed in the area where intensity is to be measured. Adjust the f-stop for proper exposure. Each f-stop has an approximate corresponding foot-candle reading (see the table below).

Equivalences

Other measures of illuminance are the lux and EV (exposure value)

L [lx] = 2.5 x 2^EV

1 foot-candle = about 10.764 lux;

EV 1 roughly equals to 0.46 foot candle or 5 lux.

Note that EV is a linear progression while foot candle and lux are not. Also note that the below table is hypothetical, the f/stop or aperature will change depending on the film speed.

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