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How to Convert a Candela Foot-Candle

Radiometry and photometry are the specialized practices related to measuring light. Radiometry measures light in terms of the power it carries, while photometry measures light in terms of the response of the average human eye. To add to the complication, there are dozens of different measurements that can be made: intensity, flux, power, radiance, irradiance --- all are different ways of expressing information about light. The candela (cd) and the foot-candle (fc) both measure light, but the candela measures how much light a source puts out into a given angle, while the foot-candle measures how much light strikes a given area.

Instructions

- 1
  Place a surface at some distance from a light source. This can be done virtually (on paper), or done in a physical system, depending upon your needs. You can choose any distance you want, but you need to choose one because the measure of foot-candles on the surface depends on the distance.
  
  Take as an example a surface 6 feet from the light source.
- 2
  Convert candelas to a measurement of lumens per area. A candela is defined as one lumen projected into a solid angle of one steradian. A steradian is the angle that covers an area of radius^2 at a distance of radius.
  
  For example, if the source puts out 300 candela, or 300 lumens per steradian, then 6 feet away those 300 lumens are spread over 6^2, or 36 square feet.
- 3
  Calculate the illuminance per square foot at the distance you have chosen. This is simply the total number of lumens per steradian divided by the area projected at that distance.
  
  For the example, the 300 lumens are spread over an area of 36 square feet. This is 300/36 lumens/square foot = 8.3 lumens per square foot. This is the measurement of foot-candles; so at a distance of 6 feet, a source with an intensity of 300 candelas would provide an illuminance of 8.3 foot-candles.

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