Candlepower, Lumens, & Footcandles

The Illumination Engineering Society of North America (IESNA) defines candlepower as luminous intensity expressed in lumens per steradian. Such a complex definition may be easier to understand by tracing the evolution of the term footcandle. Clearly the most practical early light source was a candle. Logic suggests that some early scientist came up with a standard candle of a certain diameter, with a standard wick and using standard bees wax, if there was such a thing. The effort resulted in an early standardized light source. The power of a standard candle to produce light was termed candlepower, and the term luminance is used when referring to a light source. If one looks directly at the candle flame, a luminance of one candlepower (candela) will be seen. If one imagines that light spreading out to pass through an imaginary frame (aperture) of one square foot, placed exactly one foot from the flame, the light passing through that area is called illuminance and is equal to one foot candle. If that one square foot of area is not transparent, that surface will be illuminated with the light of one candle at one foot or one footcandle.

The solid angle between the four light rays shown in the drawing which form an area equal to the square of the distance is called a steradian. Therefore, the light of one candle radiating through one steradian is one lumen. It follows that one footcandle is equal to one lumen per square foot since in the definition we said that one candle power is equal to one lumen per steradian. Light from a candle radiates in nearly a global range so one solid wedge (steradian) in only a portion of the light energy in a candle. If, in this case, one steradian contains one lumen, how many lumens are there in a candle? Since the area of a sphere is 12.57 times the radius, it follows that one candle has the power of 12.57 lumens.

It is important to remember that a lumen is like an energy source. When the lumens are directed onto or through an area, you get lumens per square foot or footcandles. Lux is simply the metric unit for lumens per square meter. 10.76 lux equals 1 footcandle.

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