Lumens and candlepower (cp) are slightly different ways of measuring the brightness of light, for instance light bulbs. Candlepower relates to the strength of the light at it's source and is measured in a single beam or direction, while lumens refer to the level of illumination received by the human eye over an area. It is possible to do a mathematical conversion between the two, although it's not actually that helpful. However, it helps to understand the difference, and reason for the different measures, especially when redesigning a home's lighting and deciphering the information provided by manufacturers.

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## Candlepower

Candlepower is actually and outdated term, and today is normally used as a synonym for a "candela." Candela is measured at the bulb, and quantifies how much light the bulb produces. The units, as both names suggest, are based upon the output of light provided by one candle; this has become refined over time as our ability to both manipulate and measure light sources has advanced. The current definition for a candela is "the luminous intensity of a light source producing single-frequency light at a frequency of 540 terahertz (THz) with a power of 1/683 watt per steradian, or 18.3988 milliwatts" according to Russ Rowlett from the University of North Carolina. The key point to understand is that it is the brightness of the light at its source.

## Lumens

Lumens refer to the amount of light that would be perceived by the human eye at a given point; this could be on a surface at a distance, at the actual source or the light being provided in an area. It is a measurement that is based on the candela, but working in a spherical space, rather than at a point, so the math isn't too hard.

## The Math

In geometry, a circle has 2pi radians, whereas a sphere has 4pi steradians; where we use 2pi to calculate the size of a circle from its radius, we use 4pi to calculate the size of a sphere. A bulb that creates one candela of light shining in all directions would create 4pi lumens. 4 times pi is a difficult number to work with, so most conversion tables use 12.57. Dividing the lumens at the source of a light by 12.57 (or as close to 4pi as you prefer) will give you the candlepower; therefore 100 lumens is around 7.95 candlepower.This only really works when the light is emitted uniformly in all directions, which it normally isn't due to way light bulbs are manufactured, as he base of a bulb contains mechanisms so isn't emitting light.

## Considerations

Where a light's function is directional, like a flashlight (or hand torch), knowing the candlepower is more relevant. For general house lighting, in which we want light to be emitted less directionally, the quantity of lumens is the relevant measurement to be working with.

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#### References

- University of North Carolina at Chapel Hill: How Many? A Dictionary of Units of Measurement: Candle Power
- University of North Carolina at Chapel Hill: How Many? A Dictionary of Units of Measurement: Lumen
- University of North Carolina at Chapel Hill: How Many? A Dictionary of Units of Measurement: Steradian
- LightSearch.com: Lighting Metrics: Quantity, Quality, Efficiency