Candlepower (abbreviated as cp or CP) is a unit of measurement for luminous intensity. It expresses levels of light intensity relative to the light emitted by a candle of specific size and constituents. The historical candlepower is equal to 0.981 candelas. In modern usage, candlepower is sometimes used as a synonym for candela.—an implicit increase from its old value.

Calibration of lamps

To measure the candlepower of a lamp, a person judged by eye the relative brightness of adjacent surfaces—one illuminated only by a standard lamp (or candle) and the other only by the lamp under test. They adjusted the distance of one of the lamps until the two surfaces appeared to be of equal brightness. Then they calculated the candlepower of the lamp under test from the two distances and the inverse square law.

Modern use

"Candlepower" is largely an obsolete term. However, people still sometimes use it to describe the luminous intensity of high powered flashlights and spotlights. Narrow-beamed lights of all sorts can have very high candlepower specifications, because candlepower measures the intensity of the light on a target, rather than the total amount of light it emits. A given lamp has a higher candlepower rating if its light is more tightly focused.

Candlepower is still used today in law. For example, it is presently used in the California Vehicle Code to define the legal requirements for headlamps and other lamps, including accessory lamps.

Only a few artificial light sources, such as military photoflash bombs, have the very high candlepower ratings characteristic of narrow-beamed spotlights but, simultaneously, a wide unfocused distribution of light.

See also

  • Candela
  • List of obsolete units of measurement
  • Lumen (unit)

Notes and references

Further reading

  • International candle at Sizes.com Last revised: 27 June 2007. Accessed July 2007
  • Candle History - Candlepower 2003 Bob Sherman at Craftcave. Accessed July 2007.
  • Brief History Of Lighting 2004 by The Wolfstone Group. Accessed July 2007.
  • A History of Light and Lighting by Bill Williams Edition: 2.3 - (2005) Accessed July 2007.