Candlepower (abbreviated as cp or CP) is a unit of measurement for

luminous intensity In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the huma ...

. It expresses levels of light intensity relative to the light emitted by a

candle A candle is an ignitable candle wick, wick embedded in wax, or another flammable solid substance such as tallow, that provides light, and in some cases, a Aroma compound, fragrance. A candle can also provide heat or a method of keeping time. ...

of specific size and constituents. The historical candlepower is equal to 0.981

candela The candela (symbol: cd) is the unit of luminous intensity in the International System of Units (SI). It measures luminous power per unit solid angle emitted by a light source in a particular direction. Luminous intensity is analogous to radi ...

s. In modern usage, ''candlepower'' is sometimes used as a synonym for ''candela''.

History

The term candlepower was originally defined in the

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...

, by the Metropolitan Gas Act 1860, as the light produced by a pure spermaceti candle that weighs and burns at a rate of . Spermaceti is a material from the heads of

sperm whale The sperm whale or cachalot (''Physeter macrocephalus'') is the largest of the toothed whales and the largest toothed predator. It is the only living member of the Genus (biology), genus ''Physeter'' and one of three extant species in the s ...

s, and was once used to make high-quality candles. At the time the UK established candlepower as a unit, the French standard of light was based on the illumination from a Carcel burner, which defined the illumination that emanates from a lamp burning pure colza oil (obtained from the seed of the plant '' Brassica campestris'') at a defined rate. Ten standard candles equaled about one Carcel burner. In 1909, several agencies met to establish an international standard. It was attended by representatives of the '' Laboratoire Central de l’Electricité'' (France), the National Physical Laboratory (UK), the Bureau of Standards (United States), and the '' Physikalische Technische Reichsanstalt'' (Germany). The majority redefined the candle in term of an electric lamp with a carbon filament. The Germans, however, dissented and decided to use a definition equal to 9/10 of the output of a Hefner lamp. In 1921, the ''Commission Internationale de l'Eclairage'' (International Commission for Illumination, commonly referred to as the CIE) redefined the international candle again in terms of a carbon filament

incandescent lamp An incandescent light bulb, also known as an incandescent lamp or incandescent light globe, is an electric light that produces illumination by Joule heating a filament until it glows. The filament is enclosed in a glass bulb that is eith ...

. In 1937, the international candle was redefined again—against the luminous intensity of a blackbody at the freezing point of liquid

platinum Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...

which was to be 58.9 international candles per square centimetre. In 1948, the international unit ( SI)

replaced candlepower. One ''candlepower'' unit is about 0.981 candela. In general modern use, a ''candlepower'' now equates directly (1:1) to the number of candelas—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 In science, an inverse-square law is any scientific law stating that the observed "intensity" of a specified physical quantity is inversely proportional to the square of the distance from the source of that physical quantity. The fundamental cau ...

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

headlamp A headlamp is a lamp attached to the front of a vehicle to illuminate the road ahead. Headlamps are also often called headlights, but in the most precise usage, ''headlamp'' is the term for the device itself and ''headlight'' is the term for t ...

s and other lamps, including accessory lamps.Se
California Vehicle Code § 25105
among other sections. 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.

History

Calibration of lamps

Modern use

See also

Notes and references

Further reading