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 human eye. ...

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

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

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

candela The candela ( or ; 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 ...

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 Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and North ...

, by the

Metropolitan Gas Act 1860 The Metropolis Gas Act 1860 (23 & 24 Vict c 125) was an Act of the Parliament of the United Kingdom. The Bill for this Act was the Gas (Metropolis) Bill. The Metropolis Gas Act 1860 regulated the operation of gas supply companies in London. As ...

, as the light produced by a pure

spermaceti Spermaceti is a waxy substance found in the head cavities of the sperm whale (and, in smaller quantities, in the oils of other whales). Spermaceti is created in the spermaceti organ inside the whale's head. This organ may contain as much as of ...

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 ''Physeter'' and one of three extant species in the sperm whale famil ...

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 The Carcel lamp was an efficient lighting device used in the nineteenth century for domestic purposes and in France as the standard measure for illumination. The lamp was invented by the French watchmaker Bernard Guillaume Carcel (1750–1818) to ...

. They defined the unit was that illumination that emanates from a lamp burning pure

colza oil Close-up of canola blooms Canola flower Rapeseed oil is one of the oldest known vegetable oils. There are both edible and industrial forms produced from rapeseed, the seed of several cultivars of the plant family Brassicaceae. Historically, i ...

(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 A standards organization, standards body, standards developing organization (SDO), or standards setting organization (SSO) is an organization whose primary function is developing, coordinating, promulgating, revising, amending, reissuing, interpr ...

(United States), and the ''

Physikalische Technische Reichsanstalt The Physikalisch-Technische Bundesanstalt (PTB) is the national metrology institute of the Federal Republic of Germany, with scientific and technical service tasks. It is a higher federal authority and a public-law institution directly under fe ...

'' (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 The Hefner lamp, or in German ''Hefnerkerze'', is a flame lamp used in photometry that burns amyl acetate. The lamp was invented by Friedrich von Hefner-Alteneck in 1884 and he proposed its use as a standard flame for photometric purposes with a ...

. 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, incandescent lamp or incandescent light globe is an electric light with a wire filament heated until it glows. The filament is enclosed in a glass bulb with a vacuum or inert gas to protect the filament from oxid ...

. In 1937, the international candle was redefined again—against the luminous intensity of a

blackbody A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name "black body" is given because it absorbs all colors of light. A black body ...

at the freezing point of liquid

platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Platinu ...

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 a specified physical quantity is inversely proportional to the square of the distance from the source of that physical quantity. The fundamental cause for this can be understoo ...

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 The California Vehicle Code, informally referred to as the Veh. Code or the CVC, is the section of the California Codes which contains almost all statutes relating to the operation, ownership and registration of vehicles (including bicycles and eve ...

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 ...

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 bomb Flashbombs are loaded into a photo-reconnaissance Melsbroek_Air_Base.html"_;"title="De_Havilland_Mosquito_at_Melsbroek_Air_Base">Melsbroek,_Belgium._c.1944 A_photoflash_bomb,_or_flash_bomb,_is_bomb.html" ;"title="Melsbroek Air Base">Melsbroek, B ...

s, 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