Candlepower (abbreviated as cp) is an obsolete unit expressing luminous intensity, equal to 0.981 candela. It expresses levels of light intensity in terms of the light emitted by a candle of specific size and constituents. In modern usage candlepower equates directly to the unit known as the candela.
The term candlepower was originally defined in England by the Metropolitan Gas Act 1860 as the light produced by a pure spermaceti candle weighing one sixth of a pound and burning at a rate of 120 grains per hour. Spermaceti is found in the heads of sperm whales, and was once used to make high-quality candles.
At this time the French standard of light was based upon the illumination from a Carcel burner. The unit was defined as that illumination emanating from a lamp burning pure colza oil (obtained from the seed of the plant Brassica campestris) at a defined rate. It was accepted that ten standard candles were about equal to one Carcel burner.
In 1909 a meeting took place to come up with 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.
In 1937, the international candle was redefined again against the luminous intensity of a blackbody at the freezing point of liquid platinum which was to be 58.9 international candles per square centimetre.
In 1948 the term candlepower was replaced by the international unit (SI) known as the candela. One old candlepower unit is about 0.981 candela. In general modern use, a candlepower now equates directly (1:1) to the number of candelas1—an implicit increase from its old value.
The candlepower of a lamp was measured by judging 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. The distance of one of the lamps was adjusted until the two were judged to give equal brightness. The candlepower of the lamp under test could then be calculated from the two distances and the inverse square law.
"Candlepower" is largely an obsolete term. However, it is still sometimes used to describe the luminous intensity of high powered flashlights and spotlights.
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