Candlepower
Candlepower (abbreviated as cp) is an obsolete unit expressing luminous intensity, equal to 0.981 candelas. 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 use of candle term candlepower was originally defined in England by the Metropolitan Gas Act 1860 as the light produced by a pure spermaceti candle weighing 1⁄6 pound (76 grams) and burning at a rate of 120 grains per hour (7.8 grams 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.
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