Candlepower
Encyclopedia
Candlepower is a now-obsolete unit which was used to express 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
.
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 head of sperm whale
s, and once was 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.
Since 1948 the term candlepower was replaced by the international unit (SI
) known as the candela
. One old candlepower unit is about 0.981 candela. Less scientifically, modern candlepower now equates directly (1:1) to the number of candelas—an implicit increase from its old value.
Candle
A candle is a solid block or cylinder of wax with an embedded wick, which is lit to provide light, and sometimes heat.Today, most candles are made from paraffin. Candles can also be made from beeswax, soy, other plant waxes, and tallow...
of specific size and constituents. In modern usage Candlepower equates directly to the unit known as the candela
Candela
The candela is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function . A common candle emits light with a luminous intensity of roughly one candela...
.
History
The term candlepower was originally defined in England by the Metropolitan Gas Act 1860Metropolitan Gas Act 1860
The Metropolitan Gas Act 1860 was a United Kingdom act of parliament that regulated the operation of gas supply companies in London.-Background:...
as the light produced by a pure spermaceti
Spermaceti
Spermaceti is a wax present in the head cavities of the sperm whale . Originally mistaken for the whales' sperm , spermaceti is created in the spermaceti organ inside the whale's head and connected to its nasal passage...
candle weighing one sixth of a pound and burning at a rate of 120 grains per hour. Spermaceti is found in the head of sperm whale
Sperm Whale
The sperm whale, Physeter macrocephalus, is a marine mammal species, order Cetacea, a toothed whale having the largest brain of any animal. The name comes from the milky-white waxy substance, spermaceti, found in the animal's head. The sperm whale is the only living member of genus Physeter...
s, and once was used to make high quality candles.
At this time the French standard of light was based upon the illumination from a Carcel Burner
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 unit was defined as that illumination emanating from a lamp burning pure colza oil
Colza oil
Colza oil is a nondrying oil obtained from the seeds of Brassica rapa, var. oleifera, a variety of the plant that produces turnips. Colza is extensively cultivated in France, Belgium, the United States, the Netherlands and Germany and Poland. In France, especially, the extraction of the oil is an...
(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
National Physical Laboratory, UK
The National Physical Laboratory is the national measurement standards laboratory for the United Kingdom, based at Bushy Park in Teddington, London, England. It is the largest applied physics organisation in the UK.-Description:...
(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
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 luminous intensity unit of the Hefnerkerze...
.
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
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
which was to be 58.9 international candles per square centimetre.
Since 1948 the term candlepower was replaced by the international unit (SI
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...
) known as the candela
Candela
The candela is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function . A common candle emits light with a luminous intensity of roughly one candela...
. One old candlepower unit is about 0.981 candela. Less scientifically, modern candlepower now equates directly (1:1) to the number of candelas—an implicit increase from its old value.