Ben Carter
Candlepower, an obsolete unit of measurement, expresses levels of light intensity relative to the light emitted by a candle of specific size and constituents. In modern usage, candlepower is sometimes used as a synonym for candela, the unit of luminous intensity in the International System of Units (SI). One candlepower is equal to about 0.981 candelas, and a common candle emits light with a luminous intensity of roughly one candela. Candlepower is a measure of the brightest part of the beam of a flashlight, while lumens measure the total amount of light emitted. Candlepower was originally defined in the United Kingdom 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. Today, candlepower is sometimes used to describe the luminous intensity of high-powered flashlights and spotlights.
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What You'll Learn
Candlepower is a unit of measurement for luminous intensity
Candlepower, abbreviated as cp or CP, is a unit of measurement for luminous intensity. It expresses the level of light intensity relative to the light emitted by a candle of a specific size and composition. In 1948, the international unit (SI) candela replaced candlepower. One candlepower unit is about 0.981 candela.
The term candlepower was originally defined in the United Kingdom by the Metropolitan Gas Act 1860. It was defined as the light produced by a pure spermaceti candle weighing one-sixth of a pound (76 grams) and burning at a rate of 120 grains per hour (7.8 grams per hour). Spermaceti is a material from the heads of sperm whales that was historically used to make high-quality candles.
Candlepower is now considered an obsolete term, although it is still sometimes used to describe the luminous intensity of high-powered flashlights and spotlights. It is particularly relevant for narrow-beamed lights, as candlepower measures the intensity of the light on a target rather than the total amount of light emitted.
Peak Beam Intensity (Candlepower) is a measure of the brightest spot in a focused beam. It is influenced by both the output of the lamp and the efficiency of the reflector. For example, a flashlight with 20,000 beam candlepower will project the same amount of light within its "hot spot" as a bare lamp of 20,000 candela at the same distance.
It is important to note that all light rating systems, including candlepower, depend on the perception of the human eye and are therefore subjective.
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The term was defined in the UK in 1860
The term "candlepower" was originally defined in the United Kingdom by the Metropolitan Gas Act of 1860. It was defined 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 a material derived from the heads of sperm whales and was 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. This was a lamp burning pure colza oil obtained from the seeds of the plant Brassica campestris. Ten standard candles were roughly equivalent to one Carcel burner.
In 1909, several agencies from France, the UK, the US, and Germany met to establish an international standard for light measurement. However, it wasn't until 1948 that the international unit (SI) candela replaced candlepower. One candlepower unit is approximately 0.981 candela.
Today, the term "candlepower" is largely obsolete. However, it is still occasionally used to describe the luminous intensity of high-powered flashlights and spotlights. Candlepower measures the intensity of light on a target rather than the total amount of light emitted. For example, a lamp with a higher candlepower rating has light that is more tightly focused.
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It is now largely an obsolete term
The term candlepower is now largely obsolete, replaced by the internationally recognized SI unit candela in 1948. The modern candela is based on a physical process and can be precisely calibrated, unlike candlepower, which relied on a basic candle as the basis for measurement.
Candlepower (abbreviated as cp or CP) was originally defined in the United Kingdom 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, a material derived from sperm whale heads, was used to make high-quality candles.
However, as science and technology progressed, lighting technology also advanced, and sources of illumination became significantly brighter and safer than candles, which required open flames. The candlepower unit was adjusted several times to different light sources, but it eventually became obsolete.
Today, candlepower is sometimes still used to describe the luminous intensity of high-powered flashlights and spotlights, particularly in product packaging. However, its use is inconsistent and can create confusion when comparing the intensity of light sources. For example, a flashlight with a 20,000 beam candlepower would have a "hot spot" with the same intensity as a bare lamp of 20,000 candela at the same distance.
While the term candlepower may still be encountered, it is important to note that it is no longer an official unit of measurement and has been replaced by the more precise candela.
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One candlepower is about 0.981 candela
Candlepower, abbreviated as cp or CP, is a unit of measurement for luminous intensity. It measures the brightest part of the beam of a flashlight. In 1948, the international unit (SI) candela replaced candlepower. One candlepower unit is about 0.981 candela.
The term candlepower was originally defined in the United Kingdom 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 a material from the heads of sperm whales that was historically 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 was defined by the light emanating 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.
Today, candlepower is largely an obsolete term, but it is still used in certain contexts, such as in the California Vehicle Code to define the legal requirements for headlamps and other lamps. In modern usage, candlepower is sometimes used interchangeably with candela.
It's important to note that candlepower measures the intensity of light on a target, rather than the total amount of light emitted. This distinction is important when comparing candlepower to other units like lumens, which measure the total amount of light output.
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Candela is the unit of luminous intensity in the International System of Units
The candela (cd) is the unit of luminous intensity in the International System of Units (SI). It measures the luminous power per unit solid angle emitted by a light source in a particular direction. Luminous intensity is analogous to radiant intensity, but instead of simply adding up the contributions of every wavelength of light in the source's spectrum, the contribution of each wavelength is weighted by the luminous efficiency function, which models the sensitivity of the human eye to different wavelengths. The word candela is derived from the Latin word for candle, and the unit was defined in 1948 by the 9th CGPM as:
> "...the luminous intensity, in the perpendicular direction, of a surface of 1/600,000 square meter of a black body at the temperature of freezing platinum under a pressure of 101,325 newtons per square meter."
The candela replaced the standard candle or lamp as a unit of luminous intensity in calculations involving artificial lighting. The old candle-based unit was defined in the United Kingdom 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 a material from the heads of sperm whales that was historically used to make high-quality candles.
In modern usage, candlepower is sometimes used as a synonym for candela, with one candlepower unit being approximately equal to one candela. Candlepower is a measure of the brightest part of the beam of a flashlight, and it is calculated based on the output of the lamp and the efficiency of the reflector.
The candela is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540x10^12 Hz, Kcd, to be 683 when expressed in the unit lm W^-1, which is equal to cd sr W^-1, or cd sr kg^-1 m^-2 s^3. This frequency is in the visible spectrum near green, corresponding to a wavelength of about 555 nanometres, and it is the frequency to which the human eye is most sensitive when adapted for bright conditions.
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Frequently asked questions
1 candlepower is equal to 0.981 candelas.
Candlepower is a unit of measurement for luminous intensity. It expresses levels of light intensity relative to the light emitted by a candle of specific size and constituents.
To measure candlepower, a person judged by eye the relative brightness of adjacent surfaces. One surface is illuminated only by a standard lamp (or candle) and the other only by the lamp under test. The distance of one of the lamps is adjusted until the two surfaces appear to be of equal brightness.
