Jessie Taylor
Candlepower, a unit of measurement for luminous intensity, is defined as the light produced by a pure spermaceti candle weighing 1/6 pound and burning at a rate of 120 grains per hour. The candle power of a lamp can be determined by using polar curves to represent the luminous intensity in all directions. The mean horizontal candle power (mhcp) and mean spherical candle power (mscp) can be determined from the horizontal and vertical polar curves, respectively. The candle power of a source in any given direction can be measured by comparing it to a standard or substandard source using a photometer bench and a photometer in a dark room to eliminate errors due to reflected light.
| Characteristics | Values |
|---|---|
| Definition | Candlepower is a unit of measurement for luminous intensity. |
| Synonyms | Candela |
| Historical Definition | Equal to 0.981 candelas, or 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). |
| Modern Usage | Synonymous with candela. |
| Determining Candle Power of a Lamp | The luminous intensity of a lamp can be determined by using polar curves. The mean horizontal candle power (mhcp) and mean spherical candle power (mscp) can be determined from the horizontal and vertical polar curves, respectively. |
| Standard Source Comparison | The candle power of a source can be measured by comparing it to a standard or substandard source using a photometer bench and a photometer in a dark room with black walls and ceiling. |
| Calculation | To calculate lumens from candlepower, multiply the candlepower by 12.57. To calculate candlepower from lumens, divide the lumens by 12.57. |
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What You'll Learn
Luminous intensity
Prior to the definition of the candela, a variety of units for luminous intensity were used in various countries. These were typically based on the brightness of the flame from a "standard candle" of defined composition or the brightness of an incandescent filament of specific design. One candlepower was the light produced by a pure spermaceti candle weighing one-sixth of a pound and burning at a rate of 120 grains per hour. In modern usage, candlepower is sometimes used as a synonym for candela.
Candlepower (abbreviated as cp or CP) is a unit of measurement for luminous intensity. It expresses levels of light intensity relative to the light emitted by a candle of a specific size and constituents. The historical candlepower is equal to 0.981 candelas. To calculate lumens from candlepower, multiply the candlepower by 12.57. To calculate candlepower from lumens, divide the lumens by 12.57.
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Mean horizontal candle power
Candlepower (abbreviated as cp or CP) is a unit of measurement for luminous intensity. It expresses levels of light intensity relative to the light emitted by a candle of a specific size and composition. The modern candlepower is equal to 0.981 candelas.
The mean (or average) candle power of a source in all directions in a horizontal plane is known as mean horizontal candle power (MHCP). The candle power of a source varies in different directions. The average candle power of a source is, therefore, the mean of candle powers in all directions.
The mean (or average) of candle power in all directions and in all planes from the source of light is termed as mean spherical candle power (MSCP). The ratio of the mean spherical candle power to the mean horizontal candle power of a source of light is known as the reduction factor of the source of light.
The ratio of the total number of lumens reaching the working plane to the total number of lumens emitting from the source is known as the utilization factor. The utilization factor can be calculated using the following formula:
$$\mathrm{\mathrm{Utilization\: factor}\:=\:\frac{\mathrm{Total\: lumens\: reaching\: the\: working\: plane}}{\mathrm{Total\: lumens\: emitting\: from\: the\: source}}}$$
The specific consumption of a light source is defined as the ratio of input electric power to average candle power. This can be calculated using the following formula:
$$\mathrm{\mathrm{Specific\: consumption}\:=\:\frac{\mathrm{Input \:electric \:power}}{\mathrm{Average\: candle\: power}}}$$
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Mean spherical candle power
Candlepower (abbreviated as cp or CP) 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. The modern usage of candlepower is sometimes used as a synonym for candela, which is a more standard unit for light intensity. One candlepower is equal to 0.981 candelas.
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 1/6 pound (76 grams) and burning at a rate of 120 grains per hour (7.8 grams per hour). Spermaceti, a material from the heads of sperm whales, was used to make high-quality candles.
Over time, the definition of candlepower has evolved internationally. In 1909, several agencies, including representatives from France, the UK, the United States, and Germany, met to establish an international standard. The majority of agencies redefined the candle in terms of an electric lamp with a carbon filament. However, the German representatives used a definition equal to 9/10 of the output of a Hefner lamp.
In 1921, the Commission Internationale de l'Eclairage (CIE) or the International Commission for Illumination redefined the international candle again in terms of a carbon filament incandescent lamp. Finally, in 1937, the international candle was once more redefined against the luminous intensity of a blackbody at the freezing point of liquid platinum, resulting in 58.9 international candles per square centimetre.
To calculate lumens from candlepower, multiply the candlepower by 12.57. Alternatively, to calculate candlepower from lumens, divide the lumens by 12.57.
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Standard source
The standard source is a key component in determining the candle power of a lamp or light source. Candlepower, also known as candela, is a unit of measurement for luminous intensity, expressing the level of light intensity relative to the light emitted by a candle of specific size and composition.
To measure candle power using a standard source, the standard source with known candle power (referred to as 'S') and the source under test are set at a distance apart with a screen or photometer head between them. The Bunsen and Lummer Brodhun photometers are commonly used for this purpose, especially when the sources emit light of similar colours. If the light sources differ in colour, a Flicker photometer is more suitable.
The photometer head or screen is then adjusted until the illumination on both sides is equal. This involves moving the screen or photometer head to specific points where there is a perceptible difference in illumination from each side, and then determining the midpoint where the illumination is equal. This process adheres to the inverse square law, which takes into account the squares of the distances between the sources and the photometer head or screen.
Another method involves using the Bunsen Grease spot photometer, where the photometer is fixed in one position, and the illumination from the test lamp and standard lamp is measured independently. The distance of each lamp from the photometer head is noted when the grease spot disappears. This approach utilises two mirrors placed behind the paper to simultaneously observe the illumination on both sides.
It is important to note that candlepower only expresses the intensity of light at its source and does not indicate how strong the light is at a distance or how it is perceived by the human eye.
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Calculating lumens
Lumens are a crucial unit of measurement in the lighting industry, as they help us understand how much visible light a source emits. This is particularly important when comparing different lighting products, as it provides a holistic view of their illuminating capabilities. Lumens are especially valuable when selecting light fixtures or bulbs, as they enable us to make informed choices by predicting how bright a light source will be.
The number of lumens a light source emits determines its overall brightness and intensity. This is in contrast to candlepower, which measures the intensity of light emitted in a specific direction from a light source. Candlepower takes into account the performance of the whole fixture, making it more relevant for searchlights and other focused light sources used to illuminate specific targets at long ranges.
While candlepower was historically defined as the amount of light emitted by a standard candle, modern measurement techniques employ more precise instruments. Today, candlepower is more accurately described as equal to 0.981 candelas, which is the standard unit for light intensity. To convert candelas into candlepower, simply multiply the number of candelas by 0.981.
To convert lumens to candlepower, a specific formula is used: Candlepower = (Lumens x 2π) / Solid Angle. The solid angle is typically measured in steradians (sr), which represent the extent of the surface of a sphere covered by a given angle. For a point source, the solid angle is 4π steradians, representing the entire sphere surrounding the source.
For example, let's consider a light source with an output of 10,000 lumens, emitting light uniformly in all directions. To determine the equivalent candlepower rating, we use the formula: Candlepower = (10,000 x 2π) / 4π. Simplifying this equation, we get: Candlepower = (10,000 x 2) / 4. This further simplifies to: Candlepower = 20,000 / 4. Therefore, the final answer is that a light source with an output of 10,000 lumens is equivalent to a candlepower rating of 5,000.
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Frequently asked questions
Candlepower (abbreviated as cp or CP) 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.
The candle power of a lamp can be determined by using polar curves to measure the luminous intensity in all directions. The mean horizontal candle power of a lamp can be determined from the horizontal polar curve by taking the mean value of the candle power in a horizontal direction.
To calculate lumens from candlepower, multiply the candlepower by 12.57. To calculate candlepower from lumens, divide the lumens by 12.57.
