Jessie Taylor
Candles have been used for over two millennia, and they were a significant form of indoor lighting until the invention of other light sources. The word candle comes from the Latin candēre, which means 'to shine'. A candle flame has three to five regions or 'zones', one of which is the non-luminous, coolest part of the candle flame located around the base of the wick. The unit of luminous intensity in the International System of Units (SI) is called a candela (cd). It measures the luminous power per unit solid angle emitted by a light source in a particular direction. A candlepower (cp or CP) is a unit of measurement for luminous intensity that expresses levels of light intensity relative to the light emitted by a candle of specific size and constituents. This leads us to the question: is a candle luminous or illuminated?
| Characteristics | Values |
|---|---|
| Definition of Luminous | Scientifically, luminosity is the amount of light that a luminous source provides |
| Luminous Source | The sun, stars, fire, glow worms, jellyfish, fireflies |
| Illuminated Source | The moon, an object that reflects light from a luminous source |
| Unit of Measurement for Luminosity | Candela (cd) |
| Candela Definition | The candela is the unit of luminous intensity in the International System of Units (SI) |
| Luminous Intensity of a Common Wax Candle | One candela |
| Luminous Intensity of a Modern Candle | 13 lumens |
| Luminous Efficacy of a Modern Candle | 0.16 lumens per watt |
| Luminous Intensity of a Traditional Candle | 0.981 candelas |
| Luminous Intensity of a Candle (in Zones) | Zone I: non-luminous; Zone II: blue; Zone III: dark; Zone IV: luminous (yellow/white) |
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What You'll Learn
Luminous intensity and the Candela unit of measurement
Luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle. The SI unit of luminous intensity is the candela (cd), a base unit of the SI system. The candela is derived from the Latin word for candle, and a common wax candle emits light with a luminous intensity of roughly one candela. The unit was adopted in 1948 by the 9th CGPM, and the definition was amended in 1967 to specify the atmospheric pressure applied to freezing platinum.
Prior to 1948, various standards for luminous intensity were in use, typically based on the brightness of a "standard candle" of a defined composition. One of the most well-known standards was the English standard of candlepower, which 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. Germany, Austria, and Scandinavia used the Hefnerkerze, a unit based on the output of a Hefner lamp. These units were superseded by the definition of the candela.
In 1884, Jules Violle proposed a new standard based on the light emitted by 1 cm2 of platinum at its melting point, which was roughly equal to 60 English candlepower. This proposal was notable as the first unit of light intensity that did not depend on the properties of a particular lamp. However, impurities on the surface of the platinum could affect its emissivity and luminous intensity by altering the melting point. In 1937, the Commission Internationale de l'Éclairage (International Commission on Illumination) and the CIPM proposed a "new candle" based on a hollow shell of thorium dioxide suspended in a bath of molten platinum, which served as a black body emitting black-body radiation.
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. This frequency is in the visible spectrum near green, corresponding to a wavelength of about 555 nanometres, to which the human eye is most sensitive when adapted for bright conditions. The candela is a photometric" unit that compensates for human visual sensitivity, as it measures light that is visible to the human eye, coming directly from a source.
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The non-luminous zone of a candle
A candle is an ignitable wick embedded in wax or another flammable solid substance, which provides light. A candle flame is widely recognised to have between three and five regions or "zones".
The wick of a candle, typically made from braided cotton, draws up the melted wax through capillary action to be burnt, thereby maintaining a constant flame. The rate at which the melted wax is drawn up is determined by the wick's capillarity. If the capillarity is too great, the molten wax will run down the side of the candle.
To prevent this, wicks are often infused with chemicals to modify their burning characteristics. For example, chemicals such as ammonium nitrate and ammonium sulfate are used to prevent the wick from glowing after the flame is extinguished.
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The luminous zone of a candle
A candle is an ignitable wick embedded in wax or another flammable substance, which provides light. The candle flame is widely recognised to have between three and five regions or "zones". The fourth zone is the middle or luminous zone, which is yellow/white and is located above the dark zone. This is the zone where the wax vapourises and combines with oxygen in the air to form a flame.
The light produced by a candle is about 13 lumens, with a luminous efficacy of about 0.16 lumens per watt, almost a hundred times lower than an incandescent lightbulb. The unit of luminous intensity in the International System of Units (SI) is the candela (cd), which measures luminous power per unit solid angle emitted by a light source in a particular direction. One candela is equal to roughly the luminous intensity of a common wax candle.
The term candlepower is now largely obsolete, but it is still sometimes used to describe the luminous intensity of high-powered flashlights and spotlights. Candlepower expresses levels of light intensity relative to the light emitted by a candle of a specific size and composition. In modern usage, candlepower is sometimes used as a synonym for candela. One candlepower is equal to 12.57 lumens and 0.981 candelas.
The light emitted by a candle spreads in all directions, while the light from a flashlight is highly directional, which makes direct comparisons of their intensities difficult. However, a candle with a higher candlepower rating will be brighter than one with a lower rating.
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Candlepower and its modern usage
Candlepower, a unit of measurement for luminous intensity, expresses levels of light intensity relative to the light emitted by a candle of specific size and constituents. Historically, candlepower was defined 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. This definition was established in the United Kingdom by the Metropolitan Gas Act of 1860.
Over time, the definition of candlepower has evolved. In 1921, the Commission Internationale de l'Eclairage (International Commission for Illumination) redefined the international candle in terms of a carbon filament incandescent lamp. In 1937, the international candle was again redefined against the luminous intensity of a blackbody at the freezing point of liquid platinum.
In 1948, the international unit system (SI) candela replaced candlepower as the standard unit of luminous intensity. One candlepower unit is approximately 0.981 candela, and in modern usage, candlepower is sometimes used as a synonym for candela. The candela measures luminous power per unit solid angle emitted by a light source in a particular direction.
While the term "candlepower" is largely obsolete, it is still occasionally used to describe the luminous intensity of high-powered flashlights and spotlights. Narrow-beamed lights can have very high candlepower specifications because candlepower measures the intensity of light on a target rather than the total amount of light emitted. Candlepower is also still used in certain legal contexts, such as in the California Vehicle Code to define the legal requirements for headlamps and other lamps.
In summary, candlepower was historically used as a unit of measurement for luminous intensity, but it has since been replaced by the candela in the international unit system. However, the term candlepower still sees some modern usage as a synonym for candela and in specific contexts such as high-powered lighting and legal requirements.
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The illumination of a candle compared to a flashlight
A candle is an ignitable wick embedded in wax or another flammable substance, which provides light. The light produced by a candle is about 13 lumens, for a luminous efficacy of about 0.16 lumens per watt. This is almost a hundred times lower than an incandescent light bulb. The candle flame has three to five regions, with the lowest part of the flame being non-luminous and coolest, located around the base of the wick where there is insufficient oxygen for the fuel to burn.
The candela (cd) is the unit of luminous intensity in the International System of Units (SI). It measures luminous power per unit solid angle emitted by a light source in a particular direction. A common wax candle emits light with a luminous intensity of roughly one candela. Candlepower (cp or CP) is a unit of measurement for luminous intensity, expressing levels of light intensity relative to the light emitted by a candle of specific size and constituents. One candlepower is equal to 0.981 candelas.
A flashlight, on the other hand, is designed to be highly directional. A 1-candela flashlight with a beam that illuminates 1 square meter from a 1-meter distance will have an illumination of 12.57 lumens, compared to 1 lumen for the candle. This makes direct comparisons between the two light sources tricky. However, the candle spreads its light in all directions, while the flashlight is more focused, making the latter more useful in certain situations, such as finding something in the dark.
In summary, while a candle has a luminous intensity of approximately one candela, the comparison between a candle and a flashlight is complex due to the directional nature of flashlight beams. The flashlight's ability to focus its light in a specific direction makes it more effective in certain situations, despite the candle's higher candela rating.
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Frequently asked questions
A luminous source is something that emits its own light. The sun, stars, and fire are examples of luminous sources that do not use artificial energy.
An illuminated source is something that reflects light from another source and can therefore be seen. The moon is an example of an illuminated object as it reflects the light of the sun.
A candle is a luminous source as it emits its own light. The flame of a candle is formed because wax vaporizes when burned. The middle or luminous zone of a candle flame is yellow/white and is located above the dark zone. A candle typically produces light with a luminous intensity of about 13 lumens.
