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A candela is a measurement of the intensity of light and is used in the International Standard (SI) measurement system. Historically, a candela has been roughly equal to the intensity of the light emitted by a regular candle. It has come to be technically defined as the intensity in a given direction of a source that emits monochromatic radiation of a frequency of 540 x 1012 hertz and which has a radiant intensity in the same direction of 1/683 watts per steradian. A light measured at 120 candela is roughly equal to the light emitted by a 100-watt light bulb.
The candela as a unit of measurement grew out of the desire to adopt a standard method to measure light. During the 19th century, each country had its own measure of light, and most of them were extremely difficult to reproduce with accuracy. At the beginning of the 20th century, groups of nations began adopting new standards, such as the international candle, which were based on a single type of lamp and were much more consistent. Eventually, in the 1930s, it was decided that a more precise definition was needed.
Initially, the candela was defined as the luminous emission of a Planckian radiator, a type of blackbody, at the temperature of freezing platinum, 3,223.4 degrees Fahrenheit (1,773 degrees Celsius). This correlated roughly to the light emitted by a typical candle, making it a suitable measure. In the late 1970s, it was determined that the experimental difficulties in creating a Planckian radiator at such high temperatures made the existing definition of a candela less than desirable. Breakthroughs in radiometry allowed scientists to have a more specific definition, so the definition was adopted.
One reason the watt was not initially tied to the candela is the eye's differences in seeing various wavelengths of light. The human eye is less observant of blue and red light, so more wattage is needed to produce the same result in the brain as with yellow or green light. This is why the definition of a candela is defined as a very specific hertz that is unachievable in actual common lighting. Some people criticize the modern definition of the candela for this reason, arguing that it has removed any common-sense understanding of the candela as an actual emission of light.
The reason why the modern definition of candela includes the phrase "1/683 watts per steradian" is to make it conform to the previous definition and to avoid any confusion when comparing the modern unit to the historical candela. In this context, a steradian is the cone of light as it emerges from the source, such that it would light up 1 square meter of the inside of a sphere with a 1-meter radius.
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