The candela is a unit of light intensity used in the SI system of measurement. It was created to standardize the measurement of light, replacing various country-specific measures. Initially defined as the light output of a Planckian radiator, it was later redefined to be more specific. The modern definition includes the phrase “1/683 watt per steradian” to conform to the previous definition and avoid confusion.
A candela is a measure of light intensity and is used in the International Standard (SI) system of measurement. Historically, a candle has been approximately equal in intensity to the light emitted by a regular candle. It is technically defined as the intensity in a given direction of a source emitting monochromatic radiation of frequency 540 x 1012 hertz and having a radiant intensity in the same direction of 1/683 watt per steradian. A light measured at 120 candelas is approximately equal to the light emitted by a 100 watt light bulb.
The candela as a unit of measurement was born from the desire to adopt a standard method for measuring light. During the 19th century, each country had its own measure of light and most of them were extremely difficult to reproduce accurately. In the early 20th century, groups of nations began adopting new standards, such as the international candle, which were based on a single lamp type and were much more consistent. Finally, in the 1919s, it was decided that a more precise definition was needed.
Initially, the candela was defined as the light output of a Planckian radiator, a type of black body, at the freezing temperature of platinum, 3,223.4 degrees Fahrenheit (1,773 degrees Celsius). This correlated roughly to the light emitted by a typical candle, making it an adequate measure. In the late 1970s, it was determined that the experimental difficulties of creating a Planck radiator at such high temperatures made the existing definition of a candle less than desirable. Advances in radiometry allowed scientists to have a more specific definition, so the definition was adopted.
One reason why watt was not initially related to candela is the eye’s difference in seeing the various wavelengths of light. The human eye is less attentive to blue and red light, so more power is needed to produce the same result in the brain as yellow or green light. This is why the definition of candela is defined as a very specific hertz which is unattainable in actual common lighting. Some criticize the modern definition of a candle for this reason, arguing that it has removed any commonsense interpretation of the candle as the actual emission of light.
The reason the modern definition of candle includes the phrase “1/683 watt per steradian” is to make it conform to the previous definition and to avoid any confusion when comparing the modern unit to the historical candle. In this context, a steradian is the cone of light emerging from the source, such that it would illuminate 1 square meter of the interior of a 1 meter radius sphere.
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