A lumen is a unit of measurement for light, defined as 1 candela multiplied by 1 steradian. It is part of the photometry group, which includes units such as candela and lux. The footcandle is a nonstandard measure of illuminance, while lux is the standard measure. Candela and lux use a measure of luminous intensity called a candela. A steradian is a unit of measurement used to define a solid angle. Luminous efficacy is determined by combining power and luminous flux emitted.
A lumen is a standard unit of measurement used to describe the amount of light contained in a given area. It is part of a group of standard measurements known as the photometry group, which measure different aspects of light. This group also includes units such as candela, which measures luminance, and lux, which measures illuminance.
Strictly speaking, a lumen is defined as 1 candela multiplied by 1 steradian, which can be expressed as . A related unit of measurement, although not part of the standard units, is the foot candle, which is often used in photography and film. To really understand this, it’s important to understand these units: the candela, the candela, the steradian, and the lux.
There is an important distinction to be made between measurements of radiance and measurements of illuminance. When measuring radiance, someone is basically looking at how much energy a direct light source releases. He’s not interested in what happens to that energy as it leaves the source, only what is at the source itself. When measuring illuminance, by contrast, a person is looking at how much of that energy gets to a given object.
The footcandle, also called a footcandle, is the nonstandard measure of illuminance, broadly defined as how much light would strike the inner surface of a sphere of 1 foot (30.48 cm) in radius if it were lit by a single candle in the center of that sphere. Lux is the standard measure of illuminance and can be defined simply as how much light would strike a similar sphere of 3.28 feet (1 meter) in radius. The two can be converted simply, with 10.76 lux at 1 foot-candle.
Both candela and lux make use of a measure of luminous intensity, called a candela. Candela was originally called candela and simply referred to the amount of light energy emitted by a known type of candle. Later, when it was standardized, a definition that approximated the previous definition was chosen, so that the existing equations could remain the same. The modern definition of a candela has to do with the radiation emitted by 1/60th of a single square centimeter of platinum when it is at its melting point.
A steradian is a standard unit of measurement used to define a solid angle. The technical definition is “the solid angle subtended at the center of a sphere of radius r by a portion of the surface of the sphere having an area of r2”. In this definition, subtended has to do with the relationship between the arc length and the resulting angle. The steradian is unitless and is represented by the abbreviation sr. Mathematically, using the meter as a unit of measurement, we can therefore define a steradian as 1(sr) = m2 x m-1.
So, taking all of this together, it’s easier to interpret the original definition of a lumen as 1(lm) = 1(cd) x 1(sr). A lumen, in this case, is a measure of the amount of luminous flux emitted in an area of 1/60th of a single square centimeter of platinum at its melting point at a certain angle. This measurement is often used in conjunction with power to determine the luminous efficacy of a light-emitting body.
For example, a typical 100-watt light bulb has a luminous efficacy of approximately 17.5, putting out 1750 lumens of light. A 13-watt fluorescent bulb, on the other hand, has an efficiency of about 56, putting out about 730 lumens of light. The sun, by contrast, has an efficiency of about 93.
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