Luminous intensity
From Freepedia
Luminous intensity is a measure of the perceived power emitted by a light source in a particular direction. Luminous intensity should not be confused with luminous flux, which is the total perceived power emitted across all directions.
Luminous intensity is not the same as the radiant intensity, the corresponding objective physical quantity. This is because the human eye can only see light in the visible spectrum and has different sensitivities to light of different wavelengths within the spectrum. The eye is most sensitive to light at 555nm; light with the same power at longer or shorter wavelengths has a lower luminous intensity.
The SI unit of luminous intensity is the candela (cd).
1 cd is luminous intensity perceived of a monochromatic light source of wavelength of 555 nm (or frequency 540THz) which has a radiant intensity of 1/683 watt per steradian.
In 1881, Jules Violle proposed the Violle as a unit of luminous intensity, and it was notable as the first unit of light intensity that did not depend on the properties of a particular lamp. It was superseded by the candela in 1946.
| SI light units | ||||
|---|---|---|---|---|
| Quantity | SI unit | Symbol | Notes | |
| Luminous energy | lumen seconds | lm · s | lumen seconds are sometimes called Talbots | |
| Luminous flux | lumen or (candela · steradian) | lm | also called Luminous power | |
| Luminous intensity | candela or (lumen / steradian) | cd | ||
| Luminance | candela / square metre | cd/m2 | also called Luminosity | |
| Illuminance | lux or (lumen / square metre) | lx | ||
| Luminous efficacy | lumens per watt | lm/W | maximum possible is 683 | |
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