Luminous flux
From Freepedia
Luminous flux or luminous power is the measure of the perceived power of light. On the other hand, radiant flux is the measure of the total power of light emitted, without regard to the sensitivity of the human eye.
However, luminous flux is not a direct measure of the perception of brightness.
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Units
Any unit of power can be used to express luminous flux, however the SI unit of luminous flux is the lumen (lm) and not the watt.
One lumen is defined as the luminous flux of light that passes from a light source of one candela through a unit steradian. Only luminous flux is expressed in lumens; radiant flux is expressed in simple watts.
Weighting
Different wavelengths of light are perceived as having different intensities. Thus, the luminous flux is calculated as a weighted average of the wavelengths' powers. These weighting factors are called the luminous efficacies.
Contexts
Luminous flux is often used as an objective measure of the useful power emitted by a light source. Energy conscious consumers commonly compare the luminous flux of different light bulbs. However, luminous flux is not as useful when comparing bulbs of differing brightness, since it does not directly measure brightness.
SI light units
| 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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