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Luminous energy
From Wikipedia, the free encyclopedia
In photometry, luminous energy is the perceived energy of light. This is sometimes also called the quantity of light.
Luminous energy is not the same as the radiant energy, 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. When adapted for bright conditions (photopic vision), the eye is most sensitive to light at 555 nm. Light with the same power at longer or shorter wavelengths has a lower luminous energy.
The SI unit of luminous energy is the lumen second, which is unofficially known as the Talbot in honor of William Henry Fox Talbot.
In other systems of units, luminous energy may be expressed in basic units of energy.
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| Quantity | Symbol | SI unit | Abbr. | Notes |
|---|---|---|---|---|
| Luminous energy | Qv | lumen second | lm·s | units are sometimes called talbots |
| Luminous flux | F | lumen (= cd·sr) | lm | also called luminous power |
| Luminous intensity | Iv | candela (= lm/sr) | cd | an SI base unit |
| Luminance | Lv | candela per square metre | cd/m2 | units are sometimes called nits |
| Illuminance | Ev | lux (= lm/m2) | lx | Used for light incident on a surface |
| Luminous emittance | Mv | lux (= lm/m2) | lx | Used for light emitted from a surface |
| Luminous efficacy | lumen per watt | lm/W | ratio of luminous flux to radiant flux; maximum possible is 683.002 |
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