In photometry, the talbot (T) is a nonstandard unit of luminous energy, named in honor of the early photographer William Fox Talbot. It is exactly equal to the SI standard unit, the lumen-second:
- 1 T = 1 lm · s
The use of the symbol T for talbots conflicts with T as the symbol for the tesla, the SI unit of magnetic flux density.
SI photometry units
| 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 | |||||
| SI • Photometry | ||||||||
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