Lumen second
In photometry, the lumen second (lm s or lm⋅s) is the SI derived unit of luminous energy. It is based on the lumen, the SI unit of luminous flux, and the second, the SI base unit of time.
The lumen second is sometimes called the Talbot (symbol T). This name was coined in 1937 by the Committee on Colorimetry, Optical Society of America, in honor of the early photographer William Fox Talbot.[1] The Talbot is exactly equal to 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.
An older name for the lumen second was the lumberg.
The photometric unit lumerg, also proposed by the Committee on Colorimetry in 1937, correlates with the old CGS unit erg in the same way that the lumen second correlates with the radiometric unit joule, so that 107 lumerg = 1 lm⋅s.[1]
| Quantity | Unit | Dimension | Notes | |||||
|---|---|---|---|---|---|---|---|---|
| Name | Symbol[nb 1] | Name | Symbol | Symbol | ||||
| Luminous energy | Qv [nb 2] | lumen second | lm⋅s | T⋅J [nb 3] | Units are sometimes called talbots. | |||
| Luminous flux / luminous power | Φv [nb 2] | lumen (= cd⋅sr) | lm | J [nb 3] | Luminous energy per unit time. | |||
| Luminous intensity | Iv | candela (= lm/sr) | cd | J [nb 3] | Luminous flux per unit solid angle. | |||
| Luminance | Lv | candela per square metre | cd/m2 | L−2⋅J | Luminous flux per unit solid angle per unit projected source area. Units are sometimes called nits. | |||
| Illuminance | Ev | lux (= lm/m2) | lx | L−2⋅J | Luminous flux incident on a surface. | |||
| Luminous exitance / luminous emittance | Mv | lux | lx | L−2⋅J | Luminous flux emitted from a surface. | |||
| Luminous exposure | Hv | lux second | lx⋅s | L−2⋅T⋅J | ||||
| Luminous energy density | ωv | lumen second per cubic metre | lm⋅s⋅m−3 | L−3⋅T⋅J | ||||
| Luminous efficacy | η [nb 2] | lumen per watt | lm/W | M−1⋅L−2⋅T3⋅J | Ratio of luminous flux to radiant flux or power consumption, depending on context. | |||
| Luminous efficiency / luminous coefficient | V | 1 | ||||||
| See also: SI · Photometry · Radiometry | ||||||||
- ↑ Standards organizations recommend that photometric quantities be denoted with a suffix "v" (for "visual") to avoid confusion with radiometric or photon quantities. For example: USA Standard Letter Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967
- 1 2 3 Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ or K for luminous efficacy.
- 1 2 3 "J" here is the symbol for the dimension of luminous intensity, not the symbol for the unit joules.
References
- 1 2 Loyd Ancile Jones. Colorimetry: Preliminary draft of a report on nomenclature and definitions. Journal of the Optical Society of America, Volume 27, page 211 (June 1937): “Referring again to Table I, terms (21) and (22) represent names for units which the committee has voted to adopt. The decision was almost unanimous in the case of recommending talbot as a psychophysical correlate of joule. The vote on lumerg as the psychophysical correlate of the erg was less unanimous.”