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]

SI photometry quantities
Quantity Unit Dimension Notes
Name Symbol[nb 1] Name Symbol Symbol[nb 2]
Luminous energy Qv [nb 3] lumen second lm⋅s T⋅J The lumen second is sometimes called the talbot.
Luminous flux / luminous power Φv [nb 3] lumen (= cd⋅sr) lm J Luminous energy per unit time
Luminous intensity Iv candela (= lm/sr) cd J 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. The candela per square metre is sometimes called the nit.
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 Time-integrated illuminance
Luminous energy density ωv lumen second per cubic metre lm⋅s⋅m−3 L−3⋅T⋅J
Luminous efficacy η [nb 3] 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 Luminous efficacy normalized by the maximum possible efficacy
See also: SI · Photometry · Radiometry
  1. ↑ 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
  2. ↑ The symbols in this column denote dimensions; "L", "T" and "J" are for length, time and luminous intensity respectively, not the symbols for the units litre, tesla and joule.
  3. 1 2 3 Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ or K for luminous efficacy.

References

  1. 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.”
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