Lumen second

Lumen second
Unit systemSI
Unit ofluminous energy
Symbollm·s

In photometry, the lumen second (lm⋅s) is the unit of luminous energy in the International System of Units (SI). 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.

The photometric unit lumerg[1] or lumberg,[2] 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
[nb 1] 		Notes
Name Symbol[nb 2] Name Symbol
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 (= candela steradian) lm (= cd⋅sr) J Luminous energy per unit time
Luminous intensity Iv candela (= lumen per steradian) cd (= lm/sr) J Luminous flux per unit solid angle
Luminance Lv candela per square metre cd/m2 (= lm/(sr⋅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 (= lumen per square metre) lx (= lm/m2) L−2⋅J Luminous flux incident on a surface
Luminous exitance, luminous emittance Mv lumen per square metre lm/m2 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/m3 L−3⋅T⋅J
Luminous efficacy (of radiation) K lumen per watt lm/W M−1⋅L−2⋅T3⋅J Ratio of luminous flux to radiant flux
Luminous efficacy (of a source) η[nb 3] lumen per watt lm/W M−1⋅L−2⋅T3⋅J Ratio of luminous flux to power consumption
Luminous efficiency, luminous coefficient V 1 Luminous efficacy normalized by the maximum possible efficacy
See also:
  1. ^ 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.
  2. ^ Standards organizations recommend that photometric quantities be denoted with a subscript "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
  3. ^ a b c Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ for luminous efficacy of a source.
  1. ^ a b c 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.”
  2. ^ Jerrard, H. G.; McNeill, D. B. (2012). Dictionary of Scientific Units: Including dimensionless numbers and scales (6th ed.). Springer Science & Business Media. p. 92. ISBN 9789401122948.