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. 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 .