Mole (unit) and growth chamber lighting for plants is sometimes expressed in micromoles per square meter per second, where 1 mol photons = photons. In 2011, the 24th meeting of the General Conference on Weights and Measures (CGPM) agreed to a plan for a possible revision of the SI base unit definitions at an undetermined date. The 26th meeting of the CGPM, in a formal vote on 16 November 2018, approved the proposed redefinition of SI base units mole, kilogram, ampere and kelvin. One mole of a substance will have exactly specified "elementary entities" of that substance. The definition of the mole
Cross section (physics) of the colliding system are , and , before and after the collision respectively, the differential cross section is given by where the on-shell matrix is defined by in terms of the S-matrix. Here is the Dirac delta function. The computation of the S-matrix is the main goal of the scattering theory. Although the SI unit of total cross sections is m, smaller units are usually used in practice. In nuclear and particle physics, the conventional unit is the barn b, where 1 b = 10 m = 100 fm. Smaller prefixed units such as mb and μb are also