[2.13.3] The statement that the radiant exitance (M) or radiance (L) of a blackbody radiator is proportional to the fourth power of its absolute temperature; that is, M = σT4, and L = σLT 4.
Note: The currently recommend value3 of σ is 5.67032 x 10-8 W/(m2 K4), and that of σL is 1.80492 x 10-8 (W/(m2•sr•K4)
3 “Dimensions/NBS,” NBS Technical News Bulletin, Washington DC: U.S. Department of Commerce. Jan 1974.