When the temperature of a black body is such that the maximum spectral efficiency of production of radiant energy occurs at a given wave-length there also occurs at that wave-length, both the maximum spectral rate of production and the maximum spectral efficiency of production of photons. The particular wave-length involved is the effective wave-length for the total radiation, which is defined as the wave-length at which, for a given temperature, the percentage rate of increase in spectral radiancy with temperature is the same as for the total radiation from the black body source.
A. G. WORTHING, "New λT Relations for Black Body Radiation," J. Opt. Soc. Am. 29, 101-101 (1939)