Noun
emittance (countable and uncountable, plural emittances)
English Wikipedia has an article on:emittanceWikipedia
The quantity of luminous flux emitted per unit area.
Also, the peak emittance wavelength of black-body radiation shifts to infinity at absolute zero; indeed, a peak no longer exists and black-body photons can no longer escape. Source: Internet
The top of a Planck curve ( the peak emittance wavelength ) is located in a particular part of the electromagnetic spectrum depending on the temperature of the black-body. Source: Internet