The total amount of electromagnetic radiation emitted by a surface; related to emissivity but considering the total energy output.
From Latin emittere (to send out) plus the suffix -ance/-ence, which creates nouns denoting action, state, or quality—similar to importance and residence.
Emittance is a technical cousin to emissivity: emissivity is a ratio (0-1) comparing how well something emits relative to a perfect black body, while emittance measures actual watts per unit area, so emittance = emissivity × Stefan-Boltzmann constant.
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