= Radiometric absorptance
{title2=$a=P_{\rm abs}/P_{\rm in}$}
<Radiometric absorptance> is the fraction of incident <radiant flux> converted to <heat> in the specified receiver: $P_{\rm abs}=aP_{\rm in}$, with $0\leq a\leq1$ for a passive receiver. Specify the entrance boundary, <wavelength>, incidence <angle> and receiver <temperature>. Radiation reflected or transmitted out of that receiver is not absorbed <power>. A cavity can improve <radiometric absorptance> by giving <light> repeated opportunities for <wave absorption>; a dark coating alone does not establish $a=1$.
For broadband <light>, <spectral radiant flux> matters: $P_{\rm abs}=\int a(\lambda)\Phi_{e,\lambda}(\lambda)\,d\lambda$. A single effective <radiometric absorptance> therefore depends on the incident <optical spectrum> as well as on the receiver. If the stated incident boundary lies before a window of known power transmission $\tau_w$, the receiver absorbs $a\tau_wP_{\rm before}$ when other optical losses are absent. Window loss and receiver <radiometric absorptance> are separate corrections.
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