= Optical concentrator
{title2=$C=A_{\rm in}/A_{\rm out}$}
An <optical concentrator> redirects <light> from an entrance <area> $A_{\rm in}$ to a smaller receiver <area> $A_{\rm out}$. Its geometric concentration ratio is $C=A_{\rm in}/A_{\rm out}$. Specify the admitted angular distribution and losses before interpreting $C$ as an <irradiance> gain: if a fraction $\eta$ of incident <radiant flux> reaches the receiver, the ratio of mean receiver to mean entrance <irradiance> is $\eta C$.
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