= View factor
{title2=$F_{i\to j}$}
{wiki}
The <view factor> $F_{i\to j}$ is the fraction of uniformly distributed, diffuse outgoing <radiant flux> from surface $i$ that directly reaches surface $j$. It depends on geometry, including obstruction, rather than on <temperature> or <emissivity>. All destinations in a closed enclosure have fractions summing to one. For a convex body fully enclosed by walls, $F_{\rm body\to walls}=1$; the reverse <view factor> is generally smaller. Reciprocity gives $A_iF_{i\to j}=A_jF_{j\to i}$. Two unobstructed spheres with outer <radius> ten times the inner <radius> have $F_{\rm inner\to outer}=1$ and $F_{\rm outer\to inner}=1/100$. Do not multiply both directions by an assumed identical <view factor>.<Thermodynamics and Propulsion>[§19.4, equations 19.4 and following reciprocity relation; §19.4.1, concentric spheres]{ref}
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