Line-of-sight projection
= Line-of-sight projection
The map from a three-dimensional astronomical distribution to its observed sky position or integrated surface signal. For a scalar volume emissivity $j(\mathbf x)$ in simple transparent geometry, <surface brightness> is proportional to $\int j\,dl$ along the ray. Reconstructing the spatial distribution requires additional geometry or symmetry assumptions; a projected surface distribution is not a unique three-dimensional <density>.