[] The ratio of illuminance on a surface at point 1 that is due to the flux directly received from lambertian surface 2 to the exitance of surface 2. It is used in flux transfer theory.

c_{1\rightarrow 2}=E _{1}/M _{2}

Also: the ratio of the differential flux directly received by surface 2 and due to element 1 to the total differential flux emitted by differential lambertian surface element 1:

c_{1\rightarrow 2}=(d\Phi _{1\rightarrow 2})/(d\Phi _{1})

 Note: In the literature, this term is also called angle factor, illumination factor, point configuration factor, and sky factor.

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