[9.5.7.3] The ratio of the flux directly received by surface 2 that is due to lambertian surface 1 to the total flux emitted by surface 1. It is used in flux transfer theory.

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

Also: the ratio of the average illuminance E on surface 1 to the causative exitance M of lambertian surface 2:

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

Note: In the literature, this term is also called angle factor, configuration factor, geometrical factor, I-factor, illumination factor, and shape modulus.

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