form factor, f_1→2

[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}=\left ( \phi _{1\rightarrow 2} \right )/\left ( \phi _{1} \right )

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.