For luminous quantities, the units are lux.
For luminous quantities, the units are apostilbs.(a value of emittance or luminance in apostilbs is the figure in candelas per square metre multiplied by p). It is assumed that the emitting surface is a uniform diffuser.
The configuration factor describes flux transfer between an infinitesimal surface element ds1 and a finite area s2 on another surface.
The form factor is the fraction of radiation leaving one finite surface element s1 that reaches another finite element s2.
When the surface elements are of different areas, A1 and A2, then by reciprocity Fs1-s2A1 =Fs2-s1A2