# Configuration factor and form factor definitions -- Algorithm 3.11

## a. Flux density and emittance

Flux density:

For luminous quantities, the units are lux.

Emittance:

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.

## b. Configuration factor

The configuration factor describes flux transfer between an infinitesimal
surface element *ds1* and a finite area *s2
*on another surface.

*cfds1-s2* =

=

=

## c. Form factor

The form factor is the fraction of radiation leaving one finite surface
element *s1* that reaches another finite element *s2.*

*Fs1-s2* =

=

=

When the surface elements are of different areas,* A1* and *A2,*
then by reciprocity *Fs1-s2A1 *=*Fs2-s1A2*

*Source*
- Bellchambers
*et al.*(1) (symbols changed).
**References**
- 1. Bellchambers H E, Petherbridge P and Phillips R O Nomenclature and
symbols associated with radiation transfer calculations
*Transactions of the
Illuminating Society (London) ***26** (3) 136-142 (1961)

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