The small area *d1 *is parallel to rectangle
*2* and lies on a line perpendicular to one corner of the rectangle. The
configuration factor can be used to compute the illuminance at a point on a
surface facing a rectangular source or the mean illuminance from a small area
source onto a rectangle.

*Input*- Parameters
*X*&*Y*where

- as in diagram
*Equation**Note*- Angles are expressed in radians
*Source*- Siegel(1).
**References**- 1. Siegel R and Howell J R
*Thermal Radiation Heat Transfer*(New York: McGraw Hill) (1972)

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