13.2.7 Final Illuminance

Determining the exitance The area density of the luminous flux leaving a surface. at each subsurface after all interreflections (called the final exitance The area density of the luminous flux leaving a surface.) involves solving a system of equations that contain the initial illuminances, diffuse Having hte properties of diffuse reflection or diffuse refraction. reflectances, and radiative exchange form factors. Details are in the IES Lighting Handbook, Chapter 10.

If any of the subsurfaces are diffusely transmissive, the system of equations is expanded to include the flux that back-to-back diffusely transmissive subsurfaces exchange with each other, modeling the flux that is transmitted through the original transmissive surface.

The system of equations is solved iteratively and the result is the final illuminance The area density of the luminous flux incident on a surface. on each subsurface, accounting for all interreflections. Multiplication by the reflectance The ratio of the flux actually reflected by a sample surface to that which would be reflected into the same reflected-beam geometry by an ideal, perfectly diffuse standard surface irradiated in exactly the same way as the sample. gives the final exitance The area density of the luminous flux leaving a surface..