Synopsis
Reverse Raytracing - what is it about?
Solution
"The concept of "Reverse Raytracing" is designed to make raytracing more efficient, that is, to allow TracePro to trace rays of interest and not consume resources tracing rays that do not contribute to the solution of the engineering/design problem.
One simple example of Reverse Raytracing would be in the case of a lamp emitting onto a planar target surface. While "conservation of flux" suggests that the lamp needs to emit rays into the entire 360 deg sphere, the only rays of interest are those that hit the planar surface. Reverse rays would "emit" from the planar surface back toward the source, and the flux of those rays would be calculated to account for the conservation of flux truly emitted from the lamp.
In the case above, let's say the planar surface is a 10x10 mm target. If we trace 100,000 Reverse Rays, that represents a sampling of the target surface of 1000 rays/mm^2. A further extension of the use of Reverse Raytracing comes from the symmetry of the simple model. We know that the irradiance will be greatest in the center of the target, and decrease gradually toward the 4 corners of the target. So is it absolutely necessary to fill the entire target surface with rays? Instead, you could create 2 1x1 mm objects, one at the center of the target surface and one at one corner. Now you can trace Reverse Rays from each of these. To achieve sampling of 1000 rays/mm^2, we would only need to trace 1000 reverse rays from each surface, for a total of 2000 rays. These targets would represent the max and min Irradiance of the original 10x10 mm Target Surface but at 1/50 the ray trace time."