I would like to know how the relative illumination graph is drawn in Zemax.
If you can answer the following two questions during the process of drawing the RI graph, the rest of the process will be deducible.

1. What spec of Light Source does zemax use?
In general, renderer's light has various specs such as Infinity Area Light and Point Light.

2. Can you tell me how to get the energy of the final potons that reached the Image Plane of zmax?
For example, is it calculated as I (x,y,λ) at the bottom right on page 2 of the paper below?
https://web.stanford.edu/~jefarrel/Publications/2010s/2012AppliedOptics.pdf