I am using automatic importance sampling and for some surfaces, the Prescription is being set to -1. Why does this happen?

Synopsis Auto Importance Sampling - Prescription is set to -1

Solution

The Auto Importance Sampling function assigns the Prescription numbers in the order that the ray intersects them during the Automatic Data Selection ray trace (Define/Apply Properties/Prescription). Surfaces that are assigned a Prescription number -1 were not intersected by the Automatic Data Selection ray trace and are not included in the Importance Sampling. I recommend that you go back to the Define/Apply Properties/Prescription dialogue box and make sure that the Origin and Direction Vector of the rays are set to pass through all of the elements of your model, and then repeat the Define/Auto Importance Sampling step.