I am trying to make a Point-In-Time simulation for a room with a light duct using the maximum number for -ab (which is 8 according to the manual ), as you proposed. Following are drawings and the model of the room and the duct.
Is it indeed possible to simulate something like this using only instruments available in the grasshopper plug-in, if not - what would be the easiest method using SDK?
Make sure that all your mirror surfaces are inward-facing, i.e., the surface normal should be pointing inwards. I flipped some surfaces which you can see in the attached file.
I changed the location in your file to Copenhagen since I am more familiar with the sun path there, and ran the simulation with a sunny CIE sky on March 21 12:00. I added sensor points just above the duct openings to the room. The image below shows the illuminance, and as you can see the new sensor points receive light.
When using mirror modifiers in Radiance you should select the dr parameter carefully. In my simulation I used the default parameters for detail level 1 but with increased ab (left image) - the default dr value is 1. If I add a custom dr value, in this case the same as ab, we can see that the sensors in the room receive light from the duct openings, more significantly for the larger opening in the back of the room.
This is one the applications where photon mapping is probably more suitable, and you might want to increase the parameters further than I did (detail level 2, and custom value for ab and dr).
