How to create a illum material with zero radiance?

radiance-general
1

In the book of “Rendering with radiance”, a illum material with zero radiance was employed to leave the side surfaces out of the computation when modelling the radiance distribution of window with blinds using mkillum. However, how to create a illuminance material with zero radiance?
image

2

I think you just use a material like this:

void illum ind_blocker
0
0
3 0 0 0

Hope this helps.

-Greg