I have a vray light material with a bitmap applied to a plane. I want to mask some of it using a black and white alpha map so that part of it is transparent. However the transparent area still retains some opacity. Even though the map is pure black, it never goes completely transparent in these areas.
Make the color of the VrayLightMtl black instead of the default white. That will make the alpha work as expected. This took me the longest time to figure out. Someone should add it to the help files.
You will have to mask the color texture separately, in addition to the opacity map. This is because a VRayLightMtl material might be perfectly transparent, but still emit light.