I have a little problem with my area lights being parallel to the floor of my bathroom. It casts an annoying light rim on the wall, where the light would emit 90° offset to the lights plane’s normal. But i don’t find a solution myself nor on the web.
It looks like artifacts or two planes lying ontop of each other but i proved it is being caused by the arealight. If you move the camera closer it is easier to perceive as a light rim. Is this the wrong way how to use an area light? Or is there maybe a little option i oversaw, where you can change the angle of the area light ( like the spot light).?
I use area lights as well but for big indirect lights (like your full ceiling/floor in this case), and I have the same issue. Upcoming version will have UV coords for area lights, so that might help by making edges less bright. Texturing the walls can also help mask (not fix) the problem. If everything fails you can choose to use a textured plane emitter instead but that may take longer to clean up.
I only use this technique to speed up rendering, not to make it more realistic. For simulating bounced cove lighting (direct only for the actual lightsource) you may also want to angle the plane light, which can make it look more weird (but still worth it in my view).
