Academic Phoenix Plus share some insights into modifying Arnold lights by using a light blocker.
Monica from Academic Phoenix Plus is covering all the light sources in Arnold along with some tips and tricks for using them. Her latest tutorial covers working with an Arnold lighting feature called light blockers. You can use this feature as a way to mask out the effects of a light in a scene without the overhead of adding additional geometry.
You can find the Blockers in the light filter section of the attribute editor. Monica’s tutorial shows how to access the blockers and how they might be used in a scene.