After Cls make sure you aren't calling RenderWorld until you want the graphics to show up again. 3D graphics are rendered to the backbuffer whenever you call RenderWorld.
You shouldn't have to change anything about the lights if you Cls to erase the backbuffer and then don't call RenderWorld until you want graphics again.
You shouldn't have to change anything about the lights if you Cls to erase the backbuffer and then don't call RenderWorld until you want graphics again.