For example, let's say you owned a tourist attraction that was a cavern that lead to the outside world by a tunnel. Tourists would typically enter the cave via this tunnel. To prevent vandalism, theft, and for general safety you decide to install a digital video camera surveillance monitoring system. The entrance camera is outside, directly across from the tunnel entrance. Standing in front of the tunnel, the sunlight on this bright sunny day is just about directly overhead such that the only outside light that enters the tunnel is diffuse light and the lighting system of the tunnel itself.