A lighting system distinguishes between light from natural and manmade sources to prevent the system's light source from being inadvertently deactivated due to light received from manmade light sources. A sensor converts received light into electrical energy. The system light source and/or some other external light source can emit light with a modulation. The system includes a modulation detector that can detect the modulation in the electrical energy received from the sensor. A system controller selectively adjusts the flow of electrical energy to the system light source responsive to the electrical signals from the sensor. The controller adjusts the flow of electrical energy in response to the non-modulated component of the electrical signals from the sensor. The controller does not adjust the flow of electrical energy in response to the modulated component of the electrical signals from the sensor.