Abstract: A demand defrost system is provided for detecting the existence of frost build-up on the surface of the heat exchanger of a cooling system. A light beam is directed through the heat exchanger such that frost build-up blocks the path of the light from reaching a photocell. When the light received by the photocell falls below a selected value, a control signal is produced which discontinues cooling operation of the cooling system and initiates defrost heaters which heat the cooling coils and remove the frost build-up thereon. A temperature sensing switch is also provided which reinitiates the cooling cycle after a temperature indicates that frost has been removed from the cooling coils.