Abstract: A continuously active artificial light source, such as a light-emitting diode (LED), is supported in a housing that also contains a photocell, and is directed at the operating detection surface of the photocell. A light-transmitting diffuser, such as made of translucent plastic, covers the detection surface of the photocell and allows ambient light to pass to the surface, which thereby detect a combination of external ambient light and light from the artificial source. The light source thereby biases the photocell, so that, in normal operation, the photocell produces an output voltage that does not exceed a predetermined value. The predetermined value is set at a level approximating the output voltage measured during total darkness outside the photocell housing. If an open-circuit condition occurs, the voltage of the photocell will exceed that predetermined value, thus permitting the checking circuit to recognize a faulty photocell.

Abstract: To suppress electromagnetic interference across a wide range of frequencies, in both magnetic and electrical fields, an emissive wire is surrounded by a ferrite core having an outer surface covered with a metal foil. The metal foil on the core or cores is grounded. If multiple cores are used in proximity, the foils of the several cores are electrically coupled together. In one embodiment, an omni-directional strobe light like the one shown in U.S. Pat. No. 5,155,666 is shielded by placing pairs of foil-covered ferrite cores around the wires at each apex of the polygon formed by the interconnected flash tubes, grounding the cores to the nearest lamp reflector, and grounding the reflectors together.