Abstract: Closed loop fiber optic link means, transferring energy from a radiant source to a receiver, includes serially connected fiber optic links and detectors. Energy cessation causes alarm. Detectors include: (1) A pair of optical transducers oriented whereby light, air transmitted, is received with insignificant loss. In water, light received is so insignificant that energy to the receiver ceases. The first transducer provides a light beam within a constant index of refraction medium to the second transducer spaced and oriented to ordinarily receive the light beam. When light passes through media having different refraction indices at non-normal angles, light is refracted whereby receiver energy ceases. The transducers are oriented in a straight inclined path. (2) Optical fiber, stable to approximately 135.degree. F., which melts above approximately 155.degree. F., provides fire detection. (3) A bimetallic element attached to one of axially aligned optical fiber holders, when heated, mis-align the fibers.

Abstract: In a frost detector mounted proximate cooling fins of refrigeration apparatus, an emitter directs pulses of infrared radiation in the direction of the fins. When there is frost buildup on the fins, the frost reflects the radiation back towards a detector, the detector and emitter being mounted at a slight angle to each other.