Abstract: A device for detecting undesired, light-producing events in a protected installation, for example arcing of electrical apparatus or the outbreak of fire in a building, comprises a detector for receiving light produced directly or indirectly by the undesired event and directing at least part thereof into the field of view of fiber optic means which transmits at least part of the received light to a monitoring unit. The detector is in the form of material applied to an end of the fiber optic means situated at the protected installation, the detector material being in the form of at least one light-transmitting body with a large number of light-refracting surfaces within the same which refract received light and conduct at least part of the refracted light into the field of view of the fiber optic means.