A radiation sensitive sensor (1) which detects electromagnetic radiation within a narrow band of the electromagnetic spectrum using a single, fixed infrared detector (12) to cover a 360.degree. area in a plurality of segmented sectors obtained by rotation of a mirror (19) for each of the sectors with detection of the radiation from each sector providing an indication of the presence of a physical phenomena (50) such as a forest fire. The use of a single fixed detector and the mirror rotation allows for a solar powered unit which is able to be employed either singularly or in a system of grid locations to cover a wide sensor area in order to provide continuous operation and reliable alarm indications.