Abstract: Apparatus for sensing occupancy of a room detects the infrared energy that is radiated by persons and may be used actuate any of a variety of devices such as electric lights, heating systems, air conditioners or alarms. Detection of persons throughout a broad area is made possible by a dome shaped lens assembly having an array of Fresnel lens segments which face in different directions and which focus intercepted infrared towards one or more infrared sensing components. Economical fabrication of the lens is realized by forming the Fresnel lens grooves in a sheet of material while it is in a flat condition and then sandwiching the material between nested dome shaped frameworks. In the preferred form, the sensor includes an adapter which can support different numbers of infrared sensors in any of a plurality of different orientations to accommodate to the configurations of different rooms.