Abstract: A system for identifying and locating objects of known chemical composition and discriminating against a background having a different chemical composition comprises a source of radiation for illuminating the object and the background and a receiver responsive solely to the fluorescent radiation from the object. The receiver comprises a filter which passes radiation from the object and a photodetector array which receives the filter output and produces an output containing the coordinates of the location of the received signal or signals on the array. This location information is applied to utilization apparatus which may comprise a marking mechanism which marks the location of the object on the background or automatic tracking apparatus which tracks the object of interest or counter apparatus which identifies the quantity of such objects that are illuminated.The invention also comprehends the method of deriving position data by causing a known substance to fluoresce at wavelengths (centered at .lambda..sub.