Apparatus for detecting the presence of a targeted group of hydrocarbons, such as diesel/fuel oil, lube oil, motor oil, hydraulic oil, jet fuel, mineral oil and crude oil in a highly reliable manner even though present at only extremely low concentration. A high power, pulsed light source is focused into a collimated beam that is reduced by a set of filters to a band of pulsed light within a precise set of wavelengths and directed vertically onto a target surface, such as a body of water. All but a precise band of light wavelengths returning to the apparatus are blocked so that substantially all light which then reaches an internal photodetector is within such precise band of wavelengths; as a result receipt of such light programmed intervals following such pulses is indicative of the presence of a member of the targeted hydrocarbon group.