Abstract: A sensing system for the detection of trace gases in the atmosphere or contaminates on a surface. Particular chemical substances are detected at remote points by placing a sensor unit in the vicinity of the area to be examined and illuminating the area with a laser beam generated by the sensor unit. An optical detector, also carried by the sensor unit, registers the fluorescence produced by the substance illuminated and relays this information by a telemetry link back to the base site. The utility of the system is broadened by providing a chemical reactant selected to react with the substance to be detected to produce a reaction product that fluoresces strongly at the wavelength of the laser light. The chemical reactant is carried by the sensor unit and is sprayed into the area to be examined.