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The Fraunhofer line discriminator Mark II (FLD-II) is an airborne photometric instrument for the remote measurement, on a precise and quantitative scale, of solar-stimulated luminescence. The luminescence may originate in such diverse sources as oil spills and chemical pollutants, the chlorophyll of normal or stressed vegetation, and fluorescent tracer dyes used to study current flow and dispersion in large bodies of water. Work now in progress by the U. S. Geological Survey expresses the concentrations of various fluorescent substances in terms of equivalent concentrations of the tracer dye, rhodamine WT, in water that the FLD-II can reliably measure down to levels of 0.1 part per billion (1 part in 1010). Detailed calculations show that this is within a factor of about 21/2 to 5 of theoretical, photon-limited performance. The instrument measures luminescence in a scene by quantitative distinction between reflected sunlight, which is "coded" by Fraunhofer absorption bands, and luminescence, which is not. Narrow-band optical filters provide in-band and out-of-band light samples from the scene and from pure sunlight to a photomultiplier tube. Digital conversion and computation render a reading of the fluorescence coefficient.