Use of an airborne Fraunhofer line discriminator for the detection of solar stimulated luminescence

Abstract

Luminescence is the property of some materials to emit light when excited by external stimuli such as ultraviolet or visible light or by chemical or mechanical action. The Fraunhofer line discriminator (FLO) is an airborne electro-optical device which operates as a non-imaging radiometer and permits detection of solar stimulated luminescence several orders of magnitude below the intensity detectable with the human eye. A prototype FLO, used in 1969 to monitor the dynamic of water currents in the San Francisco area, was than 5 parts per Airborne tests o a redesigned FLO, featuring an order of magnitude improvement in sensitivity, permitted measurement of differences in the luminescence of trees containing geochemically anomalous concentrations of Colorado) and molybdenum (near Gardnerville, Nevada) trees growing in background areas nearby. In the tests near Denver, luminescence-contrast between stressed and non-stressed trees tended to be greater during periods when cloud cover was 10 percent or less; in most cases, contrast insignificant when cloud cover exceeded 10 percent. In other airborne test the FLO distinguished luminescing phosphate rock and gypsum from sands one and siltstone near Pine Mountain, California; dispersal of oil in a natural seep from uncontaminated sea water in the Santa Barbara Channel; cattle feed lot and municiple sewage effluents near Denver; and paper and phosphate processing effluents in eastern and central Florida. Procedures employing a laboratory fluorescence spectrometer permit prediction of the detectivity with an FLO prior to monitoring an airborne test. Luminescence spectra of the material are corrected for wavelength variation in spectrometer source, detector sensitivity, and solar illumination. By comparing these results with the luminescence of a rhodamine WT dye standard, the luminescence .of the material may be expressed in terms of rhodamine dye equivalency at the wavelength of several Fraunhofer lines. The FLO detectivity may be assessed at each Fraunhofer line, and the optimum line for field observation of the material may be selected. Future work will include the integration of the FLO with a line scan imaging system in order to assess the contribution of two-dimensional spatial resolution to the interpretability and usefulness of luminescence data. It should also include 1) investigation of luminescence polarization of some materials, particularly metal stressed plants, 2) an assessment of the use of pulsed lasers to stimulate phosphorescence decay time in the nanosecond and microsecond ranges; and 3) a study to determine the feasibility of conducting an FLO experiment from the Space Shuttle or other spacecraft.