Application of ultraviolet reflectance and stimulated luminescence to the remote detection of natural materials

Abstract

Carbonate rocks and evaporite deposits commonly exhibit relatively high reflectance in the ultraviolet (<3600A), and some outcrops of these materials are selectively shown on ultraviolet imagery. Other materials which are commonly strongly imaged on ultraviolet imagery and photography include water, snow, concrete, and metallic objects, particularly aluminum. Outdoor tests of an active ultraviolet imaging system have demonstrated the feasibility of imaging ultraviolet stimulated luminescence of such minerals as talc, dolomite, and deweylite along quarry faces from distances of several hundred feet. The system features a cathode ray tube transmitter, an image dissector receiver, and a video monitor on which the distribution of luminescent material is imaged as the outcrop is scanned. A pulsed ultraviolet laser emitting at 3371A has been used successfully in the laboratory to stimulate phosphorescence of selected rock and mineral specimens and to discriminate between them on the basis of decay time which ranges from less than three to more than 10 microseconds. Initial results suggest that sodic feldspars have longer decay periods than calcic feldspars. Some granites also appear to have decay periods significantly different from other types of granite in the same locality. Outdoor tests with a high-resolution grating spectrometer were successful in detecting luminescent materials by means of the so-called "Fraunhofer line-depth method". This method is advantageous because it uses the sun so an ultraviolet source and therefore ifs independent of low-powered artificial sources such as cathode ray tubes, lasers, and mercury vapor lamps„ A Fraunhofer line- discriminator suitable for aircraft operation has been constructed, and initial on-the- ground tests are being performed on Rhodamine WT, a luminescent dye used by hydrologists and oceanographers in studies of current dynamics in streams and estuaries.