Luminescence is the emission of light from a solid which is 'excited' by some form of energy. The term
broadly includes the commonly-used categories of fluorescence and phosphorescence. Fluorescence is said
to occur where emission ceased almost immediately after withdrawal of the exciting source and where there
is no thermal cause, whereas in phosphorescence the emission decays for some time after removal of
excitation. The distinction between these so-called types of luminescence is somewhat arbitrary and
confusing; for example, many minerals have very long post-excitation decay times. Confusion is avoided by
using the term luminescence, and specifying the activating energy as a descriptive prefix. Thus
roentgenoluminescence is produced by X-rays, photoluminescence by light (e.g. ultra-violet) and
cathodolminescence(CL) results from excitation by electrons. Thermoluminescence results from heating.
Ultra-violet fluorescence microscopy is a well established technique for petrographic study of petroleum fluid
inclusions and often used in examination of hydrocarbon residues in sediments. The interpretation of
observed fluorescence intensities and colors is strongly influenced by the type of light source and filter
combinations in the microscope. Polished thin section surfaces are required, and a special microscope with
UV source and qaurtz lenses is needed, such as used for immunological work in many biological
laboratories. Various wavelenths of UV can be selected by means of filters, and filters can be interposed
when viewing the emission. Hydrocarbon inclusions show strong luminescence, the color varying with the
gravity of the oil. Recrystallized organic-rich fossils, such as renalcid micro-organisms in the reefs, may
show up very well under UV, whereas they may be invisible in transmitted light and CL. Davis &
Yurewicz(1985) have shown that in some limestones, cement generations and fine crystal growth zoning can
be revealed by UV. Certainly UV microscopy is attractive because it does not require elaborate vacuum
arrangements, but inorganic materials such as calcite often show only very weak UV luminescence, so UV
microscopy is not a general substitute for CL work.
Cathodoluminescence petrography is now a routine technique that can provide essential information on
provenance, growth fabrics, diagenetic textures and mineral zonation, in addition to enabling more precise
quantification of constituents and fabrics. Without the support of CL spectroscopy, however, CL petrography
can only remain a fabric analysis technique. Although subtle variations in CL color recorded on film give
important information, describing luminescence intensity and color from a photographic record is a dubious
and subjective affair. The actual CL color is determined by the number and type of emission and quenching
centers present. Superposition of several luminescence bands of different intensities can provide quantitative
dasta on the wavelength and intensity of luminescence and the nature of the luminescing centers. CL
spectroscopy should become a standard technique used by the luminescence petrographer becuase it is
the only means of recording CL colors and emission intensity objectively and quantitatively, in addition to
providing unique information on the nature of luminescence centers.
Fig. Schematic representation of the energies produced from elecron beam interaction with solid matter.
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