The Principles of Fluorescence - Aquatic Organic Matter Fluorescence - page 3

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The Principles of Fluorescence
Darren M. Reynolds
1.1 Luminescence
Although this chapter is concerned primarily with the fluorescence process it is important
to understand that fluorescence is one of several phenomena that give rise to the collective
term luminescence . Broadly speaking, luminescence is defined as the emission of light by
a substance, where the emitted light cannot be attributed to incandescence, that is, thermal
radiation. In the case of fluorescence, light is emitted from a substance after it has been
irradiated, usually with visible or ultraviolet light. There are other important kinds of lumi-
nescences and some of these are described in the following paragraphs.
Phosphorescence is a light emission that often lasts minutes or sometimes hours after
irradiation. The slow release of light characterized by phosphorescence is the result of the
storage of energy in metastable states and is often thermally activated. In this instance
“metastable” refers to the local stability of an energy state with respect to surrounding
systems that exhibit a different energy state. Photoluminescence is a more general term that
encompasses both fluorescence and phosphorescence.
Chemiluminescence is light emitted during cold chemical reactions whereas biolumines-
cence is essentially chemiluminescence from living organisms. Thermoluminescence is a
type of phosphorescence but one that occurs at elevated temperatures. Thermoluminescence
is not related to incandescence, as thermal excitation is used to initiate the release of energy
from another source.
Electroluminescence is light emission initiated by electric influences. For example, in
cathodoluminescence, the emission of light is initiated by excitation with an electron beam.
Radioluminescence is caused by excitation with nuclear radiation or X-rays, whereas tribo-
luminescence occurs when certain materials are mechanically altered, such as when frac-
tured or polished.
Although several investigators reported luminescence phenomena during the 17th and
18th centuries, it was British scientist Sir George G. Stokes ( 1852 ) who first described “fluo-
rescence” in his reference to the light-emitting properties of the mineral fluorite (fluorspar).
It was not until the discovery of the electron by Sir Joseph John Thomson ( 1897a , b ) and the
discovery of the quantized nature of matter by Planck ( 1900 , 1902) and light by Einstein
( 1905 ) that our comprehension of matter and energy was spectacularly revolutionized, and
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