When a molecule is exposed to light, some of it can be absorbed and one of the molecule's electrons are excited in the process. As the amount of light that a solution absorbs is proportional to the concentration of the molecule absorbing it, it is possible to use this technique to measure concentrations.
Fluorimetry is similar in spirit to the experiment described above, but much more sensitive. In an absorption experiment, a beam of light is passed through a sample and a relatively small fraction of that light is absorbed. That difference can then be related to the sample's concentration. In contrast, a fluorimeter shines a beam of light through the sample which will excite those molecules. When the excited electrons return to the ground state, they radiate light of the energy that they had absorbed. Thus, instead of measuring the fraction of light absorbed, a fluorimeter measures the amount of radiated light.
Perkin Elmer LS-50 Luminescence Spectrometer
Page last modified January 3, 2012