Figure 3 is a graphical representation of what happens when a fluorescing sample is observed with a fluorescence microscope. The filtered UV light illuminates the specimen, in this case a crystal of fluorspar, which emits fluorescent light when illuminated with ultraviolet light. Visible light emitted from the specimen, red in this case, is then filtered through a barrier filter that does not allow reflected UV light to pass. It should be noted that this is the only mode of microscopy in which the specimen, subsequent to excitation, gives off its own light. The emitted light re-radiates spherically in all directions, regardless of the direction of the exciting light.
Fluorescence microscopy is a rapidly expanding and invaluable tool of investigation.
Its advantages are based upon attributes not as readily available in other optical microscopy techniques. What is more, the fluorescence microscope can reveal the presence of fluorescing material with exquisite sensitivity. In a given sample, through the use of multiple staining, different probes will reveal the presence of different target molecules. Although the fluorescence microscope cannot provide spatial resolution below the diffraction limit of the respective objects, the presence of fluorescing molecules below such limits is made visible. Autofluorescence has been found useful in plant studies, coal petrography, sedimentary rock petrology, and in the semiconductor industry.