Fluorescence is the property of certain substances which when illuminated by light of a specific wavelength will re-emit this light at a longer wavelength. In fluorescence microscopy, the exciting radiation is usually in the ultraviolet wavelength, approximately 360nm or, the blue region, approximately 400nm, although longer wavelengths can be used with some modern dyes. A substance which possesses a fluorophore will fluoresce naturally. This is known as primary fluorescence or autofluorescence. Ultraviolet excitation is required for optimum results with substances such as vitamin A, porphyrins and chlorophyll. Dyes, chemicals and certain antibiotics added to tissues produce secondary fluorescence of structures and are called fluorochromes. The majority of fluorochromes require only blue light excitation and this is the most common use of fluorescence in microscopy. Induced fluorescence is a term applied to substances such as catecholamines which, after treatment with formaldehyde vapor, are converted to fluorescent quinoline compounds.

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