Multi-wavelength epi-illumination in fluorescence microscopy

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Fluorescence is a process where a substance after having absorbed light (photons) emitts a radiation the wavelength (colour) of which is longer than that of the absorbed light, and where this emission stops immediately after cessation of the excitation. This phenomenon is the basic element of fluorescence microscopy and its application.
Besides the "classical” excitation of fluorescence in a light microscope it is possible today to obtain the same emission effect via the modern technology of confocal laser scanning microscopy by an excitation with two or multiple photons having longer wavengths than those of the emitted ones. Fluorescence occurs either as autofluorescenc of biological and/or inorganic structures or as so called secondary fluorescence after a treatment of the specimen with special dyes (fluorochromes, fluorescent markers).