Certain substances, when reached by light with high energy and low wavelength, have the property to emit radiation with a wavelength greater than the exciting radiation.
The substances that behave in this way are said to have a primary fluorescence. Such are the organelles, the chromatophores in photosynthetic bacteria, cellulose, etc.
In substances that do not have this characteristic it is possible to induce a secondary fluorescence by colouring them with fluorescent substances called fluorochromes, which are organic compounds capable of reacting chemically with the substrate without altering and have an observable fluorescence in the specific localization sites even at a minimum concentration.

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