Organic substance that fluoresces in the visible light range (excitation wavelength 490 nm, fluorescence wavelength 515 nm).
The exceptional strength of its fluorescence makes it possible to detect the tiniest concentrations down to the ppb range. Fluorescein has long been used in hydrogeology for studying watercourses. When it is added at a certain point of a river or stream, the flow can be observed over long distances.
Fluorescein is also used in industry as tracer of substances that are difficult to detect. For example, adding a small amount of fluorescein to the coolant glycol permits localization of leaks in refrigeration circuits.