A "fluorescent dye" can be defined as a dye that on excitation at particular wavelengths transits from a higher to a lower electronic state within a span of short duration, emits light.
Some of the important types of Fluorescent Dyes along with their chemical structures is shown below.
Unlike other traditional dyes a fluorescent dye not only absorbs wavelengths, but the resultant energy of this process gets dissipated in the form of yellow-green light emissions. Let's see the working process by taking the example of fluorescent yellow dye like Coumarin 7. Here the molecules of Coumarin 7 because of absorption of blue light appears yellow, consequently it also emits yellow-green light. This is depicted in the form of a graph:
This type of light that gives supplementary emission often results into an apparent impression that more yellow-green light is transmitted or reflected than what actually falls on the dye. Thus emanating the characteristic bright glow of fluorescence. The light emitted this way is of longer wavelength than the absorbed light.
Coumarin 7 is a special Fluorescent Dyes as their yellow colour because of absorption, is very well complemented by a yellowish-green fluorescence. This results in their specially brilliant yellow-green appearance.
Following features of Fluorescent Dyes makes them unique and productive in many respects:
Fluorescent dyes are used in a variety of applications. As penetrant liquids for crack detection, plastics, synthetic resins, printing inks, non-destructive testing (NDT), sports ball dyeing. A fluorescent dye that is particularly useful for textile applications are Merantine Brilliant Yellow 8G.