The photo-chromic behavior of dyestuff is known as Photochromism.
The Photochromism is defined as the reversible transformation of a chemical species between two forms by the absorption of electromagnetic radiation, where the two forms have different absorption spectra.
Thus we can say that photo chromic dyestuffs are the substances, which demonstrate the phenomenon of reversible colour change, which are induced by the UV-Light. When sunlight or UV-Light falls on the surface the colour is revealed and the colour disappears when the light source is removed. The strength of the colour depends on the intensity of the light source.
Photochromism is the characteristic of huge number of organic and inorganic chemicals. In Photochromism based photochemical reaction, the reaction either occurs by a rearrangement of valence bonds or by a change in the configuration of the atoms in the molecules, which is also called as cis-trans Isomerism.
The photochromic colors are plastisol-based inks, which are generally off-white when not exposed to UV-lights, it gain color when exposed to Sunlight/UV radiation.
The active photochromic dye is contained in a micron-size microcapsules dispersed in water to make-up the pigment. The pigment so formed has consistency of thick cream. When such pigments are mixed with a binder like acrylic base, it can be applied to almost any surface. The Intensity for color change depends on the consistency of the pigment. The more diluted is the pigment, more less is the change in the color. So, the mixing is generally done on the trial & error bases.