Photochromics (PCs) change from a near Invisible state when not exposed to UV Light (e.g. indoors) to a coloured state when exposed to UV Light (e.g. outdoors). They have a Fast Reversible Transition with Intense colour in 15 seconds returning to clear after 5 minutes indoors.
Specifically, PCs change colour in response to ultraviolet (UV) light, usually from the sun or a black light. Photochromic behaviour can be either Reversible or Irreversible. (Technically all photochromism is defined as reversible but, for simplicity, we will use the term irreversible photochromic to describe irreversible a photochemical reaction that yields a visible colour change.)