Stable UV indicator

A colourimetric indicator comprises an ultra-violet radiation-driven acid release agent; a pH indicator which displays altered colour between deprotonated and acid forms; a proton scavenging agent; and a UV stabiliser. In an aspect, the indicator also comprises a binder and a solvent or carrier. The acid release agent, e.g. diphenyl iodonium choride, may release a proton when exposed to UVA, UVB or UVC. The pH indicator may comprise a pH-sensitive dye, e.g. thymol blue. The proton-scavenging agent may comprise a base. The UV stabiliser may comprise a hindered amine light stabiliser, such as a sebacate diester or other derivative, especially bis(2,2,6,6-tetramethyl-4-(piperidine) sebacate, perhaps at a concentration of 0.05-0.1 w/v % or 0.14-0.71 wt% by solid content. The indicator may be in the form of an ink, and may be applied to a self-supporting substrate layer 10, to form a wearable article.