Abstract— A mid-UV dosimeter has been developed based on the irreversible photochemical conversion of 3′-[p-(dimethylamino)-phenyl] spiro-[fluorene–9,4′-oxazolidine]–2′,5′-dione (1) to p-dimethylamino-N-fluoren-9-ylidine aniline (2), carbon monoxide and carbon dioxide. Formulations of 1 in a polycarbonate film matrix are colorless, thermally stable, and undergo photolysis only when exposed to wavelengths of less than 350 nm. The red photoproduct 2 is both thermally and photochemically stable under dosimeter use conditions. The depth of red color developed depends solely on the degree of exposure to mid-UV radiation. In outdoor studies color intensity steadily developed during an initial 2 h of exposure to noontime Miami June sunshine. Color comparators representing consecutive 15–20 min exposure increments of this sunshine were readily distinguishable from each other by visual inspection. The comparators could be visually matched to colors generated by the dosimeter film formulation, and covered the exposure range of interest for avoidance of sunburn. This photochemical mid-UV dosimeter is seen to fulfill all the requirements of a practical sunburn dosimeter.