A narrowband luminescent coating for ultraviolet optical fibre sensors has been developed. A mixture of phosphor and epoxy is used as a coating for these sensors. The cladding and jacket are both removed from the fibre, exposing the core. The coating thus replaces the cladding of the fibre. The combined absorption spectrum of the phosphor and the transmission spectrum of the epoxy result in a narrow band of wavelengths being detected, in particular 254nm. The absorption of light from ultraviolet lamps incident on the phosphor is emitted as visible light at a higher wavelength. This paper describes the development and testing of this narrowband coating and its use in monitoring the output of UV lamps, to ensure the lamps are operating at the optimum germicidal output of 254nm.