A marker is arranged to indicate the presence of ultraviolet radiation. The marker includes at least an emitting layer having a fluorescent material arranged to emit visible light in a first range of wavelengths in response to excitation by ultraviolet radiation in a second range of wavelengths. The emission of visible light is reversible, and the marker is reusable. The marker may be a warning sign, and may be adhered to a surface or incorporated into a freestanding portable apparatus.