Abstract: A photochromic material which undergoes a change in appearance when exposed to ultraviolet (UV) radiation in the UVC band provides a visual indication of the degree of sterilization achieved by a UV emitting device. The photochromic material is placed either on the UV source or in or adjacent the sample being sterilized. The photochromic material is formulated to undergo a certain degree of change in appearance when it has been exposed to a level of UV which corresponds to effective sterilization of the sample being sterilized. An optional reference card bears a color or opacity reference for comparison with the exposed photochromic material.