Abstract: An ultraviolet light sensitive emulsion material, which becomes transparent to the user only after the ultraviolet light sensitive emulsion material is exposed to a predetermined amount of ultraviolet light, to expose a colored backing surface to view. In one form of the invention, an ultraviolet light inhibiting material is disposed in covering or blocking relationship relative to the emulsion material arranged in a graduated pattern of varying opacity, for inhibiting passage of ultraviolet light selectively to the ultraviolet light sensitive emulsion material to enable the emulsion to be rendered transparent in a series of graduated indications when the device is exposed to progressively longer periods of time.