Abstract: A photographic film cartridge has a word imprinted on the exterior surface of the cartridge visually indicating exposure status of film in the cartridge. A thermally responsive coating of a phase change medium overlies a portion of the word such that when the medium is rendered opaque, the word meaning is changed to indicate that the film is in an exposed state. The camera is provided with a heat generating element that is activated when images are exposed onto the film to render the phase change medium opaque. The camera is also provided with an opto-sensor and control means responsive to the phase change medium to allow loading of the film when the medium is in its unchanged condition and to prevent loading of the film when the phase change medium is changed to indicate exposed film in the cartridge.