Abstract: A device for attachment to an X-ray film cassette to indicate whether or not the film has been exposed to X-radiation comprises a phosphor screen (12) which fluoresces on exposure to X-radiation. A photocell (14) detects the light emitted by the screen and generates a control signal which is used to alter the display status of a liquid crystal display (24) from a first condition which indicates that the film in the cassette has not been exposed to X-radiation to a second condition where it indicates that exposure has taken place. A latch circuit (20) is used to retain the altered display even after exposure to X-radiation ceases. The device therefore gives a clear indication of whether or not the film in the cassette has been exposed to X-radiation. The circuit is powered from a nickel cadmium cell (26), the power supply circuit of which incorporates a micro switch (28).