A film cassette comprises light blocking means supported for movement to a light blocking position for preventing ambient light from entering the cassette interior and to a non-blocking position. The light blocking means has a cavity for receiving a driver element in driving engagement to allow the driver element to move the light blocking means from its light blocking position to its non-blocking position. A locking pawl protrudes into the cavity to secure the light blocking means in its light blocking position and is adapted to be displaced from the cavity to release the light blocking means when the driver element is received in the cavity.