extending vertically to the direction of displacement the permanent magnet together with the spring cam 21 can be displaced in the data illuminating device 17 by way of a drive 24 here illustrated schematically along a guide path 25 on the two guides 18 19 for the x ray film cassette 2 there is provided a sensing element 28 or 29 respectively coupled with a switch 26 or 27 for the complete insertion of the x ray film cassette 2 the switches 26 27 are mutually connected in series to the drive 24 for the permanent magnet 20 and the spring cam 21 an optical system for the data illumination not illustrated here is adjusted to the position of the window 3 of the x ray film cassette 2 when the cassette is fully inserted into the data illuminating device 17 on complete insertion of the x ray film cassette 2 into the illumination device 17 the window 3 is adjusted to the optical system of the data illumination device and the slide 5 closing the window is disposed below the permanent magnet 20 of the data illuminating device 17 such that the permanent magnet 20 is located exactly above the side of the counterpoise 14 dipping into the window opening of the cassette over 1 the side of.