Method of and device for positioning an X-ray cassette

In apparatus for positioning an X-ray cassette in relation to an X-ray source that emits X-rays in a specific preferred direction (A), wherein another radiation source emits a secondary beam parallel to the preferred direction at a specific distance (d) therefrom, the improvement comprising position detecting means secured to the cassette for producing a signal indicating whether the angle of incidence of the secondary beam exceeds a specific threshold angle (.theta.) with respect to the normal to the cassette, said position detecting means including several adjacent light channels, each with a specific available cross-section and through each of which incident radiation can be conducted from an entrance to an exit,

• whereby the light can arrive at the exit only when the angle of incidence of the light does not exceed said specific threshold angle (.theta.).