Patent Number: 
Section: claims

1. A displacement detecting element for detecting displacement in a leaf of a multi-leaf collimator, the multi-leaf collimator defining a shape contour of a collimated radiation beam, the displacement detecting element comprising: a voltage source;  a displacement dependent voltage pick-off; and  a resistive strip connected between said voltage source and said voltage pick-up, said resistive strip having a length which is greater than or equal to a maximum length of displacement of the leaf, wherein one of said pick-off and said strip is connected to the leaf and the other one of said pick-off and said strip is stationary, with both said strip and said pick-off being disposed outside of the collimated radiation beam. 2. The displacement detecting element of  claim 1 , wherein the element is directly connected to the leaf. claim 1 3. The displacement detecting element of  claim 1 , wherein said pick-off and said strip are disposed on a side of the leaf facing away from a radiation source. claim 1 4. The displacement detecting element of  claim 1 , wherein said pick-off and said strip are disposed in a rear region of said leaf which is not exposed to the collimated beam. claim 1 5. The displacement detecting element of  claim 4 , wherein said rear region is shielded by a pre-collimator. claim 4 6. The displacement detecting element of  claim 1 , wherein a linearity of said strip is adjusted through partial removal of material from said strip. claim 1 7. The displacement detecting element of  claim 1 , further comprising a housing which can be mounted to the leaf. claim 1 8. The displacement wherein detecting element of  claim 7 , wherein said housing is inserted into a recess in the leaf. claim 7 9. The displacement detecting element of  claim 8 , wherein said housing defines a housing recess which extends along a length thereof, and further comprising a fixed tongue displaceably disposed in said housing recess with an amount of displaceability which is at least a maximum length of leaf displacement, wherein one of said strip and said pick-off communicates with said housing and the other one of said pick-off and said strip communicates with said tongue. claim 8 10. The displacement detecting element of  claim 9 , wherein said strip is located in a gap between said housing and said tongue. claim 9 11. The displacement detecting element of  claim 9 , wherein said strip is disposed on said tongue and said voltage pick-off is disposed on said housing. claim 9 12. The displacement detecting element of  claim 11 , wherein said voltage pick-off is effected via a window in said housing. claim 11 13. The displacement detecting element of  claim 12 , wherein said housing and said tongue are formed as flat plastic parts. claim 12 14. The displacement detecting element of  claim 13 , wherein said strip is connected to said voltage source by connecting one terminal of said voltage source to one end of said strip with the other end of said strip communicating with a conductor path which is disposed parallel to said strip and which is connected to the other terminal of said voltage source. claim 13 15. The displacement detecting element of  claim 14 , wherein said strip and said conductor path are disposed on said tongue with said strip being formed as a first slide contact path, and further comprising a second slide contact path disposed parallel to said strip, wherein said voltage pick-off comprises two electrically connected wipers disposed on said housing which slide along first and second slide contact paths, and further comprising converting means connected between said second slide contact path and said voltage source for converting a signal into displacement information. claim 14 16. The displacement detecting element of  claim 15 , wherein said wipers are disposed on an outer side of said housing and slide on said first and said second slide contact paths by passing through said window of said housing. claim 15 17. The displacement detecting element of  claim 16 , wherein said means for converting said signal into displacement information is a voltage meter calibrated to a displacement to be detected, said voltage meter evaluating a voltage across a resistor communicating with a terminal of said voltage source. claim 16 18. The displacement detecting element of  claim 17 , wherein said housing has a shielding conducting layer on an outer side thereof. claim 17 19. The displacement detecting element of  claim 18 , wherein said housing has a dove-tailed outer contour for mounting to the leaf, wherein the leaf has a complementary recess for insertion therein. claim 18 20. A multi-leaf collimator comprising displacement detecting elements according to  claim 1 , wherein each leaf has at least one displacement detecting element and all displacement detecting elements can be connected to a control of the multi-leaf collimator. claim 1