Patent Number: 
Section: claims

1. An X-ray apparatus with an X-ray source for producing a beam of X-ray radiation, an X-ray detector to detect the X-ray radiation and a filter comprising filter elements containing an X-ray absorbing medium and positioned between the X-ray source and the X-ray detector to absorb the X-ray radiation, in which the X-ray absorption properties of filter elements are individually controllable by changing a length of a column of the X-ray absorbing medium in a propagation direction of the X-ray beam, electrical means being provided, comprising a voltage supply means for the filter elements and an electrical measuring means to measure a quantity which is related to the X-ray absorption properties of individual filter elements, wherein the filter elements are tube-like structures with an inner volume for containing the X-ray absorbing medium being comprised in a fluid filling having electrically conducting and X-ray absorbing properties, the X-ray absorption of each filter element depending on the length of the column of the X-ray absorbing fluid in the inner volume, each filter element comprising a first electrode for supplying a first electric potential to its wall and a second electrode for supplying a second electric potential to the electrically conducting fluid, the first electrode being separated from the electrically conducting fluid by means of an isolator layer so that an electric capacitance is defined between the first electrode and the electrically conducting fluid, the electrical measuring means being further embodied to measure the electric capacitance in individual filter elements. 2. An X-ray filter to be used in the X-ray apparatus of  claim 1 , wherein the electric measuring means comprises an electric circuit with an AC source for providing an AC current to the first and second electrodes of individual filter elements and an AC detector to measure an AC signal induced by said current within individual filter elements. claim 1 3. An X-ray filter as claimed in  claim 2 , wherein the electric measuring means comprise a further circuit element connected in series with the respective electric capacitance to form a voltage divider. claim 2 4. An X-ray filter as claimed in  claim 3 , wherein the further circuit element is an additional electric capacitance. claim 3 5. An X-ray filter as claimed in  claim 2 , wherein the voltage supply means comprise a high impedance for supplying the first electric potential to individual filter elements. claim 2 6. An X-ray filter as claimed in  claim 2 , wherein the first electrodes of individual filter elements comprise a stack of segments in the X-ray propagation direction, wherein the neighbouring segments are mutually electrically isolated, the first electric potential being switchable between the segments by the voltage supply means to alter the length of the column of the X-ray absorbing fluid in the X-ray propagation direction, the amplitude of the AC signal being a function of time, control means being provided for controlling the switching of the first electric potential between the segments in dependence of the behavior of the AC signal with time. claim 2 7. An X-ray filter as claimed in  claim 6 , wherein the control means are conceived to switch the first electric potential between the segments triggered by the AC signal reaching a saturation region. claim 6