Patent Number: 
Section: claims

1. An X-ray apparatus which includes an X-ray source ( 1 ) for producing X-rays ( 2 ), an X-ray detector ( 4 ) for detecting the X-rays, and a filter ( 14 ) which is arranged between the X-ray source and the X-ray detector and includes a plurality of tubular filter elements ( 13 ) having a longitudinal direction z and a circumference, wherein each filter element has an internal volume ( 21 ) for receiving a liquid filling ( 22 ) which contains at least one electrically conductive and one X-ray absorbing liquid component, the X-ray absorptivity of said filter element being dependent on the quantity of X-ray absorbing liquid component present in the internal volume ( 21 ),  each filter element is provided with a first electrode ( 23 ) for applying a first electric voltage to a wall ( 28 ) of the filter element and a second electrode ( 29 ) for applying a second electric voltage to the internal volume ( 21 ) of the filter element,  the first electrode is electrically isolated from the internal volume ( 21 ) of the filter element by means of an isolator layer ( 34 ) in such a manner that an electric capacitance per unit of surface area of the filter element exists between the first electrode ( 23 ) and the electrically conductive liquid component when a quantity of the electrically conductive liquid component is present in at least a part of the internal volume of the filter element ( 13 ),  the X-ray absorptivity of each filter element ( 13 ) is adjustable by step-wise control of a surface level of the X-ray absorbing liquid component in the longitudinal direction z of each filter element,  characterized in that the electric capacitance per unit of surface area of the wall ( 28 ) of the filter element ( 13 ) varies substantially in the longitudinal direction z of the filter element. 2. An X-ray apparatus as claimed in  claim 1 , wherein the first electrode ( 23 ) includes a number of electrically interconnected first and second electrode segments, each of which extends at least over a part of the circumference of the tubular filter element ( 13 ), the first and the second electrode segments being arranged so as to succeed one another in the longitudinal direction z of the filter element and that the first electrode segment ( 37 ) extends over a larger part of the circumference of the filter element in comparison with the second electrode segment ( 39 ). claim 1 3. An X-ray apparatus as claimed in  claim 1 , wherein the isolator layer ( 134 ) includes a number of first and second isolator segments, the first and second isolator segments succeeding one another in the longitudinal direction z of the filter element ( 13 ), the first isolator segment ( 136 ) having a dielectric constant which is higher than that of the second isolator segment ( 138 ). claim 1 4. An X-ray apparatus as claimed in  claim 1 , wherein the isolator layer ( 234 ) includes a number of first and second isolator layer segments, the first and second isolator layer segments succeeding one another in the longitudinal direction z of the filter element ( 13 ) and the first isolator layer segment ( 236 ) having a layer thickness which is larger than that of the second isolator layer segment ( 238 ). claim 1