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This document specifies a method for the measurement of effective focal spot dimensions below 0,1 mm of X-ray systems by means of the reconstruction technique, applied to digital images taken from hole type or disk type gauges. The imaging quality and the resolution of X-ray images depends highly on the characteristics of the effective focal spot, in particular the size and the two-dimensional intensity distribution as seen from the detector plane. This method can also be applied for the measurement of effective focal spot dimensions above or equal to 0,1 mm, but the application of the pin hole method of ISO 32543-1 is preferable. This document provides instructions for determining the effective size (dimensions) of mini and micro focal spots of industrial X-ray tubes for users in applications where the pin hole method of ISO 32543-1 is not practicable. This determination is based on the measurement of profiles of an image of a hole or disk type gauge and the reconstruction of the 2D distribution. The use of the pin hole method (ASTM E 1165, ISO 32543-1) requires the application of special pin hole cameras. This procedure describes, how the 2D spot shape and size of focal spots can be determined from hole or disk exposures, which are easier to produce in comparison to pin holes and the related cameras. Microfocus (1 µm spot sizes 0,1mm) and nanofocus tubes (spot sizes 1 µm) have a significantly lower photon flux than mini and macro focus tubes (spot sizes 0.1 mm). Furthermore, pin holes (see ISO 32543-1 or ASTM E 1165) with diameters in the nm and µm region are difficult to manufacture and will not permit a sufficient photon statistic for digital imaging with the required magnifications. Nano- and micrometre pin hole methods would require very long exposure times for obtaining a sufficient SNR and CNR.
In order to cover the full range of effective focal spot sizes, three different methods are described in EN 12543-2: 2021, EN 12543-4: 1999, and EN 12543-5: 1999, which are actively used. EN 12543-2 has already been adopted as ISO 32543-1. EN 12543.4 and EN 12543-5 are under preparation for parallel procedure as ISO 32543-2 and ISO 32543-3. The measurement method of ISO 32543-1 is intended for effective focal spot dimensions above 0,1 mm of X-ray systems up to and including 1000 kV tube voltage by means of the pinhole camera method with digital evaluation. The edge method of EN 12543- 4: 1999 (future ISO 32543-2) is intended for effective focal spot sizes of macrofocus tubes with spot sizes of Y> 0,5 and up to 500 kV. The edge measurement method of EN 12543-5 (future ISO 32543-3) is intended for measurement of effective focal spot sizes between 5µm and 300µm and mainly applicable for the use with microfocus tubes. During the last 10 years a new generation of X-ray tubes was developed with focal spot sizes 5 µm down to 200 nm. This range is not covered by any international standard. New test gauges and procedures were developed in an European project to cover this range with sufficient measurement methods. An international Round Robin Test was conducted to evaluate and verify the new gauges and procedures. The concept of these measurement procedures permits covering a focal spot in the range from 0,2 µm to 100 µm. Complementary to the draft of future ISO 32543-4 (edge method) this new draft was developed to extend the application range of the pin hole method (EN 12543: 2020) for focal spot sizes 100 µm. Instead of pin holes with a diameter 10 µm, which are difficult to manufacture, hole gauges were developed and tested together with a reconstruction algorithm, which converts the hole radiographs to an image, equivalent to a pin hole image, based on an algorithm similar to the reconstruction of a computed tomogram. The new test gauges and procedures were developed in an European project to cover the range 100 µm with sufficient measurement methods. An international Round Robin Test was conducted to evaluate and verify the new gauges and procedures. The concept of these measurement procedures permits covering a focal spot in the range from 0,1 µm to 100 µm. An Annex is added which describes the determination of the measured spot shapes, which is important for the performance of the X-ray tubes.
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