The complete title did not fit into the headline, here it is again:
EN 12543-5, Non-destructive testing - Characteristics of focal spots in industrial X-ray systems for use in nondestructive testing - Part 5: Measurement of the effective focal spot size of mini and micro focus X-ray tubes.
With mini and micro focus X-ray tubes is meant focal spot dimensions within the range of 5 μm to 300 μm of X-ray systems.
The voltage is limited to 225 kV - the newer 300kV tubes are not mentioned already, but a note opens the door for higher kVs with the hint for lower precision.
The object producing the edge is alternativly a cross wire or a ball consisting of highly absorbing material. The diameter has to be 1mm +/- 100µm. No prefilter is allowed. The magnification (m) shall be between 20 and 100.
Like in part 4 of the EN the edge of the wire/ball produces an unsharpness. To avoid the influence of penetration of the wire/ball only the upper half of the signal is evaluated (this is the first difference to part 4). The signal from 50% to 90% is taken on both sides and added:
and this is the second deviation to , where 5% to 95% shall be taken.
As it may be difficult to measure the distance "a", the magnification can be calculated from the known diameter of the wire/ball and the size on the detector. In this case the measurement with cross wires have to be repeated because of their different magnifications.
Two evaluations have to be done, for length and width. The larger of both values have to be taken as focal spot size.
At this point I want to express that the results are only valid for a dedicated setting of the µ-focus tube. A new filament or slightly rotated target may lead to different values and also the vacuum may influence the results. Last but not least the temperature of the tube head also may defokus the optical system inside the tube with the result of a larger and/or moving focal spot and different focal spot shape.
The edge method is restricted to focal spots larger than 5µm, in 2010 at the DGZfP Meeting in Wunstorf at GE it was shown that the deviations are too high when the focal spots are smaller. In the consequence a new method was discussed and - beginning with the JIMA masks - a line pair profile was discussed. But the restrictions with JIMA are, that the contrast is too low with higher energies and the outer lines are shown with lower modulation which reduces the amount of lines to be evaluated. Additionally the fabrication of high absorbing structure with a precision of <<0.1µm is difficult.
Since 2014 the EU project works on creating a new standard for µ- and nano-focus tubes.
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