ASTM E1165 is intended to measure the size of focal spots of industrial X-ray tubes with an estimated focal spot size larger than 50µm. It was first published in 1987 from ASTM Committee E-7 in Subcommittee E07.01. There was a major update in 2012 when
- the pin hole method was aligned to EN 12543-2 with a similar pinhole method but still film-based
- a new method for digital evaluation was introduced (ILP-Integrated Line Profiles)
- a table for focal spot classes introduced (Table 3)
- annex A1 with an alternate focal spot measurement method for end users is included that end users can measure the unsharpness due to the focal spot with a simple hole penetrameter
- a test report in accordance with ASTM E691 was done to give a Precision and Bias statement.
For taking the image a pinhole camera is required. There are strong requirements to the pinhole ...

... and it's sizes:

Up to a focal spot size of 300µm the smallest pinhole diameter of 10µm is required. From >300µm to 800µm a 30µm pinhole diameter shall be taken and for larger focal spots a 100µm pinhole is sufficient.
The next requirement is the minimum magnification; it is given in table 2 based on the Focus Detector Distance of 1m to

Up to a 2mm focal spot the magnification is 3 and the distance between focal spot and pinhole shall be 25cm. The 1m FDD shall be used if possible. Sometimes the cabinett where is tube is installed does not allow 1m distance between focal spot and detector; a footnote opens the door to smaller distances:
When using a technique that entails the use of enlargement factors and a 1 m focal spot to detector distance (FDD = m+n) is not possible, the distance between the focal spot and the pinhole (m) shall be adjusted to suit the actual focal spot to detector distance (FDD) used
(for example, if a 600 mm FDD is used, m shall be 150 mm for 3:1 enlargement, 300 mm for 1:1 enlargement, and the like).
For the digital image there are some more requirements:
(a) SNR in digital image: 50
(b) pixel value of focal spot maximum: >30% and <90% of max pixel value
(c) grey value resolution: 12 Bit and more (>4095 shades of grey)
(d) coverage of focal spot in digital image: more than 20 pixel (more than 40 pixel when using a 3x3 median filter for outliners)
(e) basic spatial resolution of the detector (SRb) shall be half of the pinhole diameter multiplied with the magnification v = 1+n/m
(f) maximum unsharpness due to the pinhole is
Ug = P(1+n/m) definitions of P, m and n in the tables above
With the requirements (d), (e), and (f) we get some interesting results from the physics concerning the required SRb of the detector and the deviation due to unsharpness of the system (pinhole, geometry, and detector)


An example for a 250µm focal spot size using the 10µm pinhole: The deviation due to the unsharpness of the system is 19µm which is 8%. This is a quite good value and within the tolerance of the focal spot fabrication process. The only limitation will be the requirement of 20µm for the spatial resolution of the detector: SRb<20µm. But already for the 320µm focal spot the 30µm pinhole shall be used; now the requirement to the SRb is much lower (1/2 of the Pinhole diameter) and with this value the deviation due to the unsharpness of the system increases to 15% (hint: all values above 10% are marked with red color in the table).
With smaller focal spots the higher requirements results from the 20 pixel on the focal spot; to fulfill this requirement for a 100µm focal spot a SRb of 12µm would be required - but there is no detector like this avaliable for reasonable money .
The requirement to the SRb of the detector is nearly double as high if you have to use a Median filter to eleminate outliners or peaks in the image.

When we spend some time and look at the formulas, we could see a way out of this limitations. The example of a 1mm focal spot with magnification 4 (= n/m+1) and a 30µm pinhole will help us to understand the limitations.

For a focal spot of Class FS7 (1000µm) a pinhole with 100µm diameter and a geometry with n/m=3 would be required. As this would lead to a deviation of 16% we decide to take a 30µm pinhole and get a projected unsharpness Ugp of the pinhole of
Ugp = P*v = P*(n/m+1)
of 120µm. The total projected unsharpness UT,proj is the square root of the quare values of projected unsharpness from the pinhole and spatial resolution of the detector:

which is 170µm, which we had to devide the factor n/m to get the unsharpness of the image UIm which is 57µm or 5.7%
For the focal spots around 1mm we decrease the deviation with the smaller pinhole. If we require the 30µm pinhole also for the 1000µm and 1270µm focal spots we could reduce the deviation below 10%.
But for smaller focal spots than 320µm there is no option for smaller pinholes as 10µm is the minimum which could be manufactured. On the other hand we know that there are no detectors with better SRb as 20µm. In the formular we could see that the influence of the detector resolution will be reduced with higher magnifications. If we double the factor n/m of 3:1 to 6:1 we could bring the 160µm focal spot into the 10% limit of deviation due to the system unsharpness. Additional we should improve the detector unsharpness to 25µm and we end up with a deviation of 14µm (9%).
This changes in the requirements to the different focal spot classes improve the accuracy of the pinhole method for several focal spot classes:

The yellow boxes show the changes to the table above.
As nearly nobody would buy a separate detector for each focal spot class a good standard detector could be selected. The Kowospot focal spot camera uses a DDA with 20µm pixel size and a basic spatial resolution of 25µm. If you always use this detector you can extend the range of focal spot classes with a deviation <10% to nearly all classes above FS16 (127µm) - if you use the 10µm pinhole for the classes FS11 and FS12, too.


There is one drawback of the larger magnification (n/m): You would need more space. Therefore the elements of 25cm can be reduced to 15cm each and now even a n/m = 6:1 focal spot camera would need just a little bit more than 1m (105cm) and should fit into the most cabinets.
If you do not like deviations of much more than 10% you should not use the pinhole method for focal spots of <100µm. Try the instead.
(Hint: EN12543-2 is intended for focal spots larger than 200µm )

Set-up before image capture using a digital detector
First you have to assure the conditions (b) to (d) above and additionally
(e) Calibration of pixel geometry with a precision of 2µm or 1% of pixelsize.
As it is difficult to put an object with known size of this precision into the beam (should be inside the focal spot camera ) , you should spend some effort on exact geometry of you system; most important is the distance from the focal spot to the surface of the pinhole. Most tubes have a colored dot at the position of the focal spot; verify the geometry with high precision and if you see a deviation from the 25cm (or 15cm in the compact version) take for calibration of the effective pixel size of the system the measured value.
E.g.: With a 20µm pixel size of the detector, m=15cm and n=45cm (mag=3:1) the effective pixel size of the system would be 20µm/3=6.67µm. If you measure instead of 15cm 14.5cm only, the magnificaion would be 3.103 and effective pixel size of the system would be reduced to 20µm/3.103=6.44µm what would lead to a difference of 3.3% in the result.

It is also very important that you position the focal spot camera in a way that the central axis of the beam is aligned directly to the pinhole. Note 2 of E1165 recommends to use a special collimator to ensure conformance even with +/-1° alignment tolerance. You shall have the maximum intensity in the center of your detector (if the detector is centered in the camera); small deviations should be corrected, otherwise an elliptical shape of the focal spot may occur.

The voltage of the tube shall be 75% of maximum but not more than 200kV. If the intensity on the detector is too high you may use prefilter with a homogeneous material like copper or brass. To avoid to much signal from the lower energies I recommend to use a 0.5mm brass filter in front of the detector for voltages up to 150kV and 1mm for higher voltages. Additionally the detectors are more sensitive to lower energies and most of the high energy quants will pass the detector without producing any signal and the filter will correct that a little bit.

If you have more than 40 pixel on the focal spot you are allowed to remove outliners by a 3x3 Median filter ...

... but due my experience there is not a big difference as the method of integrated line profiles is quite robust.

Image capture and evaluation of the images
Some focal spot cameras are equipped with a fixture for photostimulable luminescence storage phosphor imaging plates (commonly called IP’s) for the dental systems which mainly have the same size as the older films. The only way to controll the required PV is the exposure time - a few exposures for testing are required.
Using DDAs you see the image on the screen just after expousre. The reuqired PVs can also be set by the exposure time, e.g. a smaller pinhole requires a longer exposure time. To test it, an image with 1s exposure time can be done; the 90% of maximium PV of the detector is divided by the maximum PV of the 1s exposure and the intergration time is multiplied with this value. If the SNR does not reach the requires SNR=50, several image may be taken with same conditions and accumulated in a computer (hint: four accumulated images double the SNR value).
Be sure the offset value is substracted all the time and sometimes it is required to do a gain calibration. For details refer to and the topic about .

For evaluation a special software tool within the line profile function of the imaging software is required.

Two line profiles with about three times the width of the focal spot are drawn and the integrated signal in both directions is used for evaluation. E1165 offers to download the iSee software in a demo version for free from the BAM server but the link is no more valid; now the software can be downloaded with full functionality directly from - the only restriction is that the images can not be stored.
Before giving a "hands-on" with the ISee! software, the procedure shall be explained.
First calibrate the pixel size. Draw a line profile with about three times the width of the focal spot across the horizontal direction pinhole focal spot image:

Integrated (accumulate) the line profile signal along the profile:

with the result of an "S-curve". Set marker in the steep area – at 16% and 84% following the Klasens method:

The distance from 16% to 84% is 388µm; it has to be extrapolated to 100% by the factor of 1.47 Hint: 100%/(100-16-16)%=1.47:

The width of the focal spot is 571µm and the size in the pinhole image:

matches the calculated value quite well.

Now the same procedure has to be done for the vertical direction:

and here the calculated size is 546µm:

which matches also the pinhole image quite well:

The final result of the evaluation of this pinhole image give a size of 571µmx546µm:

That is the theory of the measurement method.

Here is the hands-on version with ISee!:
First thing to do is the calibration of the pixel size. This can be done with Image - Input Pixelsize and SRb... and in the bos the system pixel size shall be put. For the example this is 8µm or 0.008mm. A detector with 24µm pixel size was used at magnification of 3:1. Put in for SRb the same value.
With Mode - Profiler (or just F3) the line profile function will come up. Draw a horizontal line profile with a length of threetimes the size of the focal spot.

The line profile of a single line is shown in the graph.
Now some adaptions to the task have to be done. First increase the line width (1) to a value of about threetime the height of the focal spot - here 99 pixel (hint: use odd numbers for a central line). Then Detrend the line (2) and apply the integration function (3).
Finally switch from manual measurement to 16%-84% edge unsharpness (4). This will use the Klassens Method for evaluation as required for E1165 evaluation.

Obvious this was the horizontal size but the vertical size is required also. Draw a new line profile perpendicular to the one before. You have to adjust the width - here to 199 pixel. If you activate OSD you will see the results in the image.

For your report a screenshot could be done (CNTL Print). Additionally ISee offers to list the ROIs (and a profile is an ROI also).
Open the list of ROIs (third icon from the right) and now the results are shown here.

You should enter a description to document the direction of measurement. (Hint: This evaluation was done with a second image I did with same parameters - a small deviation is always possible but will not change the focal spot class.)
Last thing to do is to match the results with the focal spot classes in Table 3 of E1165:

and create a report as shown in Table 4 of E1165.
Measured width (X): 0.69mm
Measured length (Y): 0.3mm
Reported width (X): 0.8mm
Reported length (Y): 0.32mm
Focal Spot Class: FS 8

If you operate an YXLON Image 3500 system this functionality is available, too. The pictures in Fig. 2 of E1165 were done with an YXLON system and the steps you have to follow are the same as with ISee.
If you would like to get a graph as shown in Fig.2d you should start ImageJ with the focal spot image, klick Plugins - 3D - Interactive 3D Surface Plot and you will get a diagram like this:


In the left 3D diagram of the small focal spot you can see the side wings beside the central spot quite well. The right diagram of the large focal spot shows the two edges of the filament with higher intensity - if you rotate the graph to the best direction.

Difference to the evaluation of EN12543-2 (version from 2011)
From more than 200 focal spots the pinhole images were available and evaluated with both method - the 10% threshold of EN2543-2 and the ILP method of ASTM E1165 (and may be EN12543-2 2020 )
In the result the focal spot sizes are slightly smaller with ILP method than with EN12543-2:


For the Precision and BIAS statement in 9. ASTM requires an evaluation following E691 for repeatability. In E1165 chapter 9.1.1 the parameter of a round robin test are given for a standard tube with a focal spot size of about 550µm - similar to the tube in the beginning of this thread. The integrated line profile method was tested with more than 100 pin hole images (focal spot length) using 5 different CRs, different positions and evaluation at three different Labs and the consistency is given:

Agenda:
Images taken with MXR160HP11 at COMET in Flamatt, Switzerland; Evaluation at
YXLON CRx Software Evaluation with Image 3500 and IP x at YXLON, Hamburg
COMET CRx Software Evaluation with Matlab and IP x at COMET, Flamatt (CH)
BAM CRx Software Evaluation with ISEE visual and IP x at BAM, Berlin

Now as an optimized focal spot camera with a high resolution detector (DDA) is available, some issues had occured.
a) beside the focal spot signal there is more than 50% scatter radiation from the camera itself - using a standard camera designed for film or CR
b) there is a "hill" around the focal spot in the image, which makes is nearly impossible to perform a suitable background subtraction
c) with higher kV (>160kV) the evaluated focal spot increases with the energy (which is in conflict with the physics)
d) smaller focal spots (<200µm) are more or less independed to their own size with the standard technique (always about 120-200µm)

Also some observations could be noted:
A) The signal on the DDA does not increase with the focal spot size, it is vice versa - as the signal of the DDA is the dose per square mm and smaller focal spots tend to have a higher signal as the density of the dose is higher
B) The signal when using the smaller pinholes does not decrease as the smaller hole would indicate
C) Already with a single image the SNR of the focal spot camera is above the requirement of the standards when measuring a standard tube with a focal spot of 0.4 to 1mm. This allows to perform a focal spot measurement following the standards within a few seconds.

In the next postings I will share my experience with you.

Let us look at the first issue - the high scatter signal. This is an image using a 20µm DDA with CsI scintillator inside a standard focal spot camera designed for film or CR use. At the position of the film the DDA is mounted.

In the line profile can be seen that the base signal (signal beside the focal spot) is about 1900; the peak intensity value is less than 2600 - having 700 gray values for the focal spot only.
The first step to improve the situation is a collimator in front of the pinhole (as recommended in E1165); this reduces the scatter radiation down to 1200 gray values.

A much bigger effect is a shielding behind the pinhole to avoid any scatter from the walls of the camera itself. The benefit is a background signal level of about 120 gray values and the "peak to backgound" ratio is a dimension better with both improvements.
If everything is alined perfect, the improvement can be optimized. The following left graphic (images taken at 150kV) shows the mean signal beside the focal spot (scatter signal); on the left a standard camera, next column the standard camera equipped with a collimator, next column without collimator but the anti scatter behind the focal spot, and last column the mean signal with both optimations. The right graphic shows the achievable SNR based on the maximum signal of the focal spot devided by the Sigma (noise) beside the focal spot.

As larger the focal spot is, as more necessary the anti scatter protection will be to ensure that the SNR level is sufficient.
The background signal also influences the result of the focal spot evaluation, already with 150kV it is visible:

With higher energies the effect is much stronger (shown later) which means that the shielding is quite necessary for reliable results with a DDA inside the camera.
Beside a too large result value also the dynamic range of the DDA is limited with the high background signal from scattering; without there is more usable signal for the evaluation.

The picture shows left the optimised camera on the left with about 4300 levels of grey for the focal spot and on the right side the same camera without the scatter protection behind the pinhole where only 2800 levels of grey are usable for the evaluation.

The next topic is the plateau around the focal spot. It is concise with smaller pinholes and more intensive with higher energies.
Here an example using the 30µm pinhole and the 75% of 225kV from max. tube voltage (168kV). The plateau is slightly visible.

The standard allows or requires that the backgound is subtracted. The value for subtraction is a little bit unsharp, as the plateau shows gradients, and these are the possible regions where the background subtraction is performed:

and with the different box sizes a variance of about 50% of the focal spot value can be calculated with the 0.4mm focal spot of an HP11 tube from COMET.
Hint: First row shows the focal spot image left, in the middle the horizontal line profile and on the right the vertical line profile. Next row shows the possible sizes of the box for the background subtraction, in the middle the results using the different box sizes in horizontal direction and right in the vertical direction. Last row shows the resulting focal spots from the row before and the Integrated Line Profiles (ILP) for horizontal and vertical direction.
The evaluation was done by Laurin Mordhorst. Thanks a lot, Laurin!

As mentioned above the effect increases with higher energies; same situation but now 200kV was used (the max. voltage for tube with more than 225kV)

Important hint: For the visualization here a Gamma of 0.15 was used to make the plateau visible; in real applications you would hardly see it like this.


Using the line profile function it could be seen that the plateau value is 124 gray values only. But the influence to the evaluation following the standard is really big

A factor of four is possible depending on the size of the box for background subtraction.
An expert would see that one standard value would not solve the problem as the value depends on the physics of the camera, the noise in the image and the DDA correction.
A second approach was done from Laurin with a 3 percent noise based background subtraction. The results with the same 200kV image are much better

but still a variance of about 30% for the focal spot size can be noted.
As source of the plateau we could identify a penetration of the pinhole itself and the alignment with the collimator (the collimator hole diameter is 2.5mm and the 2.5mm are exposed at the pinhole giving a small signal - about 120 gray value out of 4500 [2.6%] at 200kV).
The penetration of the pinhole could not be avoided as the manufactoring is already at the limit (more details later) and the alignment to the 90° angle of the tube head is quite difficult, a movement of 20µm would cause another shape of the plateau.

The third topic is about the energy sensitivity of the results of focal spot measurements.
For the evaluation an optimized iterative background subtraction was applied. For the measurements a COMET HP11 tube with the large focal spot (LF) and small focal spot (SF) was used. The results can be transfered to different tubes.
Let us start with the large focal spot and it is obvious that the signal level goes down when the energy is reduced (ISO Watt) - but down to about 140kV the signal is quite okay. The focal spot width reduces with higher energies and the length is nearly constant.

With the small focal spot with it's unusual shape the results are similar, the signal level goes down but down to 140kV it is usable. Different to the large focal spot the width is nearly independend to the energy but the length is quite different - the side wings give a higher contribution to the signal with higher energies.


Here comes the idea to use a prefilter - which was recommended in E1165 for higher energies to reduce the signal or to avoid saturation. With the digital system we do not have an issue with saturation as we could decrease the exposure or integration time of the DDA. And the influence to the focal spot size is not really positive:

Not unexpected the signal level goes down - less than half of the signal with a 0.5mm Cu filter. This reduced the SNR of the camera, not a good effect. The difference in the result of the focal spot size is negligible in both directions (about 20µm at 500µm size).

The drop of signal is the same with the small focal spot, also the reduced SNR. The result of the measurement is a little bit better/constant without the filter, specifically at the focal spot length (where the wings contribute more to the result).

In the result we would not recommend to use a filter inside the camera.
If you have different results please share with us here in the forum. Thanks.

The last topic is about the limited minimum focal spot size when using the magnifications as required in Table 2 of E1165:


A tube was selected with focal spot sizes of 50µm, 130µm, and 200µm, measured with the edge method (user method of E1165) and additinally with the duplex wire. Here is the result with magnification 3 for this three foci. The normative value is displayed in transparent green.

It is strange that the results from the 130µm focal spot are larger than from the 200µm focal spot. Also the value for the 50µm focal spot is about 130µm. To verify the influence of too low magnification the measurement was repeated with double magnification (6) and now the results seem to be resonable down to 130µm.

Recently we did some measurements with (very) old double line focal spots. This type of focal spot was explained in a sketch in E1165/2001:

For this large focal spot with two strong lines the ILP Method shows some restrictions, the direction orthogonal to the lines are calculated to large and the other direction to small compared to the 10% film evaluation which gives 5.5mmx4mm:


The problem is not a problem of the Kowospot Software used for the image above, also ISee and the YXLON Image 3500 Software show the same problem:

We identified the problem occur as a results of the main areas of intensity are not mainly inside the 16%-84% area of the line profile - a high part of the total intensity comes from the edged, marked with blue arrows. The interpolation from the ILP Threshold 16% down to 0% and 84% up to 100% does only fit for focal spots where the major intensity is in the center.
The line profile of this double line focal spot shows the main intensities outside the (green) area of 16% to 84%:
.
To use the ILP function of the standards for this type of focal spots would require a different threshold to get the main intentity into the evaluation.
We did a test with the thresholds of 16/84%, 12/88%, 8/92%, and 4/96% - each set should give 100% in totoal. With this thresholds we run the ILP method and - of course - interpolated with the correct values to 100% focal spot value as given in E1165. And we evaluate in both directions.


With larger threshold range the results come closer to the 10% film evaluation values. The 4/96% thresholds show a good fit.
But this thresholds are not suitable to "normal" focal spots as they have longer lines at the end - which was the reason to take 16/84%.

To verify what would happen if you use different thresholds to a set of focal spots we did the same evaluation also with the small focal spot (also a double line style, but with less intensities in the two lines) and with a COMET 225kV HP11 tube, which is very common.
First the second focal spot of the traditional tube - left with the normative thresholds 16/84% and right with 4/96%:

Here it is clear that the normative thresholds are more suitable then the 4/96% - which should be as the main intensity is inside the 16% to 84% area:


It is the same with the HP11 focal spots:




In a graph it could be seen that only with the large double line focal spots the extended thresholds are suitable for width and length:

The graph shows the difference in mm from the result of the evaluation with the different thresholds to the nominativ value.
If we put the results in a second graph we see that the large double line focal spot is a outlier - if we would use for this type of focal spots the extended thresholds if would be more suitable.

Here the idea is, to take the minimum of the threshold results from 16/84% and 4/96% as result of the evaluation. Only for focal spots which have the high intensity outside the 16-84% area the extended thresholds of 4/96% would be used for the result. When taking the minimum, the minimum shall be calculated from the sum of width and height for each focal spot.

Before we change a standard we should look what else is in the standards. For smaller focal spots (5µm to 300µm) the uses the two edges from 90%->50% and 50% to 90% - which is in total 80% of the "photons".
When I got the hint I repeated the evaluation with the 10% to 90% threshold of ILP - but with NO interpolation to 100% (as requested). The "new" evaluation is the second column in the following graphs:

The width of the large double line focal spot is better as ILP but the deviation in height is even higher.
If I put the results in a column graph with % the difference is better visible:

As the ILP method does only "fail" with the large double line focal spot in width, the 10/90% solution "fails" with the small focal spot of this tube. The minimum method is best in width.
In height, the ILP method only "fails" again with the large double line focal spot; the 10/90% method "fails" with the small HP11 focal spot with the satellites and also with the large double line focal spot - even higher than ILP. Here again the minimum method is best in width.
From my point of view the minimum proposal still fits best and a good software should offer to use the minimum method or should offer the different threshold 4% - 96% for such double line focal spots.

Thanks Klaus for the details.

Could you share a few images of normal x-ray tube spot images... and images of what not normal!
I've seen customers who have melted the lead shielding on their x-ray tube due to poor maintenance (cooling not working = tube overheated), and are still using their x-ray tube with films.

Should we measure their focal spot(s), we would get what kind of images???

Eric,
normally the user of the Kowospot Camera measure the focal spots and keep their images. And the best address to ask, if a focal spot is "gone" is the manufacturer or supplier of the tube.
Nevertheless I had the opportunity to see some focals spots of good and not so good quality.
Let's start with the COMET HP11 with the large focal spot (1mm) - from 225kV and 320kV tubes:

Left is a quite new tube with a nearly perfect spot; the other two focal spots are already melted a bit or a bit more ...

and here is the small focal spot (COMET states 0.4mm) again from 225kV and 320kV tubes:

Left is the good one and on the right you could see that some tungsten is already melted and flown to the right.
Additional pictures of this type of tube can be found for the large and small focal spot in the postings above.

This 3mm focal spot is not as good as it could be - the right side of the target has much lower power than the left side


and this 6.3 mm focal spot is also not as good as it could be, the complete target is melted and you could see cloudy structures.

A quite good focal spot of this type could be seen in .

It would be nice if you or somebody else could present some more pictures of focal spots (good and "not as good") to help the community to see what is okay and what not.

I have some for review


#1 looks descent, can easily measure lines and distance between them


#2 faint lines, still able to measure, bottom line is half of the width of top tho, what's that means?


#3 lines are extremely faint, one goes at aslant? Hard to tell


#4 I truly do not know what happened here


#5 lines again are extremely faint/invisible, dark body showed next to focal spot


#6 similar situation as with photo number #4

Extra question:
What exactly are the consequences of using X-Ray tubes with bad focal spots?

Welcome in the forum, LoneWanderer.
I am sorry to tell you that I am not an expert of focal spots. In my previous life - when I worked for a leading X-Ray system house - I was a user of focal spots and only in the digital world.
May be here is an expert who knows more about film images of focal spots?
The degradation of the focal spot may have several consequences in images. Simplest the dose goes down - you could compensate with longer exposure times (film and CR). Then the focal spot may increase (due to lower dose from the center) which could create unsharper images. The influence of unsharpness in the image is explained . Sometimes some tungsten from the target spotters on the beryllium window and the focal spot is like a donut - showing some strange ring artifacts in the image. Of course there may be some more consequences ... .
For safety critical applications you should not use tubes with bad focal spots.