2023-05-12

ISO/DIS 32543-1:2023(E)

ISO TC 135/SC 5/

Secretariat: DIN

Non-destructive testing — Characteristics of focal spots in industrial X-ray systems —
Part 1: Pinhole camera radiographic method

© ISO 2023

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Contents

Foreword iv

Introduction v

1 Scope 1

2 Normative references 1

3 Terms and definitions 1

4 Test equipment 2

4.1 Essential characteristics of the pinhole 2

4.2 Alignment and position of the pinhole camera 3

4.3 Position of the radiographic image detector 4

4.4 Requirements on the radiographic image detector 5

4.5 Image processing equipment for digital images 6

4.6 Loading factors 6

5 Measurement and determination of the focal spot size 6

5.1 Measurement procedure 6

5.2 Measurement with digital technique (preferred method) 7

5.3 Evaluation with digital technique using Integrated Line Profiles (ILP) 8

5.4 Measurement of effective focal spot size visually using film radiographs 10

6 Classification and result of focal spot size measurement 10

Annex A (normative) Values for the classification of X-ray tube focal spot sizes 12

Bibliography 16

Foreword

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This document was prepared by Technical Committee CEN/TC 138, Non-destructive testing (as EN 12543-2) and was adopted, under a special “fast-track procedure”, by Technical Committee ISO/TC 135, Non-destructive testing, Subcommittee SC 5, Radiographic testing, in parallel with its approval by the ISO member bodies.

Any feedback or questions on this document should be directed to the user’s national standards body. A complete listing of these bodies can be found at www.iso.org/members.html.

Introduction

In order to cover the large range of effective focal spot sizes, different methods are described in, EN 12543-2, , EN 12543-4 and EN 12543-5.

The pinhole method (EN 12543-2 equal to ISO 32543-1) is intended for effective focal spot sizes above 0,1 mm and mainly used for sealed standard and mini focus tubes.

The edge method of EN 12543-4 is intended for field applications when the users have to observe the effective focal spot on a regular basis and the pinhole method is non-practical.

The edge measurement method of EN 12543-5 is intended for measurement of effective focal spot sizes between 5 µm and 300 µm and mainly for the use with µ-Focus tubes (up to 100 µm) and mini focus tubes with spot sizes of 100 µm to 300 µm.

In the overlapping ranges, the different standard parts provide comparable values within ± 20 % tolerance.

ASTM E1165 describes the same pinhole procedure.

Non-destructive testing — Characteristics of focal spots in industrial X-ray systems for use in non-destructive testing —
Part 1: Pinhole camera radiographic method

This document specifies a method for the measurement of 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 tube voltage applied for this measurement is restricted to 200 kV for visual film evaluation and may be selected higher than 200 kV if digital detectors are used.

The imaging quality and the resolution of X-ray images depend 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 compared to the others in the EN 12543 or ISO 32543 series allows to obtain an image of the focal spot and to see the state of it (e.g. cratering of the anode).

This test method provides instructions for determining the effective size (dimensions) of standard (macro focal spots) and mini focal spots of industrial X-ray tubes. This determination is based on the measurement of an image of a focal spot that has been radiographically recorded with a “pinhole” technique and evaluated with a digital method.

For the characterization of commercial X-ray tube types (i.e. for advertising or trade) it is advised that the specific FS (Focal spot) values of Annex A are used.

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 19232‑5, Non-destructive testing — Image quality of radiographs — Part 5: Determination of the image unsharpness and basic spatial resolution value using duplex wire-type image quality indicators

For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminology databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https://www.iso.org/obp

— IEC Electropedia: available at https://www.electropedia.org/

3.1

actual focal spot

X-ray emitting area of the anode as viewed from a position perpendicular to the anode surface

Note 1 to entry: The actual focal spot is also called thermal focal spot in other literature.

Note 2 to entry: See Figure 3, Key 7.

3.2

effective focal spot

X-rays emitting area of the anode as viewed from the image plane of the detector

Note 1 to entry: The effective focal spot is also called optical focal spot in other literature.

Note 2 to entry: See Figure 3, Key 4.

3.3

effective focal spot size

focal spot size measured in accordance with this document

3.4

nominal focal spot size

SS

characteristic value for X-ray tubes having measured spot sizes within a defined range

Note 1 to entry: Annex A, Table A.1, defines the ranges of measured spot sizes in reference to the nominal value SS for characterization of X-ray tubes

3.5

Focal spot class

FS

number used to classify X-ray tubes based on the nominal focal spot size

3.6

basic spatial resolution of a detector

SR_b detector

smallest degree of visible detail within a digital image, determined with the duplex wire image quality indicator (IQI) according to ISO 19232-5 located on the detector (magnification = 1), from the smallest number of the duplex wire pair with less than 20% modulation depth in a linearized profile and it corresponds to ½ of the detector unsharpness

The pinhole camera shall consist of a diaphragm with a pinhole having followed essential dimensions P and H according to Table 1 dependent from the effective focal spot size.

Table 1 — Dimensions of the pinhole

The essential dimensions P and H are shown in Figure 1.

Dimensions in millimetres

Key

Figure 1 — Essential dimensions of a pinhole diaphragm

The pinhole diaphragm shall be made of tungsten or of a similar absorbent material (e.g. gold, platinum, tantalum or related alloys).

The angle between the beam direction and the pinhole axis (see Figure 2) shall be smaller than ± 1,5 °. When deviating from Figure 2, the direction of the beam shall be indicated.

Key

Figure 2 — Alignment of the pinhole camera

The incident face of the pinhole diaphragm shall be placed at a distance m from the focal spot so that the variation of the magnification over the extension of the actual focal spot does not exceed ± 5 % in the beam direction. In no case shall this distance be less than 100 mm.

The radiographic image detector [film, imaging plate (CR) or digital detector array (DDA)] shall be placed normal to the beam direction at a distance n from the incident face of the pinhole diaphragm determined from the applicable magnification according to Figure 3 and Table 2.

Key

Figure 3 — Beam direction dimensions and planes

Table 2 — Magnification for focal spot pinhole images

Digital radiographic image detectors can be used instead of film, provided sensitivity, dynamic range and detector unsharpness allow capturing of the full spatial size of the focal spot image without detector saturation. The maximum allowed basic spatial resolution (SRbdetector) of the digital detector is determined from the pinhole diameter P and magnification n/m. It is calculated according to Formula (1).

(1)

The basic spatial resolution of the digital detector (SRbdetector) shall be determined with the duplex wire IQI according to ISO 19232‑5. For correct quantitative measurements the minimum projected length and width of the focal spot image should be covered always by at least the pixel number which is equivalent to 20 x SRbdetector. The signal-to-noise ratio of the focal spot image (ratio of the maximum intensity value inside the focal spot and the standard deviation of the background signal outside) should be at least 70. The maximum intensity inside the focal spot should be above 30 %, but lower than 90 % of the maximum linear detector output value. The grey value resolution of the detector shall be at least 12 bit.

Imaging plate systems (Computed Radiography, CR) or digital detector arrays (e.g. based on CCD-, amorphous-Si- or CMOS-detectors coupled to an X-ray fluorescence screen, or direct converting detectors) may be used as digital image detectors. The pixel values (grey values) shall be linear to the dose.

If radiographic film is used as image detector, it should meet the requirements of the film system class C 4 or better according to ISO 11699‑1 and shall be used without screens. The film shall be exposed to a maximum optical density between 1,5 and 2,5. The film shall be digitized with a maximum pixel size of 50 µm or a smaller size, which shall fulfil the requirements of the above described SRbdetector condition and be evaluated according to 5.3.

If the user has no digital equipment, the film may be evaluated visually; the procedure is described in 5.4. The visual evaluation of film radiographs will be less accurate than the evaluation of digital images with the profile function as described in 5.3.

This apparatus is used to capture the images and to measure the intensity profile of the focal spot in the projected image. The image shall be a positive image (more X-ray dose shows higher grey values) and linear proportional to the dose.

The equipment shall be able:

— to calibrate the pixel size with a precision of 2 µm or 1 % of the pixel size;

— to draw line profiles and average the line profiles over a preset area;

— to integrate line profiles by the length of the line profile;

— to subtract the baseline using a linear interpolation (straight line) of both ends of the line profile using at least the average of 10 % of the line profile as support on both ends; and

— to calculate the width or length of the focal spot in the image with two threshold values of 16 % and 84 % of the integrated line profile and extrapolate to 100 % (see Figure 5 and Figure 6).

When using CR technology or digitized film where outlier pixel may occur a median 3 x 3 filter should be applied.

The X-ray tube voltage shall be 75 % of the nominal tube voltage, but not more than 200 kV for evaluation with film. For evaluation with a DDA or CR the maximum voltage is limited by the condition that the background intensity is lower than the half of the maximum intensity inside the focal spot. The X-ray tube current shall be the maximum applicable tube current at the selected voltage. For measurements with more than 200 kV an optional copper pre-filter may be used to prevent saturation of the imaging device.

If possible, use a standard 1 m focal spot to detector distance (FDD = m + n) for all exposures. If the machine geometry or accessibility limitations will not permit the use of a 1 m FDD, use the maximum attainable FDD (in these instances adjust the relative distances between focal spot, pinhole, and detector accordingly to suit the image enlargement factors specified in Table 2). For small focal spots FDD may be larger than 1 m to meet the requirements in 4.4 and 5.2. The distance between the focal spot and the pinhole is based on the anticipated size of the focal spot being measured and the desired degree of image enlargement (see Figure 3). The specified focal spot to pinhole distance (m) for the different focal spot size ranges is provided in Table 2. Position the pinhole such that it is within ± 1,5 ° of the central axis of the X-ray beam.

NOTE The accuracy of the pinhole system is especially sensitive to the relative distances between (and alignment of) the focal spot, the pinhole, and the detector. Accordingly, a specially designed apparatus could be necessary in order to ensure compliance with the above requirements. ASTM E1165, Figure 6, provides an example of a special collimator that can be used to ensure conformance with ± 1 ° alignment tolerance.

Key

Figure 4 — Exposure set-up schematic and specification of focal spot directions

Position the detector as illustrated in Figure 4. When using film as detector, the exposure identification appearing on the film (by radiographic imaging) should be:

— X-ray machine identity (make and serial number);

— organization making the radiograph;

— energy (kV);

— tube current (mA); and

— date of exposure.

When the film is digitized or a digital detector is used, this information shall be stored within the image or file name.

The X-ray dose proportional grey scale image of the radiation detector shall be evaluated to measure the dimensions of the focal spot. The detector shall be exposed as given in 4.4. When using CR or film, the maximum pixel value or density shall be controlled by exposure time only. With a DDA the internal detector settings (frame time and/or sensitivity) shall be selected that the conditions of 4.4 are met.

NOTE The required signal-to-noise ratio(SNR) can be achieved with a DDA system by integration of frames with identical exposures (frame times) in the computer.

Before evaluation the image shall be inspected for spikes or outliners (CR and digitized film only). These artefacts shall be removed using a median 3 x 3 filter. In this case the size of the focal spot in the image shall be > 40 pixels in both orthogonal directions.

The images shall be stored with the nomenclature of 5.1 in 16 bit lossless image format, e.g. TIFF or DICONDE.

The pixel size in the image shall be calibrated by a known object size in the image like a “ruler” or by measured geometry with the precision of 1 % of the pixel size.

A line profile shall be drawn in length or width direction through the centre of the focal spot. The line profile shall be accumulated perpendicular to the profile direction over about 3 times the anticipated focal spot size (see Figure 5 and Figure 6). The line profile should have a length of at least 3 times the anticipated focal spot size. With this procedure an area of nine times the size of the anticipated focal spot size is used for evaluation. The baseline shall be subtracted using a linear interpolation (straight line) of both ends of the line profile, using at least the average of 10 % of the line profile as support on both ends.

Key

Figure 5 — Example for the Evaluation of effective Focal Spot Width with the ILP method

The line profile shall be integrated as shown in Figure 5a and Figure 5b). Then the points on the resulting curve (see Figure 5b) at which the curve has 16 % and 84 % of its maximum value shall be determined (Klasens’ method, Philips 1946, see Figure 5c). The distance between these points is extrapolated to the theoretical 0 % and 100 % values of the total focal spot intensity by a multiplication with 1,47. The result is the size of the focal spot in the direction of the integrated line profile.

NOTE By using the values of 16 % and 84 % instead of 0 % and 100 % the determined size is 32 % too small. The factor 1,47 = 100/(100–32) extrapolates this to 100 %.

Key

Figure 6 — Example for the evaluation of effective Focal Spot Length with the ILP method

This evaluation shall be repeated with the orthogonal direction (see Figure 6a and Figure 6b) where the direction, as shown in Figure 5, is vertical to the electron beam direction (focal spot width) and the direction, as shown in Figure 6, is parallel to the electron beam direction (focal spot length) (see Figure 4).

If radiographic film is used as an image detector and it can’t be digitized, it shall be evaluated visually using an illuminator with a uniform luminance of 2 000 cd/m2 to 3 000 cd/m2. The visual evaluation shall be carried out using an x5 to x10 magnifying glass, with a built-in reticule, with divisions of < 10 % of the magnified spot size but always with divisions ≤ 0,1 mm. The resulting focal spot shall be defined by the visible extent of the blackened area, divided by the selected magnification factor. An example is shown in Figure 7 after digitization. The visual evaluation of films is less accurate than the digital evaluation as described in 5.3 and should be used mainly for monitoring issues.

Key

Figure 7 — Example for visual evaluation of focal spot image on radiographic film

The focal spot shall be classified according to its measured size. The nominal values of the focal spot sizes and the dedicated classes are consistent with the wire pair stepping as described in ISO 19232‑5. The values for width and length shall be measured separately and the larger of both values shall be considered as measured focal spot size d. This focal spot size shall be used to assign the corresponding nominal focal spot size and focal spot class as shown in Table A.1. An example of a dual focal spot X-ray tube is given in Table A.2.

If measured with film, the report documenting for the focal spot size measurement, determination of the nominal spot size and class shall include the film system class, the X-ray tube name and serial number, the focal spot size(s) that was measured (some X-ray tubes have dual focal spots), the set-up and exposure parameters including kilo voltage, milliamps, magnification (n, m), pin hole diameter, date, name of operator and organization, and estimated beam time hours (if available). If the digitized film is evaluated, the film scanner type and setting shall be documented.

If measured with a digital detector, the report documenting for the focal spot size measurement, determination of the nominal spot size and class shall include the image name (see 5.1), detector model, used pixel size, measured SRbdetector and serial number, the X-ray tube name and serial number, the focal spot size(s) that was measured (some X-ray tubes have dual focal spots), the set-up and exposure parameters including kilo voltage, milliamps, magnification (n, m), pin hole diameter, date, name of operator and organization, and estimated beam time hours (if available).

A print of the focal spot image or the digital image file may be added to the report for information purposes.

A specification of the measured and nominal focal spot size or class of the X-ray tube shall refer to this document.

NOTE 1 If the tube axis is not defined, then the direction of the electron trajectory is used instead.

NOTE 2 ASTM E 1165 describes a test method for determination of focal spot sizes of X-ray tubes by the pinhole method, also. The ASTM

1. 
   (normative)
   
   Values for the classification of X-ray tube focal spot sizes

The larger of the sizes for length or width (l or w) shall be used as the “focal spot size” d and this focal spot size determines the nominal spot size and focal spot class.

Table A.1 — Nominal Values of Focal Spot Sizes d and Corresponding Classes

Table A.2 — Example of a Classification Result for a Measured Nominal Value of Focal
Spot Size d, Spot Length l, Spot Width w and Corresponding Class for an Example Tube
“Company XXR 225‑22”

Bibliography

[1] EN 12543‑4, Non-destructive testing – Characteristics of focal spots in industrial X-ray systems for use in non-destructive testing – Part 4: Edge method

[2] EN 12543‑5, Non-destructive testing – Characteristics of focal spots in industrial X-ray systems for use in non-destructive testing – Part 5: Measurement of the effective focal spot size of mini and micro focus X-ray tubes

[3] ASTM E1165, Standard Test Method for Measurement of Focal Spots of Industrial X-Ray Tubes by Pinhole Imaging

[4]  ISO 11699‑1, Non-destructive testing — Industrial radiographic film — Part 1: Classification of film systems for industrial radiography

[5] Klasens, H.A. Philips Research Report 1, 1946, p.241