1、Designation: E2903 13Standard Test Method forMeasurement of the Effective Focal Spot Size of Mini andMicro Focus X-ray Tubes1This standard is issued under the fixed designation E2903; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision,
2、 the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 The image quality and the resolution of X-ray imageshighly depend on the characteristics of the focal spo
3、t. Theimaging qualities of the focal spot are based on its twodimensional intensity distribution as seen from the imagingplace.1.2 This test method provides instructions for determiningthe effecting size (dimensions) of mini and micro focal spots ofindustrial X-ray tubes. It is based on the European
4、 standard,EN 125435, Non-destructive testing - Characteristics of focalspots in industrial X-ray systems for use in non-destructivetesting - Part 5: Measurement of the effective focal spot size ofmini and micro focus X-ray tubes.1.3 This standard specifies a method for the measurement ofeffective fo
5、cal spot dimensions from 5 up to 300 m of X-raysystems up to and including 225 kV tube voltage, by means ofradiographs of edges. Larger focal spots should be measuredusing Test Method E1165 Standard Test Method for Measure-ment of Focal Spots of Industrial X-Ray Tubes by PinholeImaging.1.4 The same
6、procedure can be used at higher kilovoltagesby agreement, but the accuracy of the measurement may bepoorer.1.5 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.6 This standard does not purport to address all of thesafety concer
7、ns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E1165 Test Method for Measurement of Foca
8、l Spots ofIndustrial X-Ray Tubes by Pinhole ImagingE1255 Practice for RadioscopyE1742 Practice for Radiographic ExaminationE1815 Test Method for Classification of Film Systems forIndustrial RadiographyE2002 Practice for Determining Total Image Unsharpness inRadiologyE2033 Practice for Computed Radio
9、logy (PhotostimulableLuminescence Method)E2446 Practice for Classification of Computed RadiologySystemsE2597 Practice for Manufacturing Characterization of Digi-tal Detector ArraysE2698 Practice for Radiological Examination Using DigitalDetector Arrays2.2 European Standards:3EN 125435 Non-destructiv
10、e testingCharacteristics of fo-cal spots in industrial X-ray systems for use in non-destructive testing - Part 5: Measurement of the effectivefocal spot size of mini and micro focus X-ray tubes3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 actual focal spotthe X-ray producing
11、 area of thetarget as viewed from a position perpendicular to the targetsurface.3.1.2 effective focal spotthe X-ray producing area of thetarget as viewed from the image plane.1This test method is under the jurisdiction of ASTM Committee E07 onNondestructive Testing and is the direct responsibility o
12、f Subcommittee E07.01 onRadiology (X and Gamma) Method.Current edition approved June 1, 2013. Published June 2013. DOI: 10.1520/E290313.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume in
13、formation, refer to the standards Document Summary page onthe ASTM website.3Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036, http:/www.ansi.org.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959
14、. United States14. Summary of Test Method4.1 This method is based on indirect measurement of thefocal spot size by measuring the geometric unsharpness thenusing a geometric calculation to determine the effective focalspot dimensions (see Section 8). For this purpose, edges areimaged either on a film
15、 or by means of a radioscopic or digitalradiographic device using a relatively high geometric magni-fication. For a full description see reference below.45. Significance and Use5.1 One of the factors affecting the image quality of aradiographic image is geometric unsharpness. The degree ofgeometric
16、unsharpness is dependent upon the focal spot size ofthe radiation source, the distance between the source and theobject to be radiographed, the distance between the object to beradiographed and the image plane (film, imaging plate, DigitalDetector Array (DDA), or radioscopic detector). This testmeth
17、od allows the user to determine the effective focal spotsize (dimensions) of the X-ray source. This result can then beused to establish source to object and object to image detectordistances appropriate for maintaining the desired degree ofgeometric unsharpness or maximum magnification possible, orb
18、oth, for a given radiographic imaging application. The accu-racy of this method is dependent upon the spatial resolution ofthe imaging system, magnification, and signal-to-noise of theresultant images.6. Apparatus6.1 The following equipment is required for the measure-ment if using X-ray film:6.1.1
19、A test object as described in 6.5.6.1.2 X-ray films, without screens, of sufficient size toimage magnified test object and region around test object toobtain a profile as shown in Fig. 1.6.1.3 Film cassettes made of low absorbing material (forexample polyethylene).6.1.4 A film holder.6.1.5 A film pr
20、ocessing unit.6.1.6 A film scanner capable of reading densities greaterthan 3.0 configured such that the pixel size is appropriate forthe measurement (refer to Section 7). The film shall be ofsufficient size to image the magnified test object and regionaround test object to obtain a profile as shown
21、 in Fig. 1.6.1.7 The film system shall meet the requirements of TestMethod E1815 film system class I, II, or Special.6.2 The following equipment is required for the measure-ment if using computed radiography (CR):6.2.1 A test object as described in 6.5.4Fry, Ewert, Gollwitzer, Neuser, and Selling, “
22、Measuring microfocal spots usingdigital radiography”Materials Evaluation, Vol 70, No.8, August 2012, p. 981.FIG. 1 Profile of Test Object Image (Test Object: Pt wire 1 mm)E2903 1326.2.2 A computed radiography system, consisting of animaging plate and scanner, configured such that the pixel sizeis ap
23、propriate for the measurement (refer to Section 7). Theimaging plate shall be of sufficient size to image test object andregion around test object to obtain a profile as shown in Fig. 1.6.2.3 The computed radiography system shall meet therequirements of Practice E2446 class I, II, or Special, andima
24、ge plates shall be packed in low absorption cassettes usingno screens.6.3 The following equipment is required for the measure-ment if using a radioscopic or digital detector array device:6.3.1 A test object as described in 6.5.6.3.2 A radioscopic device, for example any image intensi-fier with video
25、 equipment or Digital DetectorArray, configuredsuch that the pixel size is appropriate for the measurement(refer to Section 7), or6.3.3 A Digital Detector Array system meeting the require-ments of Practice E2597. The digital Detector Array covershould be constructed of low X-ray absorption material
26、andshould be free of cluster kernel pixels as defined in PracticeE2597.6.3.4 The imaging area shall be of sufficient size to imagemagnified test object and region around test object to obtain aprofile as shown in Fig. 1.6.4 Image processing equipment as follows:6.4.1 An image processing device with
27、the capability ofproducing linearized intensity profiles (signal is linear withdose), integration of profiles, and profile plots within the digitalimage in two directions perpendicular to each other, and withthe capability to measure distances.6.5 The test object shall be either a set of wires or a
28、sphereconsisting of highly absorbing material (for example tungsten,tungsten alloy, or platinum). The diameter of the wire or sphereshould be greater than 20 times the focal spot dimension if thefocal spot is less than 40 m to minimize edge penetration;otherwise, the diameter should be 0.8 to 1.0 mm
29、 for focal spotsgreater than 40 m. The diameter shall be known to within61%.6.5.1 In case of using two single crossed wires they shallcross each other at an angle of 90 6 3. The wires shall bemounted across a circular aperture in a stable frame, in such amanner that the crossing point is located in
30、the center of theaperture. In case of using the sphere it shall be mounted on athin polyethylene support or placed into a thin polyethyleneenvelope.6.5.2 The Unsharpness Gauge of Practice E2002 is recom-mended as a test object of defined accuracy. Two exposuresshall be taken with the wire set in per
31、pendicular directions toobtain the length and width of the focal spot.6.5.3 The mounting frame shall be of a size that enables thetest object to be positioned very close to or on the window ofthe X-ray tube.7. Procedure7.1 Any use of additional X-ray pre-filtering should beavoided.7.2 The X-ray volt
32、age (in kV) should not exceed ten timesthe focal spot size in m for precise measurements of focalspots below 20 m.7.3 Image capture requirements:7.3.1 The distance between test object and detector shallenable projective magnification (see Fig. 2), where smallerfocal spots require larger magnificatio
33、n.7.3.2 Precision is dependent upon the spatial resolution ofthe imaging system, magnification, and signal-to-noise of theresultant images (See Footnote 4). If an estimate of the focalspot size, s, is available, then an optimal magnification can becomputed:Moptimal5 111.1 SNR P/soptimal! (1)where:SN
34、R = unattenuated Signal-to-Noise ratio outside the objectP = pixel size7.3.2.1 If the actual magnification is less than or equal toMoptimal, then the estimated precision is:Precisionestimated5 0.71=1nu1! #211nu2! #23100% (2)where:n(u1) = number of pixels across the 50 to 84 % profile onone side (E t
35、o F in Fig. 1)n(u2) = number of pixels across the 50 to 84 % profile on theother side (G to H in Fig. 1)7.3.2.2 If the actual magnification is greater than Moptimal,then the estimated precision is:Precisionestimated5 2SNR 3100% (3)7.3.2.3 In many cases the optimal magnification cannot beachieved due
36、 to the distance required. However, adequateprecision may still be obtained by using lesser magnification.In general, the diameter of the test object image should be aslarge as possible within the active area of the imaging mediumwhile leaving room to make profile measurements.7.3.3 The minimum dist
37、ance between the test object and thefocal spot (distance “a” in Fig. 2) shall be at least five times thewire or sphere diameter.7.3.4 In case of using a cross wire or Practice E2002Unsharpness Gauge, its plane shall be parallel within 63 tothe detector plane and X-ray tube output window.FIG. 2 Posit
38、ioning of Test ObjectE2903 1337.3.5 In case of using a film, the exposure time shall resultin a background density outside the test object of the radio-graph film of D = 2.5 6 0.5. If no shutter is used, the exposuretime shall exceed 30 seconds.7.3.6 In the case of using computed radiography, a digi
39、taldetector array, or a radioscopic device, the mean pixel value inthe background image outside the test object should be 75 %of saturation for the system settings selected. When using aDDA or radioscopic device, if the mean pixel value is less than75 % of saturation, then frames may be added togeth
40、er toachieve 75 %.7.3.7 Capture an image. Perform film radiography in accor-dance with Practice E1742. Operate computed radiographysystems in accordance with Practice E2033. Operate DDAsystems in accordance with Practice E2698. Operate radios-copy systems in accordance with Practice E1255. No imageq
41、uality indicators are used.7.3.8 If X-ray film is used, scan the film with a scanner asspecified in 6.1.6 in accordance with the film scanner manu-facturer instructions.8. Calculation of Results8.1 Line scans shall be produced of the image in length andwidth direction, see Fig. 1.8.1.1 The measureme
42、nt shall be made using an imageprocessor in accordance with 6.5.1. Line averaging may beused to reduce noise if using wires or the Practice E2002 gaugeensuring that the wire aligns with the pixel matrix such that thewire edge image remains within one pixel width along theaveraged section of the wire
43、.8.2 From these scans the imaging plane diameters DWandDLof the test object in width and length direction shall bemeasured at 50 % of the total image contrast (diameter is thedistance between points F and G.) The length and widthdirections should be related back by geometry to the X-ray tubehousing.
44、8.3 The geometrical magnification is shown in the followingEq 4ML,W5 DL,WDreal(4)where:Dreal= the real diameter8.4 In case of the crossed wires both wires have to bemeasured because of their different magnifications.8.5 Referring to Fig. 2 and using simple geometry, thefollowing equations are establ
45、isheds/a 5 U/b (5)s 5 Ua/b 5 U/M 2 1! (6)U is the sum of the spatial distance between S50and S84oneach side of ball or wire to average the focal spot effect on U.The outside half of the line profile is used to avoid additionalunsharpness due to edge transmission.U 5 EF1GH(7)8.6 Then, according to Fi
46、g. 1, the points A and D areobtained at 84 % of the contrast. From the projection theeffective focal spot dimensions sLand sWare calculated usingEq 8 and Eq 9:sL5 1.47 3 EF1GH!ML2 1! (8)sW5 1.47 3 EF1GH!MW2 1! (9)8.7 If the actual focal spot size differs appreciably from theestimated focal spot size
47、, then the optimal magnificationshould be recalculated to determine if a more precise measure-ment can be made. See 6.2.2.8.8 Determination:8.8.1 Each focal spot size is defined by its dimensionsL(length) in the direction of the tube axis and its size W (width)in the perpendicular direction.8.8.2 If
48、 the tube axis is not defined, then the direction of theelectron trajectory shall be used instead.8.8.3 For transmission target tubes, length and width shallbe horizontal and vertical when looking at the X-ray tube fromthe image plane.8.8.4 The larger of these sizes shall be used as the “focalspot s
49、ize s” or can be reported as L x W. It is only valid inconnection with the used and recorded operating parametersduring the measurement.8.9 A certificate of the measured focal spot size shallinclude the measurement conditions (tube energy, filters, andcurrent, detector manufacturer and type, distances, film or CRsystem scanner parameters, and object type and diametercertification).9. Precision and Bias9.1 Statement of Precision:9.1.1 There is no standard X-ray tube focal spot that can bemeasured and compared to the measurement results; theref
