·Table of Contents
Radiographic Testing - A comparison of Standards for Classical and Digital Industrial RadiologyUwe Ewert, Uwe Zscherpel, BAM-Berlin, Germany
During the last decade, several joint committees have developed European standards on industrial radiography from the previous national standards. They include guidelines on the measurement of instrumental parameters, as well as minimum requirements for instruments, practice and evaluation.
New digital radiological techniques are compared with the potential of classical film radio-graphy. The major parameters are spatial resolution, contrast sensitivity and optical density range. Derived from the properties of X-ray NDT film systems and application ranges minimum requirements are defined. An European standard committee works out a standard on radiographic film digitization systems. Due to the required international harmonization the standard reference film is taken over from ASME/ASTM. Additionally, minimum requirements for digitizer systems are derived. Three system classes are currently proposed with different requirements for density range, density contrast sensitivity and spatial resolution.
Good workmanship criteria for industrial radiography were developed during the last decades in all countries. The corresponding rules are defined in a system of radiographic standards. But these rules are not harmonized in the different countries, because of differences in the traditions and training contents. This leads to deviations in the understanding of requirements for quality and expense of testing. European countries were forced to spend extraordinary efforts for the harmonization of rules due to the process of European centralization. The traditional European schools of NDT had to agree on common standards which allow a harmonized procedure and mutual agreement of NDT quality and certificates. These standards are generally submitted to ISO on the basis of the Vienna agreement between ISO and CEN. This provokes a new process of international harmonization. Nevertheless, we still observe differences in the standards between USA, Japan, Europe and others, which complicates the international mutual recognition.
|Radiography||Radioscopy||Computed Radiography||Computed Tomography|
|Vocabulary||E 1316||EN1330- 3/ISO5576||E1316||EN1330- 3/ISO5576||E1441- Gloss.|
|Equipment, Source||E1165||EN12543, EN12544, EN12579||EN13068-1,2||Draft||Draft|
|Equipment, Detector||E1815||EN584/ISO11699, EN25580||E 1000||prEN13068-3|
|Equipment, system||E1411||E1672, E1695|
|IQI's and ist application||E2002, E142, E592, E747, E1025, E1936-Digitization||EN462||E1647, E1817|
|General Rules||E94, E1742||EN444/ISO5579, Draft-Digitization||E1000||Draft||Draft||E1441, E1570, E1931|
|Welding, ref. Radiographs||E390|
|Casting, ref. Radiographs||E155|
|Sectors (Evaluation)||[EN 25817/ISO5817]- Quality levels, EN12062-General rules|
|Table 1: Comparison of most important industrial radiological standards in Europe and USA|
A new proposal on "general principles for examination of metallic materials by Computed Radiography" was worked out in a committee of the German Society of NDT and has been accepted as European standard project at CEN TC138/WG1. The concept of the standard is the classification of systems and the definition of minimum requirements to ensure a certain spatial resolution and contrast sensitivity, which shall be similar to those as defined by ISO5579 and EN444 (film radiography). It is required to carry out all measurements with two IQI types, the wire IQI for contrast and the duplex wire IQI for the spatial resolution. This concept was taken over from the European standard proposal prEN 13068 for industrial radioscopy. Tables define the minimum wire and duplex wire read out values in dependence on the testing class, the wall thickness range and radiation energy.
The most important standards for radiography can be found in tab. 1. In Europe each radiographer must know the following basic standards:
The idea behind this standards is: Standards shall guarantee a minimum image quality by 'Minimum Requirements' for the testing procedure. This is basically:
The radiographic techniques are subdivided in Europe into two classes:
Class A: basic technique;
Class B: improved technique.
Class B technique will be used when class A may be insufficiently sensitive. All test tasks shall be preferably performed by class A or class B testing.
ASTM standards (E 94, E 1742, E 1032, E1030) do not require testing classes. The quality of testing is basically defined through the Image Quality Indicator (IQI) perceptibility, defined by the responsible person of the company (or cognisant engineering organisation). This IQI's are mostly step hole penetrameters corr. to E 1025. 1997 ASTM issued a standard for wire type IQI's, E747, which corresponds to the old British standard but does not correspond to the valid European E 462.
The ASTM standards do mainly describe the method and workmanship. They do not consequently work with minimum requirements. The background is: People widely believe that testing problems are very different and specific. Therefore, a simple subdivision into two testing classes was not accepted. For each problem a written procedure shall define the needed IQI perceptibility and geometric unsharpness. The standard is the broad basic for all testing tasks. Well educated personal shall carry out the radiography with a quality corresponding to the requirements.
An example for harmonisation in the classical field of radiography is the new set of standards for classification of industrial X-ray films. Tab.2 shows the comparison of national and international standards for film classification.
The basic difference in the testing philosophy cannot easily be overridden in the new field of DIR too. But in principle there exist no obstacles for harmonisation of the qualification and classification standards for the new methods and new digital equipment. The new digital detectors (e.g. flat panel detector, like a -Si or a -Se detector, imaging plates and others ) are world wide conquering NDT-applications. Due to the industrial globalisation, there exist also a serious need for harmonised standards.
Unfortunately several countries started already own standardisation activities. Also in USA and Europe different standard proposals were worked out. Since four years common discussions led to better harmonisation. Beside the classical film radiography three fields are of importance:
All digital methods have the problem, that their spatial resolution is considerably lower than provided by film radiography. Therefore, it was necessary to have a better measure for spatial resolution and to measure it separately of contrast resolution. For this purpose the British duplex wire IQI (CERL B) was included in the European standard EN 462-5 and in the American standard ASTM E 2002-99.
|System Class||Minimum gradient at||Minimum gradient-noise ratio at||Maximum granularity at|
|D=2 above Do||D=4 above Do||D=2 above Do||D=2 above Do|
|Table 2: Definition of film system classes|
Here, it should be mentioned that European standards do not accept EPS (Equivalent Penetrameter Sensitivity). Contrast resolution is measured by wire or step hole penetrameters. The step hole penetrameter have in Europe only 1T-holes. No 2T and 4T holes are allowed. Fig. 1 shows the comparison of the wire IQI's.
|Fig a: Marking wire IQI/EN 462-1||
Fig b: Duplex wire IQI/EN 462-5
||Fig 1: Image quality indicators. Left: Wire IQI for contrast measurement. Right: Duplex wire IQI for spatial resolution measurement. Both IQI's are required for radioscopy and Computed Radiography.
The basic new idea is the application of always 2 IQI's for radiological testing with new digital detectors:
EN 13068 is confirmed now in Europe. Part 3 requires to carry out all measurements with two IQI's, the wire IQI for contrast and the duplex wire IQI for the spatial resolution (fig. 1). Table 3 defines the minimum wire and duplex wire values, depending on the testing class (SA or SB) and the wall thickness for metallic materials. Another table is given for light-alloy materials. Advantages of real time testing are considered here.
|Testing Class SA||Testing Class SB|
|System Class||SC2||System Class||SC3|
|Wire no.||Duplex no.||Wire no.||Duplex no.|
|Wall thickness||Wall thickness|
|1.2 - 2.0 mm||W17||11D||W19||13D||-1.5 mm|
|2.0 - 3.5 mm||W16||10D||W18||12D||1.5 - 2.5 mm|
|3.5 - 5.0 mm||W15||9D||W17||11D||2.5 - 4.0 mm|
|5.0 - 7.0 mm||W14||8D||W16||10D||4.0 - 6.0 mm|
|7.0 - 10 mm||W13||7D||W15||9D||6.0 - 8.0 mm|
|10 - 15 mm||W12||7D||W14||9D||8.0 - 12 mm|
|15 - 25 mm||W11||7D||W13||9D||12 - 20 mm|
|25 - 32 mm||W10||7D||W12||9D||20 - 30 mm|
|32 - 40 mm||W9||7D||W11||9D||30 - 35 mm|
|40 - 55 mm||W8||7D||W10||9D||35 - 45 mm|
|55 - 85 mm||W7||6D||W9||9D||45 - 65 mm|
|Table 3: System performance for metallic materials testing class SA and SB|
The equipment is subdivided into three system classes SC1 - SC3, which depend on the test problem. Lower requirements for spatial resolution than in film radiography (EN 444, ISO 5579) are compensated by increased requirements for contrast.
|Radiation source||Wall thickness|
|Class IPA||Class IPB|
|Double wire |
|X-ray Up £ 50 kV||w < 4||40||> 13||30||>> 13|
|4 £ w||60||13||40||> 13|
50 < Up £150 kV
|w < 4||60||13||30||>> 13|
|4 £ w < 12||70||12||40||> 13|
|w ³ 12||85||11||60||13|
150 < Up £ 250 kV
|w < 4||60||13||30||>> 13|
|4 £ w < 12||70||12||40||> 13|
|w ³ 12||85||11||60||13|
250 < Up £350 kV
|12 £ w < 50||110||10||70||12|
|w ³ 50||125||9||110||10|
350 < Up < 450 kV
|w < 50||125||9||85||11|
|w ³ 50||160||8||110||10|
|Yb 169,Tm 170||85||11||60||13|
|Se 75, Ir 192||w < 40||160||8||110||10|
|w ³ 40||200||7||125||9|
|Co 60|| ||250||6||200||7|
|X-ray Up > 1MeV|| ||250||6||200||7|
|Table 4: Required spatial system resolution in dependence on energy and wall thickness|
Phosphor imaging plate system classes are derived from EN584 (or ISO 11699, ASTM E1815-96, JIS K 7627-97) on the basis of the Signal-to-Noise Ratio (SNR) (see Table 6). Detailed guidelines are given on how to determine the exposure time to provide the specified SNR. Even with the same CR-phosphor imaging plate (IP) and scanner, different IP-classes can be obtained by using different exposure times, if the homogeneity of the phosphor layer is
sufficient. Furthermore, a minimum lead filter thickness is specified to reduce the influence of scattered radiation (table 5), which is generally more intensive than for X-ray film.
|Screen for film||27 mm|
|Screen for IP's||100 mm|
|1 mm Pb|
0.5 mm Fe
|Specific contrast for|
IP's to film
|Table 5: Equivalent screen thickness for IP's(BAS III)|
|IP System classes|
|System class||Minimum |
|Table 6: IP-scanner system classes, depending on the minimum SNR|
Further activities for DIR-standardization are the "standard data format" and "Computed Tomography". The standard data format was the attempt to unify and simplify the manifold of available data formats for NDT technology. This is a very difficult task, because different data formats are already installed in a variety of equipment and used for different applications. The original intend to develop a standard was changed to the publication of a technical CEN report "Generic NDE data format model" published under CEN-CR13935. The ASTM has opened a new project with a similar goal. The data format DICOM, which is widely used in medical applications, shall be modified to NDT-applications under the name DICONDE based on the CEN data model.
The following ASTM - Standards are valid:
|CT Examination||E 1570-95a|
|CT Examination of Castings||E 1814-96|
|Measurement of CT System Performance||E 1695-95|
|CT Imaging||E 1441-97|
|CT System Selection||E 1672-95|
|Calibrating and Measuring CT Density||E 1935-97|
|X-Ray Compton Scatter Tomography||E 1931-97|
The most of these standards were submitted to ISO.
Beside the requirements for maximum density and SNR, X-ray films require a very high spatial resolution. The limiting structure for very low X-ray energies is the grain size of the photoactive silver based crystals, which is below 1 µm. This is particularly important for micro radiography. General NDT applications require X-ray energies between 50 and 12000 keV. In medicine, the application range is normally below 150 keV only. Due to this large energy range for NDT radiography, it was decided to reduce the requirements for spatial resolution to the unsharpness which is caused by interaction of high energy X-rays with the screen film system. Measured functions provide unsharpness values between 30 and 800 µm (Klasens criterion), depending on the energy and the screen film system. Based on these measurements, the following tables define the minimum requirements. Table 7 defines the minimum working range of the radiographic film digitization system. In this working range, the digitizer shall provide an optical density contrast sensitivity DDcs which is DDcs £ 0.02 O.D. The minimum digital resolution is given for all devices converting the digital value proportional to the optical density. If the digital value is converted proportional to the light intensity, the digital resolution must be increased by at least 2 additional bits.
|Class DS||Class DB||Class DA|
|density range* DR||0.5 - 4.5||0.5 - 4.0||0.5 - 3.5|
|digital resolution [bit]||³ 12||³ 10||³ 10|
|density contrast sensitivity DDCS within DR||£0.02||£0.02||£0.02|
|Table 7: Minimum density range of the radiographic digitisation system with a minimum density contrast sensitivity|
Table 8 specifies the minimum spatial resolution as a function of the X-ray energy.
|Energy||Class DS||Class DB||Class DA|
|MTF 20 %|
|Pixel size |
|MTF 20 % |
|Pixel size |
|MTF 20 % |
|Table 8: Proposed minimum spatial resolution of film digitisation systems|
On the basis of the image quality of film radiography and the state of the art of digitizing systems, the committee has defined three quality classes; DA, DB and DC. The user may select the testing class based on the needs of the problem:
DS - the enhanced technique, which performs the digitisation with an insignificant reduction of signal-to-noise-ratio and spatial resolution,
Application field : digital archiving of films (digital storage)
DB -the enhanced technique, which permits some reduction of image quality,
Application field : digital analysis of films, films have to be archived,
DA -the basic technique, which permits some reduction of image quality and further reduced spatial resolution, meeting the needs of radiographs according to ISO 5579 and EN 444 class A above 5 mm wall thickness.
Due to the required international harmonization, the standard reference film is taken over from ASTM for test and evaluation as well as for long term stability tests of digitizers.
Each radiographic film digitization system for NDT applications shall be identified with all working ranges of optical densities. It shall be classified corresponding to table 7 and the maximum MTF 20 % value, which can be performed by this system. So for instance, a digitization system of class DS 5 can be applied for archiving of radiographs taken with X-rays above 200 keV or gamma-rays and can be applied for all class DB and DA digitization tasks.
|© AIPnD , created by NDT.net|||Home| |Top||