TABLE OF CONTENTS

Summary 1. Introduction 2. Different Film Digitization Systems in Comparison 2.1. Different Physical Principles 2.2. Results of Investigation 2.3. Reference Radiograph 2.4. Comparison 3. European Standardisation Project for NDT film digitisation 4. Archiving 4.1. Application "Remote Inspection and Archiving of Turbine Blade Radiographs" 4.2. Latest developments 5. Digital Printing of Films for Reference Catalogues 5.1. Scanning 5.2. Rescaling of the digital data 5.3. Printing References

Summary

An overview is given about the applicability of existing film digitisation systems to non-destructive testing (NDT). The range covered are video cameras, CCD line scanners and laser scanners. Based on the physical properties of the X-ray film the parameters are defined, which have to be fulfilled by the digitisation system. As most important parameters has been chosen : dynamic range and the contrast sensitivity within, geometrical resolution and the presence of artefacts. These parameters determine the potential application field (not usable for NDT, digital analysis or digital archiving of NDT films).

In the second part the actual state of standardisation is discussed. A special test film has been developed and will be standardised world wide for the parameter evaluation of a digitisation system. Examples are given for two major applications of film digitisation : electronic archiving of films in connection with data bases and the printing of flaw catalogues. The reference catalogue ISO 5817 (assessment of weld imperfections, interpretation of arc-welded butt joints in steel) and the DGZfP catalogue D5 (Reference radiographs of castings) are produced by a digital film imager.

It can be foreseen, that improvements in this technology will considerable widen the application range during the next years.

1. Introduction

The development of high resolution digitisation systems allows the electronic handling of radiographic films in Non-destructive Testing. By now, film digitisation has become every day practice in medical applications. The problem of film digitisation is to gather the high image quality of radiographic films (optical density range and spatial resolution) in the digital image. Digitisation yields several new possibilities for conventional radiographic testing: digital archiving, quantitative evaluation, image processing, automatic image evaluation, (remote) image transfer and production of reference catalogues for flaw evaluation.

2. Different Film Digitization Systems in Comparison

Several film digitisation systems based on different physical principles have been tested for application in NDT [1]. First, the different principles will be discussed briefly, then results of the different investigated systems will be compared.

2.1. Different Physical Principles
Film digitisation systems can be classified by the object's dimension that is sampled at each measurement:

Principle	Scanner class
Point by point digitisation	Laser scanner
Line by line digitisation	CCD line scanner
Array digitisation	CCD camera

Fig. 1 to 3 show the principles for each scanner class.

2.1.1. Point by Point Digitization
Fig.1 shows the principle of a laser scanner by the example of the LS85 scanner produced by Lumisys (U.S.A). The digitisation proceeds as follows:

The film is moved in front of an collection tube. A laser beam (wavelength about 680 nm, red) with a fixed diameter (ca. 50 µm) passes the film. The diffuse transmitted light through the film is integrated by the collection tube an registered by a photo multiplier (PMT) op top of the collection tube (not shown in fig.1). During the scan the folding mirror moves the laser beam along a horizontal line on the film. The film is moved with a speed of 75 lines/sec. The resulting voltage at the photo multiplier is proportional to the light intensity behind the film. After logarithmic amplification a digitisation with 12 bit yields grey values that are proportional to the optical density of the film.

The essential difference to other digitisation principles (cf. fig. 2 and 3) is the reversed optical way. The laser scanner illuminates with focused light and measures the diffuse light intensity behind the film. All other methods illuminate with diffuse light (the film is illuminated with a diffuser) and measure the light intensity that passes the film in one direction (camera objective or human eye in classical film inspection). The laser scanner principle has two main advantages:

1. The quantity of collected light is considerably higher. The laser focuses the whole light intensity on the point to measure. The radiation passing the film is collected over a spatial angle as great as possible in the detector.
2. Scattered light from regions with low optical density passing to regions of high optical density, thereby distorting the measurement, is nearly avoided by pointwise illumination.

Fig 1: Principle of a laser scanner ( "LS 85", Lumisys, U.S.A.)

2.1.2. Line by Line Digitization
Fig. 2 shows the principle of a CCD line scanner ("NDT-Scan II", DBA Systems, USA).
The film is illuminated by a light bar to which the light of a projector lamp is passed by a light guide. A Teflon plate ensures a diffuse film illumination. The illuminated line is projected by an objective on a CCD line detector. The film is moved under the light bar to scan the whole film area.

Fig 2: Principle of a CCD line scanner ("NDT Scan II", DBA Systems)

The combination of light bar, light guide, and projector lamp is the illumination of highest power (halogen lamp with 400 W) . Simpler and cheaper possibilities to illuminate the film are fluorescence lamps as used in DTP (Desk Top Publishing) and medical scanners. But these do not have the necessary brightness for a sufficient contrast sensitivity for optical densities up to D=4. For this reason such scanners cannot be used for NDT films.

Line by line digitisation is a good compromise between speed and influence of scattered radiation at high dynamical ranges (great differences between optical densities) that are typical for NDT films. Furthermore, modern CCD line arrays have operation modes for anti-blooming (avoid to transfer charges from saturated pixels to unsaturated neighbours) and integration time control. Thus, the scanner can be adjusted optimally to the optical density range of the film to digitise.

2.1.3. Array Digitization


Fig 3: Digitisation with CCD array camera

Fig. 3 shows the principle of array digitisation. This procedure resembles classical film inspection, only the human eye has been replaced by a camera.

There are also CMOS cameras available, which will give a logarithmic output signal relative to the input light intensity. So the digitised grey value will be proportional to the film density, and do not show to exponential behaviour as for CCD chips. This is a major advantage compared with CCD chips for the digitisation of radiographic films, but the signal-to-noise ratio for CMOS detectors is considerable lower than for CCD chips. The performance of actual CMOS chips is to low for NDT film digitisation, but this may be change in the next years.

Advantages:

• simple structure, cheapest realisation (depending on camera type),
• no moving parts during digitisation,
• arbitrary pixel sizes may be chosen (depending on objective).

Disadvantages:

• The whole information of the image is projected onto the camera array. Due to scattered light only a small dynamic range of the film can be digitised. Depending on the quality of objective and camera array an optical density range between 2 and 3 (at best) can be achieved.
• Since the number of pixel for each CCD array is limited, only a small region of the film can be digitised with sufficient spatial resolution. These regions can measure from 5x5 cm² to 10x10 cm² in size. For this reason CCD cameras are not suited for archiving of complete films which are typically of a size of 8x24 cm² and larger.

2.2. Results of Investigation
Using a radiographic film of a test weld according to code EN 1435 (optical density range from 2.0 to 4.0) with many defect indications the image quality for each system can be determined visually. Fig. 5 to 8 show for each digitisation system:

1. The high pass filtered image regions of an wire type IQI (Image Quality Identifier, EN 462) and fine crack indications. High pass filtering (subtracting a background image calculated by averaging over 21x21 neighboured pixels) was chosen to reduce the dynamic range in such a way that the relevant information can be preserved in the print. EN 462-3 demands that wire # 14 has to be recognised for image class B.
2. Profile plots of the original data, whose location is marked by black lines in the corresponding image regions.
3. The density contrast sensitivity of the scanning system in dependence of the optical density. The density contrast sensitivity is calculated from the standard deviation of the pixel values of a 15 x15 pixel area in the stepped density target of the reference radiograph (see next paragraph).

2.3. Reference Radiograph


Fig 4: The EPRI reference radiograph according to ASTM E 1936 and ASME Section V, Article 2, Mandatory Appendix VI and the CEN drafts

In fig. 4 an image of the reference radiograph, developed by EPRI Charlotte (U.S.A.), is shown for the evaluation of the digitisation systems. This radiograph is used by the ASME, ASTM and CEN standards. With the help of this test film all the basic parameters of a film digitisation system can be evaluated. The reference radiograph contains five types of targets for the parameter evaluation of the digitisation system. The targets are located within a background density of D=3 :

1. Spatial resolution targets, consisting of converging line pairs in a range of 1 ... 20 lp/mm,
2. Density contrast sensitivity targets, block targets of 1 cm² with DD =0.05 at D=2 and DD=0.1 at a darker background of D=3.5,
3. Stepped density targets, a series of 13 1cm² blocks with density between D=0.5 and 4.5 : 4.5, 4.02, 4.0, 3.5, 3.02, 3.0, 2.5, 2.02, 2.0, 1.5, 1.02, 1.0 and 0.5,
4. Linearity targets, these targets provide a geometrical measurement scale in horizontal and vertical direction of 25.4 mm units,
5. Parallel line pair target, a parallel line pair gage with a resolution between 0.5 and 20 lp/mm.

The size of the reference radiograph (14"x17") may be cut to custom fit a particular digitisation system (down to 8"x10") and still contain all of the necessary targets.

Fig 5: Scan results, profile plots, transfer curves and density contrast sensitivity of the Laser scanner LS85 SDR (Lumisys, U.S.A.), 50 µm pixel size, 12 bit logarithmic digitisation

Fig 6: Scan results and profile plots of the CCD scanner "Image Master" (DBA Systems, USA), 70 µm pixel size, 16 bit linear digitisation, transfer curves and achieved density contrast sensitivity (after merging)

Fig 7: Scan results and profile plots of the medical CCD scanner "SCITRON" (MDC Kiel, Germany), 70 µm pixel size, 15 bit digitisation; achieved density contrast sensitivity with an intensity proportional transfer curve

Fig 8: Scan results and profile plots of the CCD camera "MEGA Plus" (Kodak), 70 µm pixel size, 10 bit digitisation; achieved density contrast sensitivity with an intensity proportional transfer curve

2.4. Comparison
The essential result of this comparison is the determination of the working and density ranges of each digitisation system. Within the densities of the working range the digitisation system has a density contrast sensitivity DD_CS better than 0.02. A summary of all investigated systems is shown in Tab.1.

Scanner Type	DD of the Working Range with Contrast Sensitivity DDCS < 0.02	Density Range in which the Working Range is adjustable
Video Camera Sony AVC-D5 5122 Pixel, 8 Bit	0.7	adjustable in 0 ...4
CCD Camera Kodak Mega Plus 10242 Pixel, 10 Bit	1.0	adjustable in 0 ... 4
DTP Scanner Sharp JX-610 DIN A3, 12 Bit	1.0	adjustable in 0 ... 1.3
CCD Scanner DBA NDT-Scan II 14"x17", 12 Bit	1.9	7 fixed working ranges in 0 ... 4.0
Medical CCD Scanner Scitron FD 60 14"x17", 15 Bit	2.0	fixed working range, 0 ... 2.0
CCD Scanner Vidar Diagnostic Pro 14"x17", 12 Bit	2.0	fixed working range, 0 ... 2.0
Laser Scanner CISE Alasca 14"x17", 12 Bit log	3.0	2 fixed working ranges, 0 ... 3.0 and 1.0 ... 4.0
Laser Scanner Lumisys LS 85 SDR 14"x17", 12 Bit log	3.5	2 fixed working ranges, 0 ... 3.5 and 1.0 ... 4.5
CCD Scanner DBA Image Master 14"x>17", 16 Bit	4.0 (after merging)	adjustable in 0 ... 5.0
Table 1: Ranking of investigated scanning systems according to the working range with a density contrast sensitivity better than 0.02

The results presented show that it is not possible to use simply scanners from DTP or medical applications to digitise NDT radiographs. Especially the necessary density contrast sensitivity up to optical densities of D>4 cannot be expected by these systems. These can only be obtained on scanners optimised for the NDT application.

3. European Standardisation Project for NDT film digitisation

In 1997 a new ad-hoc working group in CEN TC 138 WG1 was founded. This group CEN TC 138 WG1 /AHG 3 "NDT film digitisation" has members of 6 European countries and the U.S.A. In 1999 2 drafts have been finalised for the first 2 month inquiry within CEN TC 138. All information concerning these activities is available via Internet [2]. The two drafts are entitled :
• Non-destructive testing - Qualification of radiographic film digitisation systems - Part 1: Definitions, quantitative measurements of image quality parameters, standard reference film and qualitative control
• Non-destructive testing - Qualification of radiographic film digitisation systems - Part 2: Minimum requirements
This project was started because of the unique properties of an NDT film :
Industrial X-ray films are used with optical densities up to 4 and sometimes up to 5. Only a few available scanners are able to cover that density range (see previous section). Furthermore, the NDT X-ray films are characterised by a high signal-to-noise ratio (SNR) which is defined in EN 584, ISO 11699 and ASTM E1815-96 by maximum granularity values related to an optical density of 2 (above fog). A suitable digitizer must not only be able to reach the density of 4 or 5, it also must not increase the image noise by its own detector noise.

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 photo active 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, depending on the energy and the screen film system. Based on these measurements and the experience from the evaluation of different film digitisation systems (as shown in section 3), the following tables define the minimum requirements (see CEN draft, part 2 : minimum requirements).

Table 2 defines the minimum working range of the radiographic film digitisation system. In this working range, the digitizer shall provide an optical density contrast sensitivity DD_cs which is DD_cs £ 0.02. 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 2: Minimum density range of the radiographic digitisation system with a minimum density contrast sensitivity

Table 3 specifies the minimum spatial resolution as a function of the X-ray energy.

Energy	Class DS		Class DB		Class DA
[keV]	Pixel size [µm]	MTF 20 % [lp/mm]	Pixel size [µm]	MTF 20 % [lp/mm]	Pixel size [µm]	MTF 20 % [lp/mm]
£100	15	16.7	50	5	70	3.6
£ 200	30	8.3	70	3,6	85	3
£ 450	60	4.2	85	3	100	2.5
Se-75, Ir-192	100	2.5	125	2	150	1.7
Co-60	200	1.25	250	1	250	1
Table 2: 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 digitising systems, the working group has defined three quality classes; DA, DB and DS. 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,
Application field : digital analysis of films, films have to be archived,

Due to the required international harmonisation, the standard reference film is taken over from ASTM for test and evaluation as well as for long term stability tests of digitisation systems.

4. Archiving

Since the early Nineties BAM has been running projects together with industrial partners to study the application of film digitisation in the radiographic inspection. The central idea is the improvement of the reliability of the inspection results by means of digitisation and computer based handling and evaluation.

We have studied the following application fields of film digitisation :

1. Image Archiving.
2. Use of Telecommunication for remote expertise [3].
3. Image processing to enhance the visual detectability.
4. Computer aided film evaluation, in comparison to modelling [4].

4.1. Application "Remote Inspection and Archiving of Turbine Blade Radiographs"
The German project "TRENDT" (Transmission and Archiving of Digital Radiographs - Application of Advanced Telecommunication in NDT) had been run successfully from 1992 to 1995 [5]. The project partners have been: DeTeBERKOM Berlin, providing the Berkom network; Du Pont, Bad Homburg, providing the ScanManager system for film digitisation as a basic system for the TRENDT application; SIEMENS KWU, Berlin, providing the test problem "remote inspection and archiving of turbine blade radiographs"; and BAM Berlin as a development partner to realise this application.

The aim of this project was twofold :

• build demonstration systems, which integrates all the required functions,
• check if this new approach to turbine blade inspection has advantages over the conventional way.
During the project the following main problems had to be solved :
• The wide dynamic range of the radiographs caused by the wall thickness range (optical densities between 1 and 4). This has to be digitised without loss to the low contrast flaw signals, which has to be detected during the inspection procedure. This was solved by the development of the scanner "Image Master" by DBA as shown in fig. 6.
• The compression of this dynamic range for representation of the film image on the monitor. Because of construction limitations a video monitor is able to display only a dynamic range of 2 orders of light intensity. So a quick and easy LUT management (Look Up Table for the display of grey levels on the monitor) is necessary for the display of radiographic images on the monitor screen.
• Image processing can be applied successfully to reduce the dynamic range (especially background subtraction). This enhances dramatically the visual detectibility of flaws on the monitor.
• The archiving preserves all the selected and different LUT for the different film sections (stored at high geometrical resolution) and an overview in reduced resolution as well as the results of image processing. All this information is stored together with a flaw description on a WORM optical disk and can be retrieved all together with a simple mouse click.
• The connection of the archived film material and the electronically archived documentation (protocols etc.) is done by a database. So the user is able to search for a special film electronically in the database.
• Realisation of a suitable user interface for the remote inspection (common inspection of radiographs by locally separated inspectors) and the transmission of the images over the broadband network.

The realised user interface is shown in fig. 9.

Fig 9: User interface of the ScanManager station (TRENDT project) for archiving of the full film in reduced resolution (ROI1), annotated frames of the regions with full resolution (VIEW1) and the data base connection for film data and flaw types

4.2. Latest developments
The problem on archiving huge numbers of digitised radiographs are the amount of data to be handled in a convenient way. So one film of 35x42 cm² size digitised with 50 µm pixel size and 16 bit density resolution gives 112 MByte data. The lossless data compression used within the "Scan Manager" station for archiving on WORM media is limited to a factor of 2 caused by the noise inherent into the digital data. Lossy compression (e.g. DCT algorithm used by JPEG) is not usable for NDT applications because of the introduced artefacts. After decompression it must be ensured, that all indications originates from the digitised film. Artefacts caused by compression algorithms are not acceptable.

The solution of the TRENDT project was to reduce the spatial resolution of the complete film (overview) and to use the full resolution only in the interesting regions (ROI). But with the further development of huge storage media (RAID arrays of hard drives with several 100 GByte size, DVD-RAM's with 5 GByte and the today most used storage medium, CD-R with up to 800 MByte size) and the reducing costs for it this data size problem disappears during the next years.

An other recent development are the activities of the JPEG 2000 group [6] for standardisation of new image compression algorithms. It is expected to have a new industry standard available next year, based on the wavelet compression. This will have great influence to the image archiving community, because this state of the art algorithm allows to adjust image compression very accurately between lossless and lossy and support multi resolution too.

5. Digital Printing of Films for Reference Catalogues

Recent improvements to digital film scanning and printing technology give results which are superior to the classical contact copy with special duplicating film. This has been demonstrated in 1997 and was accepted by the IIW (international institute of welding) for further reproduction of the ISO 5817 catalogue [7]. The reproduction is a 3 step process :
1. Scanning of the original films
2. Rescaling of the digital data
3. Printing of catalogues

5.1. Scanning
Original films in the density range from D=0 up to D=4 have been scanned with the Laser scanner "LS85 SDR" from Lumisys. This scanner has a pixel size of 50 µm and an output depth of 12 Bit (up to 4095 shades of grey), the digital values are proportional to the optical density of the film.

Some films had densities higher than D>4.0. These high densities cannot be resolved by the Laser scanner "LS 85 SDR" with the necessary density contrast sensitivity. So a CCD scanner "Image Master" produced by DBA Systems was used. This scanner has a pixel size of 70 microns and reaches a contrast sensitivity DD_CS in the density range from 0 up to 4.5 O.D. This wide dynamic range is realised by a special read out mode of the CCD. Each scan line is read out twice, but with an integration time differing by a factor of 100. The resulting two images are combined after scanning to one image (scan merging) covering this wide dynamic range. The digital scanner output is 16 bit proportional to the light intensity, but after the scan merging it will be transformed to 12 Bit proportional to the optical density of the film.

5.2. Rescaling of the digital data
After scanning the raw data with 12 bit depth are rescaled to 8 bit by a linear histogram optimisation. This step was necessary for a optimum printing output. The film printer accepts only 8 bit per pixel as maximum grey scale resolution. So the minimum and maximum data values of the scanned raw data corresponding to the maximum and minimum optical density of the scanned film were found and the difference between them was divided into 256 equally spaced intervals (8 bit). This rescaling was done interactively by visual presentation of each film on a high resolution monitor. This time consuming task could not be automated because of the great differences in the original films. The resulting 8 bit grey scale images of the complete catalogue was written to a CDROM in TIFF format for printing.

No other image processing (e.g. flaw enhancement, background suppression by convolution filtering) was used.

5.3. Printing
For printing the Scopix LR 5200 High Definition Imager produced by AGFA (see fig. 10) was used. This new Laser imager developed for Mammography illuminates a special laser film with a pixel size of 40 microns and a 16 bit grayscale resolution. A normal dark room development process follows this. The imager was loaded with an appropriate density curve giving a linear density range from D=1.5 up to D=3.5 for the 8 bit pixel values of the digital input image (see fig. 11). The saturation density in the background after printing is about D=3.8.

	Laser Imager AGFA LR 5200 for Medicine, High Definition (Mammography) pixelsize: 40µm (630 dpi) 8000 x 10 000 pixel on 35 x 43 cm2 8 bit digital input via programable density function (LUT) scaled to 16 bit Dmin and Dmax between 0.2 and 3,6 OD adjustable data transmission: up to 3 min per film Laser Imaging: max 40 sec development in NDT chemistry (NDT-S): 3 min at 36°C
Fig 10: Parameters of the Laser imager used for digital film printing

Fig 11: Measured programmable transfer curves of the Laser imager LR 5200

The high background density and also the absence of any printing artefacts makes the image quality of this Laser imager superior to any other dry printing processes (Drystar by AGFA, DryView by 3M or Fuji). The printing resolution of these thermo sublimation printers is limited to 300 dpi (85 µm pixel size). This resolution is not sufficient to print all IQI's according EN 463 in original size. Also the maximum film density is limited to D_max = 3.0. This limits the application of dry film imagers for NDT usage.

References

1. U. Zscherpel, C. Nockemann, T. Wawrzinek, S. Nickisch :
   Report for the VGB project "Qualifizierung der Durchstrahlungsprüfung mit digitaler Bildverarbeitung" (Qualification of radiographic testing with digital image processing), Phase 1, VGB - Nr. 16/94, BAM Berlin, Nov. 1995
2. http://trappist.kb.bam.de/CEN-NFD
3. U. Zscherpel, C. Nockemann, S. Heine, "Industrial Application of Telecommunication in NDT", Proceedings of the ASNT 1994 Fall Conference, pp. 253 - 255
4. C. Bellon, G.-R. Tillack, C. Nockemann, "Modelling with a Global Conception of Reliability in NDE", Proceedings of the ASNT 1994 Spring Conference, pp. 48 - 51
5. U. Zscherpel, C. Nockemann, W. Heinrich, A. Mattis, "Application of Film Digitisation in the Power Plant Industry", Proceedings of the ASNT 1995 Spring Conference, pp. 145 -147
6. http://www.jpeg.org/public/jpeglinks.htm
7. U. Zscherpel, C. Jacobsen, . S. Nickisch, "Fortschritte bei der Anwendung der Filmdigitalsierung in der Schweißnahtprüfung" (New Developments in Film Digitisation for Weld Inspection), Annual Conference of DGZfP, 1998, Bamberg, DGZfP Berichtsband 63.1, pp. 83 -8

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