·Home ·Table of Contents ·Methods and Instrumentation

Imaging in Radiographic Inspection G.G.PURANIK H.E.M.R.L. PUNE 411 021 (INDIA) N.J.TOWNSEND X- INNOVATIONS LTD. BRISTOL ( U.K.) Contact

INTRODUCTION :

The radiographic inspection for defects, with an assortment of films available for recording the images, is the most popular and widely used non-destructive testing (NDT) technique even today. Though it is expensive (mainly due to costly films) and time consuming, it is simple and easy to understand. Film radiography, even today is considered as a reference for all the comparisons and many of the inspection / assessment problems. However it has its own limitations - it is not suitable for "production-line" type of job inspection or where time for inspection / evaluation is very short. It is also not suitable where a large number of sample orientations are required to be inspected.

To overcome this drawback of film radiography, real time inspection systems were developed where the image of the sample / object is available instantaneously for interpretation / evaluation. The real- time imaging systems of the sixties were "noisy' resulting in inferior image quality ( as compared to the quality of image on film) where as the present day systems are far superior due to tremendous developments / improvements in the field of semiconductors / detectors / sensors and computer based functions / fast operations.
The authors in the present paper have compared the working/ performance of conventional film radiography, the image intensifier (I I) based real-time system and the cooled "CCD" based imaging system.

IMAGING TECHNIQUES :

The most important and critical job of an imaging system is to ensure that the image produced is the most accurate possible presentation of the internal details of the object. Acquiring meaningful and quality image of the object is the main purpose of the system. In modern systems various image processing functions are provided for better perception and appreciation.

1. A simple, age old radiographic inspection set-up for film-based imaging is shown in fig. 1A. The variation in the intensity of the emergent X ray beam ( due to differential attenuation - a fundamental property of X rays) is recorded by the film. The processed x-ray film showing the internal thickness / density variations (discontinuities) of the object is known as "radiograph". The selectable parameters here are - tube voltage ( kV), current (mA), exposure time (min), film-focus distance ffd (m) and for recording image suitable combination of film & intensifying screens are chosen.

   To get a good quality image of the object on the film various codes `and practices are recommended and are followed.

2. The experimental set-up for the image-intensifier (I I) based real-time-radiography (RTR) is shown in fig. 1B, The emergent x-rays from the object are made incident on the input screen of the II tube. The II tube is an evacuated glass envelop with x-ray sensitive phosphor as the input screen, a photo cathode, a set of electro-optics incorporating accelerating potential and an output phosphor for a small bright image. This II tube is coupled to the video camera, recorder, monitor, processor etc. The I I tube is a light intensifier, camera converts light into electrical signal, monitor reproduces the image and processor carries out limited integration, subtraction, etc. functions on the video signal.

   The image of the object is seen on the monitor as soon as the x-rays are 'switched on'. It is an instantaneous image and hence is known as "real-time radiography"

3. The experimental set up of the CCD- based digital imaging system is shown in fig.1C. Here the emergent x-ray beam from the object falls on the input phosphor screen ( optimized for specific energy range and easily interchangeable ). This screen converts x-rays into 'visible light' which after reflections is received by the cooled CCD camera, the output of which is given to the computer for the reconstruction / display of the image.

The "charge-coupled device" (CCD) is a silicon chip divided into an array of photosites where electrons are accumulated. This charge pattern is similar to the image pattern. The cooling of the CCD reduces the noise electrons and hence the noise. Basically CCD is an imaging device.

Here the image is not formed instantaneously as in the previous case, but is 'delayed' by (few) seconds - the time required for integrating the charge-pattern of the CCD (signal), This is necessary for the reconstruction / display of the image.

DISCUSSION:

It is clearly seen from the above comparison that film radiography (FR) is the best option as far as image quality is concerned and if inspection time is not critical. Also for the field application or where access is limited / restricted 'FR' is the only solution.

However for large no. of objects and / or viewing the object in various angles, I I based imaging would be better option (for low and medium energy range.) It is a mature technology and there are many manufacturers in the field.

Even though image is noisy and inferior (as compared to film) it serves the purpose, in many cases, where a GO /NO GO type of inspection is involved.

In case of HEMRL's imaging system, the requirements were :

Fig 2a: The imaging system on the left, object on the manipulator and the X-ray tube on the right.

Fig 2b: Control of console of X-ray equipment and the computer controls

1. Rectangular format big screen (film like- 30x 40 cm )
2. A wide energy range (50 KeV to 6 MeV)
3. Image quality comparable to film
4. Large no. of orientations of objects for interfacial defects ( Fig.2 )
5. Real-time imaging.

So naturally the selection was in favour of an open screen, CCD - based digital imaging system ( fig. 1C) For viewing the object - for inspection of debonds around the circumference, a precision manipulator is necessary. The manipulator ( fig.2) designed for 2 m long 300 mm dia ( wt.» 250 kg) cylindrical object. It has a precision linear & rotational movements for a large no. of orientations for tangential exposures for inspection of interfacial bond / gap.

FUTURE SCENARIO:

The Scintillation detectors are fast replacing the conventional / CCD based imaging systems due to their very high sensitivity, better signal - to - noise ratio and very small size. The output data could be used for simple imaging or with some modifications / additions for computed tomography and tomosynthesis applications.

ACKNOWLEDGEMENTS :

The authors wish to place on record the support and guidance of the divisional heads and Dr. Haridwar Singh, Director HEMRL.

REFERENCES :

1. Industrial Radiography - R. Halmshaw, Appl. Sci.Publishers (U.K.)
2. Industrial Radiography - AGFA GEVAERT NV.
3. Indo- German workshop on D.I.R., Vadodara Dec' 99

© AIPnD , created by NDT.net

|Home|    |Top|