·Table of Contents
·Methods and Instrumentation
Imaging in Radiographic InspectionG.G.PURANIK
H.E.M.R.L. PUNE 411 021 (INDIA)
X- INNOVATIONS LTD. BRISTOL ( U.K.)
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.
To get a good quality image of the object on the film various codes `and practices are recommended and are followed.
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"
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.
|FEATURES||FILM BASED||I.I. TUBE BASED||CCD BASED|
|1. ENERGY RANGE||Suitable 'film / screen' combination covers entire range.||Suitable for low and medium energy ranges.||Phosphor screens - easily interchangeable - optimised for required energy ranges.|
|2. IMAGE QUALITY (CONTRAST/RESOLUTION)||- GOOD -||INFERIOR ( IMAGE IS NOISY)||COMPARABLE TO FILM.|
|3. SCREEN SIZE||NORMALLY 35 X 43 cm (max) OR ROLL FILM||300 MM CIRCULAR ( PIN CUSHION DEFECT FOR HIGHER DIAMETER TUBES)||ANY SIZE, NO PRACTICAL LIMIT.|
|4. IMAGING TIME (TYPICAL)||20 MIN. (FILM LOADING, EXPOSURE, PROCESSING)||INSTANTANEOUS.||FEW SECONDS.|
|5. IMAGE PROCESSING.||NOT POSSIBLE||LIMITED FUNCTIONS.||LARGE NO OF FUNCTIONS ARE POSSIBLE.|
|6. APPLICATION / MOBILITY||SUITABLE FOR FIELD APPLICATION FULLY MOBILE.||NOT FOR FIELD APPLICATION NOT MOBILE.||NOT FOR FIELD APPLICATION ( BEING DEVELOPED) LIMITED MOBILITY|
|7. COST/ EXPENSES/ LIFE.||CHEAPEST (RECURRING FILM COSTS HIGH)||COSTLY, BUT NO RECURRING FILM EXPENSES. LIMITED TUBE LIFE.||COSTLIER, NO FILMS, VERY LONG LIFE.|
|TABLE-1: (3) BRIEF COMPARISON(Qualitative, not exhaustive)|
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|
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.
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