Operating Manual

Stationary real-time installations
Display monitor systems, as illustrated in figure 9-17, are almost exclusively used in
stationary set-ups for production line testing of varying types of objects, in particular in
metal casting plants, pipe mills and component assembly industries. Often they provide
some image magnification and software features to improve defect detectability.
Sometimes real-time systems are utilised in the food industry to check for instance for the
presence of glass fragments or other foreign objects. Being part of a production line and due
to the necessary radiation safety provisions (such as cabins) these systems can be very
expensive. The display monitors are located at a safe distance.
The choice of a radiographic system to be used for a specific application depends on a
number of factors:
• Hardness of the radiation required and appropriate detector
• Resolution or detail discernibility required. The type of defects to be detected
in mass-production is normally known
• Magnification factor required when it concerns small defects
• Image dynamics (density range) with regard to object thickness range
• Image contrast required facilitating ease of defect detection.
Sometimes this can be “automated” when it concerns common defects
• Time restraints, number of objects to be examined per unit of time
• Budget
• Space available
• Installation and specimen dimensions
• Sufficient safety measures
A number of these factors also influence the choice of detector system.
Some of the options are:
• Phosphorescent screen (afterglow) with TV camera and display monitor
(CCTV) at a remote (safe) location
• Fluorescent screen (instant image) with CCTV-system at a safe location
• X-ray image intensifier with conversion screen, in combination with a CCTV-system
Fig. 9-17. Radiography with image magnification
X-ray
tube
object
digital
signal
digital
signal
flat panel detector
or
image intensifier
image processing
system and
archiving
display monitor
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Imaging systems for high resolution radiography
High-resolution X-ray inspection systems usually apply an image intensifier for presentation
of results as shown in figure 7-17. This electro-optical device amplifies and converts the invi-
sible X-ray shadow to visible light by means of a scintillation crystal and photo cathode.
Electrons from the photo cathode are then accelerated and focused onto a phosphor screen
where a bright and visible image is produced that is digitised by a CCD camera. In order to
avoid unnecessary losses of resolution, it is crucial to at least use 2 mega pixel high-resolu-
tion cameras. To meet highest demands, 4 mega pixel cameras are the best choice.
The advantage of the image intensifier-based digital image chain is its relatively low cost
and relatively sharp real-time image.
As an alternative, a digital flat panel detector as shown in figure 8-17 can be used. In that
cas the X-ray shadow image is still converted by a scintillator foil to visible light, which is
then directly detected by the photo diode array. This option is more expensive than the tra-
ditional image intensifier of figure 7-17.
Digital flat panel detectors provide better images with far superior contrast resolutions of
0.5% compared to 2% of image-intensifiers. This can be a decisive factor for low contrast
objects and for high quality computed tomography (CT), see section 17.3.
17.2 Fluoroscopy, real-time image intensifiers
Fluoroscopy, also known as radioscopy, is a technique whereby “real-time” detection of
defects is achieved by the use of specialised fluorescent screen technology.
At present, there are many alternatives to photographic film for making an X-ray image
visible. In addition to the CR- and DR techniques described in chapter 16, a wide range of
real-time image forming techniques using display monitors are available.
It can generally be said that the image quality of conventional X-ray film is superior to
either true digital (direct) radiography (DR) or computer-aided radiography (CR).
Therefore these new techniques cannot be considered acceptable alternatives at all times.
However, when the installation is adjusted to optimal refinement for a single application, for
example weld inspection in a pipe mill, a filmequivalent image quality can be obtained,
which would only just comply with the requirements. This would possibly require the use of
a microfocus tube, see section 5.1.
Figure 7-17. Schematic setup of an image-intensifier
digital image chain
Object
Object
X-rays
X-rays
Scintillator
Scintillator
Single component
Diode array
Photocathode
Electrons
Focusing
electrodes/coils
Phosphor
Lens
Light
Light
CCD-
camera
Fig. 8-17. Arrangement of a flat panel based
image system