Operating Manual

1514
1
Introduction to industrial radiography
Image forming techniques
In industrial radiography, the usual procedure for producing a radiograph is to have a
source of penetrating (ionising) radiation (X-rays or gamma-rays) on one side of the
object to be examined and a detector of the radiation (the film) on the other side as shown
in figure 1-1. The energy level of the radiation must be well chosen so that sufficient radi-
ation is transmitted through the object onto the detector.
The detector is usually a sheet of photographic film, held in a light-tight envelope or cas-
sette having a very thin front surface that allows the X-rays to pass through easily.
Chemicals are needed to develop the image on film, which is why this process is called the
classic or “ wet” process.
Nowadays, different kinds of radiation-sensitive films and detectors not requiring the use
of chemicals to produce images, the so-called “dry” process, are used increasingly. These
techniques make use of computers, hence the expressions; digital or computer aided
radiography (CR) or genuine (true) digital radiography (DR), see chapter 16.
A DR related technique that has been available for many decades is the one in which ima-
ges are formed directly with the aid of (once computerless) radiation detectors in combi-
nation with monitor screens (visual display units: VDU’s), see chapter 17. This is in fact is
an early version of DR.
These through transmission scanning techniques (known as fluoroscopy) the storage of
images and image enhancement are continually improved by the gradual implementation
of computer technology. Nowadays, there is no longer a clear division between conven-
tional fluoroscopy with the aid of computers and the entirely computer-aided DR. In time
DR will, to some extent, replace conventional fluoroscopy.
Summarising, the image of radiation intensities transmitted through the component can
be recorded on:
The conventional X-ray film with chemical development, the “ wet” process, or one of
the following “dry” processes:
• A film with memory phosphors and a work station for digital radiography, called
computer-assisted radiography or CR.
• Flat panel and flat bed detectors and a computer work station for direct
radiography, called DR.
• A phosphorescent or fluorescent screen (or similar radiation sensitive medium)
and a closed-circuit television (CCTV) camera as in conventional fluoroscopy,
an early version of direct radiography.
Fig. 1-1. Basic set-up for film radiography
screens
X-ray film
projection of defect on film
cavity
object
homogeneous
radiation
source