Operating Manual

147
Digitisation of these films provides an excellent alternative that also prevents
degrading. Special equipment has been developed for this purpose. Current digitisation
equipment actually consists of a fast computer-controlled flat bed scanner that scans the
film spot wise in a linear pattern, identical to the formation of a TV image, measuring
densities while digitising and storing the results.
The spot of the laser beam can be as small as 50
μ m in diameter (1 μ m = 1 micron, equi-
valent to one thousand’s of a millimetre), but the equipment can be adjusted for a coarser
scan, for example 500 microns, to achieve shorter scanning times. The values measured are
compared to a calibrated density scale and processed digitally. Density variations bet-
ween 0.05 up to 4.7 can be measured.
The scanner has part of its technology in common with the CR film scanner, of which the
schematic principle is shown in figure 3-16. Contrary to the CR film scanner which mea-
sures reflected (stimulated) light, the density measurement in the film digitiser takes
place in transmission mode using a scanning light beam synchronised with a light detector.
Usually the density (blackening/
blackness) of a film is digitised in 12
bits, equal to 4096 steps or grey
levels. For convenience, these over
4000 levels are divided by 1000,
resulting in relative digital density
values from 0 to 4. This provides a
mean comparison with traditional
film density values.
GE Inspection Technologies supplies film digitisers, made by Agfa. A desk top version is
shown in figure 1-16. In these scanners, films with a maximum width of 350 mm can be
digitised in a single run. Even for the smallest laser beam spot size of 50
μ m, approximately
4 mm of film (in length) can be scanned per second, so for the largest standard film size
(350 x 430 mm) this process would take approximately 2 minutes to complete.
Scanners exist without length limitation of film, and adapters exist for digitisation of roll
(stripe) films.
Apart from greatly reduced storage space and (almost) deterioration-free archiving,
digitising also makes it possible to (re)analyse the film’s images on a computer screen (see
work station in figure 33-16), with the possibility of electronic image adjustment (enhan-
cement), see section 16.12. Thus defect indication details not discernible on the original
film using a viewing screen can be made visible.
For use in laboratory environments only, high-resolution film digitisation systems exist
that use a scan spot size as small as 10
μ m. This is an inherently time consuming process
but enables detailed analysis of particular film areas, e.g. to make tiny cracks visible at
the work station.
146
The major parameters to compare film to digital radiography are spatial resolution,
contrast sensitivity and optical density range. The major merits of digital radiography
compared to conventional film are:
• Shorter exposure times and thus potentially safer
• Faster processing
• No chemicals, thus no environmental pollution
• No consumables, thus low operational costs
• Plates, panels and flat beds can be used repeatedly
• A very wide dynamic exposure range/latitude thus fewer retakes
• Possibility of assisted defect recognition (ADR)
Despite all these positive features, the image resolution of even the most optimised digi-
tal method is (still)not as high as can be achieved with finest grain film. A few other
limitations are also explained in this chapter.
16.2 Digital image formation
In conventional (film) radiography, the human eye is used to examine a physical record of
the radiographic image, which has recorded the intensity of X-rays incident on the film as
varying degrees of opacity (shades of grey between black and white). In digital imaging the
intensity of X-rays is first measured point by point and then individually digitised and con-
verted into many (e.g. 12 bit = 4096 levels) discrete grey values including their corresponding
coordinates. This recording process is known as mapping; a map consists of many (millions)
discrete measuring points with their individual grey levels. Finally, these grey levels and
their coordinates are displayed to form a coherent image on a video screen, or printed, as a
collection of picture elements (“pixels”) for examination by the human eye.
Because of the 1-to-1 correspondence between each final image pixel and the discrete mea-
surement area (sensor size), the areas on a digital detector are also commonly referred to as
pixels. For digital radiography using panel, flat bed or line array detectors this process of
digitisation with assigned grey levels is done at once, at the detector itself. In case of
imaging plates the digitisation and grey level assignment is done in the so-called
“reader”, see section 16.4. The mapping process allows data to be measured and stored
from a much wider dynamic range than the eye can normally perceive. After an image has
been stored, different maps can later be applied to show different thickness ranges, without
affecting the original measurements. These maps can be linear or non-linear: for example, a
logarithmic map is sometimes used to more closely mimic the response of conventional films.
16.3 Digitisation of traditional radiographs
Although the image forming of traditional film has nothing to do with digital radiogra-
phy, digitisation of such films makes use of a major part of the technology and hardware
also used for CR and DR and as such is part of this chapter of the book. Storing and archi-
ving of chemically processed X-ray films not only demands special storage conditions, see
section 10.7, but also takes up quite a bit of space.
Fig. 1-16. Desk- top film digitiser