Operating Manual

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9.2 Densitometer
A densitometer is used to accurately measure the photographic (optical) density at any spot
on a radiographic film. For most types of densitometers the size of the measured area is
approx. 1 mm
2
. The measuring range runs from density 0 to 4.
Since it is a logarithmic scale, this equals a factor 10,000 (10
4
) in density.
It is very important to regularly recalibrate these instruments, particularly around values
2 and 2.3, since those are the minimum densities (depending on class: A or B) which a film
must have in accordance with standard EN 444, to allow it to be interpreted.
Densitometers are supplied with reference material (density strips) to re-calibrate them.
Regular recalibration, at least once a year according to code, is essential.
The most commonly used density strips deteriorate quickly as a result of scratching and dis-
integration of the sealed transparent wrapping in which they are usually kept.
Their service-life, depending on use, is usually not much longer than six months. Agfa has
developed the “Denstep” density step wedge film and has succeeded in considerably
extending the service-life of these strips by supplying them in special wear proof wrapping.
These density strips are certified and have a guaranteed minimum service life of four years.
The 15 steps of the “Denstep” comprise a density range from 0.3 to 4.
9.3 Producing an exposure chart for X-rays
The step wedge
The production of an exposu-
re chart calls for either a large
step wedge or a series of
plates of different thicknesses
made from the same material
to which the chart relates.
The increase in thickness
between each consecutive
step is constant, but varies for
different materials from
0.5mm to several millimetres.
For examinations using a tube voltage of less than 175 kV the thickness of the wedge might
increase by 0.5 or 1.0 mm at each step, while for radiographs using a higher tube voltage
the increase could be in the order of 2-3 mm. In addition several flat plates made from the
same material and of a specified thickness (e.g. 10 mm) should be available.
If the thickness range of a step wedge runs from, say, 0.5 to 10 mm, the step wedge and flat
plate together would give a thickness range of 10.5 – 20 mm
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Type of X-ray equipment
Among the factors to be taken into account are: the voltage (in kV), whether alternating or
direct current, the limits of voltage adjustment and the current through the tube (in mA).
It follows that the exposure chart is unique for a particular X-ray set.
The radioactive source
Radiation intensity and half-life-time of the source have to be taken into account.
Source-to-film distance
The exposure chart for an X-ray set is produced for a specified source-to-film distance. If
another distance is used, corrections will be necessary, using the inverse square law.
Intensifying screens
When drawing up the exposure chart, intensifying screens used must be recorded and the
same type of screens used again when making radiographs.
Type of film
The type of film must be indicated on the exposure chart, since the various types of
industrial X-ray films are substantially different in sensitivity (speed).
Density
An exposure chart must be as accurate as possible. Densities indicated are to be measured
by a densitometer, see section 9.2. The radiographs that form the basis for the chart must
have been made under controlled and reproducible conditions, whereby quality monito-
ring tools such as PMC strips as described in section 10.6 are used.
Developing process
Developer formula, processing temperature and developing time all influence the final
result. The exposure chart produced will be related to a particular well-defined developing
process.
Fig. 1-9. Step wedge