Operating Manual

9
Exposure chart
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9.1 Exposure chart parameters
Codes for the inspection of welds and castings specify the maximum allowed radiation
intensity, based on the type of material and the thickness of the object. Exposure charts are
necessary to etablish the correct exposure value. A universal exposure graph or slide-rule
can be used for radioactive sources, as these have a fixed natural radiation spectrum.
The radiation spectrum of X-ray tubes varies with each tube, even if they are of the same
type. This problem is easily solved by using a universal exposure chart for the specific type
of tube, and then individualise it for each tube, the so-called “curve fitting”. The adaptation
is normally limited to a zero-point correction, based on a few measured values obtained by
trial. Sometimes the gradient of the exposure graph needs to be adjusted as well.
An exposure chart is produced by making a series of radiographs of a step wedge as
illustrated in figure 1-9.
The radiation intensity level of most X-ray equipment is expressed by the amperage of the
current through the X-ray tube, measured in milliampères (mA).
The exposure (radiation dose) is specified as the product of radiation intensity and
exposure duration in mA.min. (intensity x time).
The exposure chart shows the relationship between the thickness of the object (in mm) and
the exposure value (for X-ray tubes in kV and mA.min; for sources in GBq/h).
The exposure chart is applied for:
1. a given density, for example: 2 or 2.5
2. a given film-screen combination, for example D7 with lead screens
3. a given type of material, for example steel
The chart depends amongst others on:
4. type of X-ray equipment or radioactive source
5. source-to-film distance, usually 800 mm
6. development conditions, for example: automatic, 8 minutes at 28°C.
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X-ray photograph of a Van Gogh painting, presumably a self-portrait, on canvas.
X-rays are made to prove a paintings authenticity, check the condition of the canvas material,
or determine possible paint-overs.
Cadmium and/or lead in the paint absorb radiation, thus forming an image.
Applied X-ray process data: 30 kV, 10 mA.min, Agfa D4 without lead screens and a source-to-film
distance of 100 cm.