Operating Manual

ManualsBrandsGE ManualsTubes And GeneratorsERESCO MF4

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For desktop scanners the cassette can be opened, as

shown in figure 6-16. CR plates can be exposed to

subdued light (< 10 lux: a candle creates 5 lux) for

one minute with acceptable consequences for the

image quality. The effect strongly depends on the

type of the light source, e.g. tube light causes most

damage to the latent image. The scanned image is

ultimately or instantly made visible on a high-reso-

lution monitor (computer screen) of the worksta-

tion, see figure 33-16.

Depending on the line distance selected, typically

50 or 100 microns, the plate speed (lengthwise

progress) is 5 to 10 mm per second. This is similar

to the speed of digitisation of a traditional film. In all scanners, the latent image on the plate

is not only read but also subsequently erased (reset) which takes about one minute, and the-

refore the CR plate is almost immediately available for the next exposure.

CR cassettes

Bare CR plates are nearly as pliable as film. They can be packed in paper or vinyl

cassettes either with or without lead screens. These packages are still pliable.

Technically the plates can be used many times (up to 1000 x), provided they are

handled with utmost care while their surface despite a protective coating is very

sensitive to touching and dirt. A single scratch can make the plate unsuitable for

further use. Rigid cassettes developed especially for the NDT-market have built-in intensify-

ing lead screens at the source side, and a second lead screen at the back to absorb radiation

caused by backscatter. These multi-layer cassettes are not flexible anymore but can be re-

used more often than the flexible cassettes (even a few 1000 times).

Figure 7-16 shows a cross-section of the CR imaging plate in a rigid cassette. The steel and

magnetic plates ensure that the various active layers are evenly and closely pressed

together. For low energy exposures clip-type cassettes exist (to replace the steel- and

magnetic plate) which also ensure intimate contact between the layers. The steel- and

magnet plate would otherwise absorb the low energy radiation.

Dynamic range – Exposure latitude

CR plates have an extremely wide dynamic range (exposure latitude). In practice the

phosphor crystals on a CR plate cannot be saturated and react almost linearly to incident

radiation, while in a conventional film the silver-halide crystals react exponentially, see

the graph in figure 8-16.

As a result the dynamic

range of a CR plate is much

wider than for conventional

film, which makes exposure

times less critical, reducing

re-shoots (re-takes), and

allows various material

thicknesses to be examined

at the same time. The wide

dynamic range can also be

useful in case of under-expo-

sure, this can be compensated

for by a more sensitive read-

out scan or image adjustment

at the work station.

This wide range is illustrated

in figure 9-16. The images

have been obtained from a

stepwedge from 5 up to 25 mm

thickness, in steps of 1mm.

The digitised image of the

film shows only a portion of

the step wedge thicknesses,

the logarithmic CR image

shows all steps proportionally.

The matching analogue records,

at right handside of this figure,

confirm this behaviour.

Furthermore, those sensitivi-

ty (speed) of CR is five to ten

times higher as well, compare

point A and B at a density of

2, see also figure 27-16.

Fig . 6-16. Opened CR cassette

CR plate

Fig. 7-16. Structure of a CR cassette with storage phosphor

Plastic front of cassette

Lead intensifying foil 250

Magnetic sheet

Storage phosphor plate

Lead screen 150 μ

Steel sheet

Plastic back of cassette

Fig. 8-16. Density/intensity versus dose for film and digital methods

Fig. 9-16. Dynamic range of digitised film versus CR plate. (courtesy BAM Berlin)

Density film

Relative dose

Film

CR plate

DR panel

Intensity digital methods

Useful

dynamic

range of film

Useful dynamic range of CR/DR

Imaging plate

Digitised film

Density