Brochure

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CMOS detectors and flat bed scanners
For some applications CMOS detectors are an alternitive for temperature controlled
amorphous materials. CMOS has a lower energy consumption and the effect of tempera-
ture is less than for amorphous silicon. This in industry is an important feature because it
requires less frequent recalibration for systems using CMOS detectors versus unregulated
amorphous silicon devices. For amorphous materials with every 5°C to 10°C of tempera-
ture variation a recalibration is recommended. CMOS has a wider tolerance of up to 40°C.
With CMOS there is no risk of saturation causing blooming and edge burn-out. In addi-
tion they show no ghost/memory effect, thus no latent images.
Another fact is that the Fill Factor
(active portion of the detector) of
CMOS is better than amorphous sili-
con, see figure 14-16.
Similar to amorphous silicon, CMOS
is suitable for an energy range from
20 kV to several MeV, CMOS even to
15 MeV.
So far no true large flat panels using CMOS detectors exist, the maximum size known at
present measures 100 x 100 mm with very small pixels sizes of approximately 50 microns,
and 200 x 300 mm with pixel sizes of 100 microns. To nevertheless make use of the tech-
nical advantages a clever design provides the solution to mimic a large flat panel. Figure
16-16 shows such a virtual “panel” detector, in fact it is a flat bed scanner in which a fast
moving linear detector array (LDA) is applied. Such “panels” exist in many formats, up to
600 x 1200 mm, and can even be custom made.
For linear arrays smaller pixel sizes of 50 microns exist, this size can technically be redu-
ced further. Probably negative side effects (cross-talk and noise) would then eliminate the
advantages of such smaller pixels.
Limitations
In practice DR flat panel detectors have proven to be excellent tools for the NDT-industry,
however some limitations apply as well:
• Both true DR and flat bed CMOS scanners have a restricted lifetime cuased by the
accumulated radiation. Flat panel detectors can be used continuously for years in
mass production processes. The ultimate lifetime is determined by a combination
of total dose, the dose rate and radiation energy. Flat panel detectors are less
tolerant for high than for low energy radiation, hence extremely high energies
should be avoided.
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16.5.3 Flat panel and flat bed detector systems
There are different types, sizes and suppliers of true 2D flat panel detectors.
A variety of flat panel systems exists with a wide range of pixel sizes and resolutions.
More and smaller pixels and a high Fill Factor increase the resolution of a panel.
As an indirect sensor material amorphous silicon is in wide use. As direct sensors CCD’s
(Charge Coupled Devices) and CMOS (Complementary Metal Oxide Semiconductors) are
also applied. So far they have limited dimensions. To mimic a large flat panel detector, fast
moving CMOS linear arrays are also in use providing an almost similar solution.
Amorphous silicon flat panels
For industrial DR, flat panel detectors (knowm as DDA’s: Digital Detector Arrays) in a variety
of sizes are used, up to approximately 400 x 400 mm(maximum in 2008) as shown in
figure 15-16. Thes detectors convert incident radiation intensity into proportional and digi-
tised electronic signals. These digital signals can, by means of a computer and screen
(workstation), without intermediate steps, be presented as a coherent radiographic
image. A cable typically links the detector to the workstation from which the panel is
controlled as well.
The most common high resolution two-step flat panels, as illustrated in figure 15-16, use
amorphous silicon technology. First a scintillator made of structured Caesium Iodide(CsI)
converts incident radiation directly and instantly into light. The conversion is proportio-
nal to the radiation dose. Secondly light is converted into a proportional electric signal by
thin film transistors (TFT’s).
Each pixel contributes to the radiographic image formed on the screen of the workstation.
Each element is square in effective area, with pixel pitch typically ranging from 50 to 400
microns. The smaller the pixels the better the resolution, but the poorer the imaging
efficiency. Figure 11-16 illustrates this two-step process. Reasearch and development is in
progress to make sensor elements/pixels smaller. Depending on overall active area and
detector pixel pitch, a panel consists of up to several millions of such elements/pixels.
Fig. 15-16. DR flat panel component
with 400 x 400 mm active area.
Assembled flat panel detector
Physical data: Dimensions: ~500 x 600 x 100 mm
Weight: ~ 10 kg
Fig. 16-16. Flat bed LDA scanner/panel (courtesy Envision)