X-ray Technology Bets on the Blue Chips
CCDs are available for most applications, each with its own unique requirements. With new applications appearing almost daily, CCD makers must find improved processes and designs to make their imagers better fit customers' needs...
by Keith Wetzel
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1: CCD Sensor Functional Diagram-Many imaging apps require a large
format full-frame transfer style CCD image sensor. Fewer clocks
are required for true two-phase sensors, compared to three and four-phase.
(Photo: William Des Jardin, et al.)
2: Details of Pixel Design-The transparent gate process replaces the
second polysilicon gate with indium-tin-oxide and improves the spectral
response. (Photo: William Des Jardin, et al.)
3: CCD Sensors in Astronomy-Long exposures with blue camera lens filters
obtain correct color balance for full-color photos, but exposure times
can be reduced with Blue Plus CCDs because of increased sensitivity
at this wavelength. (Photo: Kodak)
4: CCD Sensors in Biology-Fluorescent dyes monitor calcium activity
in cells for DNA sequencing, excited with UV lights to make them
visible. Blue Plus CCDs are highly sensitive to detecting the faint
signals emitted, and exposure times are low. (Photo: Kodak)
5: Swissray Digital Optical Detector-This digital optical design detector
(above) uses mirror optics to focus four high-res CCD cameras on a
cesium iodide scintillator screen. Each full-frame CCD with 100% fill
factor captures the image of one scintillator quadrant with 10% overlap.
The final image (below) composites the four images without a seam
or dead zone. (Photo: Rex Harmon, SwissRay International)
common to all CCD chips include resolution, dynamic range, quantum efficiency
(QE), responsivity (sensitivity), readout speed, and dark current. Some
parameters, however, are more critical for certain applications than for
others. For example, CCD chips for astronomy require a large dynamic range,
high S/N ratio and low dark current over a wide color spectrum. This is
necessary in order to detect a faint star as well as a bright cluster
without saturating the photosensitive pixels.
CCDs, on the other hand, receive images from an X-ray scintillation plate
that emits predominantly green and blue light. The CCD should have the
highest QE and sensitivity in this band and a sufficient resolution in
order to find the smallest pathogen possible in the X-ray image.