CCD chip

Endoscopes containing a CCD camera replace the fiber optic bundle with a smaU monochrome CCD chip at the focal point of the fish eye lens (4). 

An x-ray area detector can be used to collect the intensities of many reflections at a time. The crystal must be oriented in many different settings with respect to the incident beam but the detector needs to be positioned at only a few positions to collect all of the data. A charge coupled device (CCD) is used as the area detector on the Siemens SMART single crystal diffractometer system. The SMART detector consists of a flat 6-cm circular phosphorescent screen that converts x-ray photons to visible light photons. The screen is coupled to a tapered fiber optics bundle which is then coupled to a one inch by one inch square CCD chip. The CCD chip has 1024 x 1024 pixels each of which stores an electrical charge proportional to the number of... 

All CCD chips have a leakage current, also called dark current, which is of thermal origin as in photomultipliers. As a result of this phenomenon, a charge slowly builds up in each pixel even when the detector is shielded from light. 

Figure 4 Principle of a back-illuminated thinned CCD chip. (Reproduced by permission from EG G Life Sciences, Evry, France.)...

Figure 5 Crosstalk between pixels due to multiple reflections between the window and the surface of the CCD chip. A photon coming from a initially should hit the pixel labeled i. If it is reflected off the surface, it bounces back toward the window and will be absorbed by pixel j as if it were coming from b.

Only one example will be given. It concerns a specific form of stray light that is observed when the entrance face of the CCD is protected by a transparent window. This situation arises mostly for cooled CCDs. A small fraction of the measured light is reflected off the surface of the CCD chip, hits the window, and bounces back toward another pixel of the CCD chip (Fig. 5). So the reading on the first pixel is lowered and that on the second is increased, the overall effect distorting the tme distribution in intensity. 

CCD detectors use a phosphor to convert the incoming X-ray to visible light, which is in turn detected by the CCD chip. During this conversion process, the apparent size of an X-ray reflection increases, a phenomenon known as the point-spread factor. Typically, this change in reflection size does not present a significant challenge. In extreme cases (i.e. large unit cell dimensions or a very short sample to detector distance), however, it can lead to overlaps between adjacent reflections. When overlaps do occur, diffraction data can be recorded... 

To increase light throughput in Raman systems, the CCD chip size can be increased vertically to match the spectrograph slit height. However, large-format CCD detectors show pronounced slit image curvature that must be corrected in preprocessing (described below). 

Frame-transfer cameras move the image quickly to a protected area on the CCD chip where it can be read out while the next image is recorded. 

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