Page 279 - Fundamentals of Light Microscopy and Electronic Imaging

262 DIGITAL CCD MICROSCOPY

Figure 14-3
CCD camera architecture. The CCD detector is mounted in a hermetically sealed chamber
charged with dry nitrogen gas or containing a vacuum. A transparent faceplate or window at
the front of the chamber is located just in front of the CCD, which is mounted on a
thermoelectrically cooled block to reduce thermal noise in the image signal. The cooling
element is backed by a fan or by circulating water to maintain the CCD at a constant
temperature of 25°C to 40°C. The body of the camera contains cooling vanes to help
dissipate heat. Depending on the type of CCD, an electromechanical shutter may be situated
in front of the faceplate. The camera head also contains several electronic components: a
preamplifier (on the CCD chip itself) to boost the signal read from the CCD, an analogue-to-
digital converter, and circuits for controlling CCD readout. (Drawing from Roper Scientific,
Inc., with permission)

CCD surface, causing all of the electron charge packets stored in the pixels in the paral-
lel register to be transferred one row at a time toward a single row of pixels along one
edge of the chip called the serial register, from which they are moved one pixel at a time
to an on-chip preamplifier and the ADC (Fig. 14-6). After the serial register is emptied,
the entire parallel register is advanced by one row to fill the serial register, the process
repeating until the entire parallel register is emptied. The function of the on-chip pre-
amplifier is to magnify the signal and transmit it as a variable voltage for a short distance
along a video cable to an ADC, which converts the signal into the 0 and 1 binary code
of the computer. For a 12 bit camera, the assignment of each pixel ranges from 0 to 4095