Page 281 - Fundamentals of Light Microscopy and Electronic Imaging

264 DIGITAL CCD MICROSCOPY

Output amplifier

Serial register

Parallel register
1
pixel

Figure 14-5
Surface design of a full-frame CCD. The majority of the surface area is occupied by an
imaging area known as the parallel register, an array of thousands of pixels that are
arranged in rows and columns with respect to the output edge of the chip near the serial
register. The serial register is composed of a single row of pixels and contains the same
number of pixels as the row at the output edge of the parallel register. Photoelectron counts
contained in the pixels are transferred serially, from pixel to pixel, until they reach an
amplifier, which sends an amplified voltage signal to a nearby ADC.

• An ADC assigns a digital code for each pixel depending on the amplitude of the sig-
nal (0–4095 ADUs for a 12 bit system).
• Pixel values are stored in a frame buffer in the computer.
• The process repeats until all 1000 rows of pixels of the parallel register are emp-
tied.
• For a 1 megapixel CCD chip processed at 2 bytes/pixel, 2 Mbytes are stored in the
computer; the image is displayed in an 8 bit (256 gray-level) format on the monitor.
• The CCD chip is cleared (reread without digitization) to remove residual electrons
prior to the next exposure.
• The total time for readout and display is 0.5 s for a 1 Mpixel camera operating at
5 MHz.

Considering the large number of operations, it is remarkable that greater than a million
pixels can be transferred across the chip, assigned a gray-scale value ranging from 0 to
4095, and stored in a frame buffer in the computer in less than a second! The accuracy
of transfer of electron packets over thousands of pixels is also nearly error-free. The
quantitative nature of a digital CCD image is shown in Figure 14-7.
Another important feature in the camera head is the elements associated with cool-
ing. As described below, CCD images are degraded by electron noises, the most serious
of which are due to heat and the electronic readout of the camera. Thermal noise refers
to the generation of electrons from the kinetic vibrations of silicon atoms in the CCD
substrate. Thermal electrons cannot be distinguished from photoelectrons and thus con-
tribute noise to the image. Cooling is essential for all scientific-grade CCD cameras, the