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Sony DFW-X700 Chapter 4
2048 x 2048 CCD array

Orangemicro iBOT Firewire
Cannon IXUS 300
Figure 4.17
Commercially available CCD chips and CCD cameras. Because this technology is relatively mature,
cameras are available in widely varying forms and costs (http://www.howstuffworks.com/digital-
camera2.htm).

that are commercially available, like the sensors discussed previously in this chapter, along
with their disadvantages and most popular applications.

4.1.8.1 CCD and CMOS sensors

CCD technology. The charged coupled device is the most popular basic ingredient of
robotic vision systems today. The CCD chip (see figure 4.17) is an array of light-sensitive
picture elements, or pixels, usually with between 20,000 and several million pixels total.
Each pixel can be thought of as a light-sensitive, discharging capacitor that is 5 to 25 µm
in size. First, the capacitors of all pixels are charged fully, then the integration period
begins. As photons of light strike each pixel, they liberate electrons, which are captured by
electric fields and retained at the pixel. Over time, each pixel accumulates a varying level
of charge based on the total number of photons that have struck it. After the integration
period is complete, the relative charges of all pixels need to be frozen and read. In a CCD,
the reading process is performed at one corner of the CCD chip. The bottom row of pixel
charges is transported to this corner and read, then the rows above shift down and the pro-
cess is repeated. This means that each charge must be transported across the chip, and it is
critical that the value be preserved. This requires specialized control circuitry and custom
fabrication techniques to ensure the stability of transported charges.
The photodiodes used in CCD chips (and CMOS chips as well) are not equally sensitive
to all frequencies of light. They are sensitive to light between 400 and 1000 nm wavelength.
It is important to remember that photodiodes are less sensitive to the ultraviolet end of the
spectrum (e.g., blue) and are overly sensitive to the infrared portion (e.g., heat).