What Are CCD and CMOS Image Sensors?

At the heart of every camera is the medium whereby it translates light into an image. Without this process, photography couldn’t exist. In the case of film photography, the light actually brings about chemical changes in the film that record the light’s information at a molecular level. All digital photography relies on using light to affect electrons and (at least at present) involve semiconductors, but not all methods of accomplishing this goal are equal.

First, the similarities: Both devices exploit the photoelectric effect, the fact that, if you “hit” a bound electron with a photon of energy greater than that with which it is bound to an atom, you can knock it free. Since the energy of a photon is dependent on its frequency, different metals will respond to different colors of light. Increasing the intensity of light increases the number of electrons dislodged, but doesn’t increase their energy. If you know what energy the electrons have when they reach you, you can figure out their color. Place individual detectors in a grid, and you can even figure out what location the photon “belongs” to.

CCD stands for Charge-Coupled Device; this detector separates the photodetector array from the device that counts the electrons, moving them first across the grid to the edge in orderly fashion, then boosting the voltage and analyzing the result. CCD’s were invented by Bell Laboratories in 1969 and thus have had a long time to be refined and improved upon; additionally, the lack of transistors on the photodetector array increases its effective area and eliminates a potential source of interference.

Complementary Metal-Oxide Semiconductor “active pixel displays”, a technology about as old as CCD but difficult to construct at first due to limitations in semiconductor technology, utilize a slightly different strategy. They include an amplifier at the site of each individual pixel, a strategy that yields increased speed, integration of image processing and detection, lower energy requirements, and decreased cost at the price of somewhat less sensitivity.

So, what’s the verdict? Integrated circuits have come a long way since the 1960’s, and both technologies have progressed significantly. CMOS has become very popular recently, primarily due to its speed, but CCD still has the edge in quality. Much of the success or failure of a sensor is due to other design considerations that will be discussed in a later article. For a detailed analysis of the pros and cons associated with these technologies, go here.