How a CCD works
Today, the best photon detectors are charge-coupled device, so-called CCDs. It enables the detection of a large fraction (typically >50%) of the photons from soft X-rays to the near-infrared. In comparison, the most sensible classical photographic devices only enable the detection of about 2 to 3% of the incoming photon besides having a smaller wavelength range.
The way a CCD works is very easy in principle. A very nice analogy, suggested by Jerome Kristian, is often used (see figure below): One can imagine a network of large buckets distributed in a regular manner on a large surface farm. After a strong downpour, the buckets filled with water are transported on conveyor belts to a weighting station where the quantity of water which poured on the field‘s ground will be measured with high accuracy. Therefore it becomes possible to determine the overall quantity of rain which fell on the field as well as its spatial distribution. A CCD detector works with the same principles: rain drops are replaced by photons, the buckets by pixels, etc.
In order to produce an image, a CCD must accomplish four functions:
- generate photoelectrons (i.e., rain drops)
- collect electrons (i.e., the buckets)
- transfer the collected charges (i.e., the conveyor belts)
- read the charges (i.e., weighting device)
The first function is based on the photoelectric effect. The light absorption in the silicate network of the CCD generates these photoelectrons, in proportion to the number of incident photons. The latter are immediately collected in “picture elements” so-called pixels (i.e., the buckets), closest to where the photons fell on the chip. Those pixels are defined by means of an electrode network which covers the surface of a CCD. The electrodes form potential wells to prevent the collected charges from escaping.