New imaging devices and image processing challenges

Electronic light sensors transform the energy of the incoming electromagnetic wave into some form of electrical energy, usually via the excitation of electrons (e.g. photoelectric effect). The photodetection process in a semiconductor material consists of interaction of a photon and an electron (absorption) and collection of the generated electron-hole pair in the external circuit, see fig. 1. The efficiency of the photodetector depends on how many photons are absorbed and detected. For the simple band-to-band absorption process from fig. 1, the quantum efficiency depends on the absorption coefficient α and the length of the detector area along the path of the incident light l. The fraction of the light absorbed in the detector is: 1 — exp[—αl]. Some of the incoming light is reflected by the surface of the detector, but by using special antireflection coatings that could be below 5%. The absorption coefficient in general depends on the light wavelength. Later on we shall see how that property was used in designing some modern image sensors.