l Light optical Photo electrical The relationship signal Source signal detector signal between a light source and an optical detector. electrical Photo optical Light electrical signal detector signal Source signal Light Sources and Detectors Radiation Absorption A photodetector is an optical detector that absorbs radiation and outputs an electrical signal that is normally proportional to the intensity of the detected electromagnetic radiation. As a refresher from Chapter 2, the energy of a photon is given by the formula where h = Planck's constant [6.626 X 10-34 J/s (joules per second) ] C = speed of light (2.9979 X 108 m/s) , which is normally rounded to 3 X 108 m/s in most publications l = wavelength The number of photons per unit area of a detector striking it per sec- ond can be expressed as N = HN hc, where H is the intensity of the inci- dent radiation in units of power per unit area. From the preceding observations and our knowledge of atoms and electrons described earlier in this chapter, we know that electrons can be excited to the conduction band through the absorption of photons. Because the bandgap between the valence and conduction bands in an atom is well defined, it becomes possible to create a semiconductor photodetector whose atoms are excited to the conduction band during photon bombardment. Because photons with an energy equal to or greater than the bandgap energy level can excite valence electrons while those with a lesser amount of energy cannot, this means that only radiation with wavelengths shorter than a critical value is capable of generating conduction electrons. This results in a critical wave- length that can be expressed in terms of the energy across the bandgap Eg as = hcl Eg. This means that it is possible to detect radiation at a specific wave- length by using semiconductor material that is fabricated with a given bandgap, and this provides the basis for the creation of photodetectors. The amount of energy required for a particular bandgap varies from material to material. For vis