Light Transducers
Light Transducers
Light transducers convert electromagnetic energy inputs in to electrical energy outputs. The wavelength range of this input energy, as shown in Table 1, can be assumed to be 100 nm to 1000 μm.
Table 1 Subdivision of the optical radiation spectrum according to DIN 5031 (courtesy Texas Instruments)
For the purpose of this book, however, we shall restrict our interest to the visible spectrum as well as the nearby sidebands both in the IR and UV regions which, as shown in Figure 1, represents a range of 300 nm to 1500 nm (0.3 μm to 1.5 μm).
Figure 1 Electromagnetic radiation spectrum (courtesy Texas Instruments)
Most robot light transducers in fact work in the visible spectrum (e.g. cameras) but the use of solid state silicon devices as optical transducers (e.g. photodiodes) also extends the useful optical range into the near IR region. Vacuum photo emissive transducers (e.g. photomultipliers) enable the use of the higher energy content of ultraviolet radiation.
Photoelectric Effect
Light transducers perform the energy conversion by absorption; that is the material atoms absorb the photon energy and use it to move one or more suitable electrons to a higher energy level, either within the valence band (i.e. to a higher atomic shell) or, by crossing the energy gap, in to the conduction band (i.e. leave the atom thereby raising the material conductivity). This phenomenon, illustrated in Figure 2, is known as the Photoelectric Effect. There are 3 main types of photoelectric effects (Chappel, 1976):
Figure 2 The photoelectric effect
(i) Internal photoeffect
This takes place when the conductivity of the bulk material is raised by the creation of electron-hole pairs (the shifting of electrons in to the conduction band in fact leaves holes back in the valence band). This is the basis for photoconductive transducers such as photoresistors.
(ii) Junction photoeffect
This is also known as the Photovoltaic Effect and takes place when the conductivity of the material is raised by the creation of electron-hole pairs within the depletion region (and within one diffusion length) of a P-N junction. In principle this is also an internal photoeffect but related to the physics of semiconductor P-N junctions and forms the basis for photovoltaic transducers such as photocells.
(iii) External photoeffect
This occurs when the electron is given sufficient energy by the incident photon(s) to overcome the photoelectric threshold value and leave the material, thereby becoming a ‘free’ electron. This is the basis for photoemis sive transducers such as photomultiplier and vacuum camera tubes. It follows therefore that for any photoelectric effect to take place the incident photon(s) must have sufficient energy to raise the electron(s) to the required level. The energy content of a photon w is given by its wavelength λ, its speed of propagation-which for most purposes can be assumed to be approximately the same as the speed of light in vacuum—c and Plank’s constant h, as shown in eqn (3.1):
