Transducers photon

Thermal Transducers Infrared radiation generally does not have sufficient energy to produce a measurable current when using a photon transducer. A thermal transducer, therefore, is used for infrared spectroscopy. The absorption of infrared photons by a thermal transducer increases its temperature, changing one or more of its characteristic properties. The pneumatic transducer, for example. 

Values of sx are a complex function of transmittance when indeterminate errors are dominated by the noise associated with photon transducers. Curve B in Figure 10.35 shows that the relative uncertainty in concentration is very large for low absorbances, but is less affected by higher absorbances. Although the relative uncertainty reaches a minimum when the absorbance is 0.96, there is little change in the relative uncertainty for absorbances between 0.5 and 2. This source of inde-... 

The technology involving light and other forms of radiant energy with a fundamental unit, the photon, is known as photonics. Transducers which have visible light as output are widely used. Examples are phosphors, which emit light. Uses of lanthanides in luminescence transducers are listed in Table 12.15. The list includes CRT Phosphors and fluorescent lamps. 

As indicated in Figure 7-, b, there arc two general types of radiation transducers. One type responds to photons, the other to heat. All photon transducers (also called phoforli ciric or quantum detectors) have an active surface (hat absorbs radiation. In some types. 

The distinction between photon and heat transducers is important because shot noise often limits (he behavior of photon transducers and thermal noise frequently limits thermal transducers. As shown in Section -SB-2. Ihe indeterminate errors associated with Ihe two types of transducers are fundaiiienlally differenl-... 

The convenient phototransducers just considered arc generally not applicable in the infrared because photons in this region lack the energy to cause photocmis-sion of electrons. Thus, thermal transducers or photo-conductive transducers (see Section 7l -4) must be used. Neither of these is as satisfactory as photon transducers. 

For a comparison of (he performance characteristics of the three most sensitive and widely used photon transducers —photomultipliers, silicon diodes, and charge-transfer devices —see W. E. L. Grossman, J. Chent. Ediic.. 1989,66.697. 

Figure 7-27 shows the relative spectral response of the various kinds of transducers that arc useful for UV, visible, and IR spectroscopy. The ordinate function is inversely related to the noise of the transducers and directly related to the square root of its surface area. Note that the relative sensitivity of the thermal transducers (curves H and /) is independent of wavelength but significantly lower than the sensitivity of photoelectric transducers. On the other hand, photon transducers are often far from ideal with respect to constant response versus wavelength. 

Several types of photon transducers are available, including (I) photovoltaic cells, in which the radiant energy generates a current at the interface of a semiconductor layer and a metal (2) phototubes, in which radiation causes emission of electrons from a photosensitive solid surface (3) photomultiplier tubes, which contain a photoemissive surface as well as several additional surfaces that emit a cascade of electrons when struck by electrons from the photosensitive area (4) photoconductivity transducers in which absorption of radiation by a semiconductor produces electrons and holes, thus leading to enhanced conductivity (5) silicon photodiodes, in which photons cause the formation of... 

The first detector for optical spectroscopy was the human eye, which, of course, is limited both by its accuracy and its limited sensitivity to electromagnetic radiation. Modern detectors use a sensitive transducer to convert a signal consisting of photons into an easily measured electrical signal. Ideally the detector s signal, S, should be a linear function of the electromagnetic radiation s power, P,... 

---