Thermal Detectors Mechanisms, Operation, and Performance

Fundamentals of Infrared and Visible Detector Operation and Testing, Second Edition. John David Vincent, Steven E. Hodges, John Vampola, Mark Stegall, and Greg Pierce. 2016 John Wiley Sons, Inc. Published 2016 by John Wiley Sons, Inc. Companion Website www.wiley.com/go/vincent/fundamentals/2e 

Infrared detectors are either thermal detectors or photon detectors. These two work in very different ways. Although the operator may not know, or need to know, which type he is using, when selecting a device for a particular application, or predicting its performance, it is important to understand the difference between the detector types. 

We discuss thermal detectors - the mechanisms, operation, and performance - in this chapter. We will discuss photon detectors in Chapters 4 and 5. 

Thermal detectors absorb radiated power, and that power causes a change in temperature that in turn causes a change in some more easily measurable property. Almost every material that responds to heat has been used as a thermal IR detector. The first thermal detector was the thermometer that Herschel used to first observe IR. In that case the height of mercury in a glass tube was the observable. Important thermal detectors and their mechanism are listed in Table 3.1. The thermal detectors of importance today are highlighted they include Golay cells, microbolometer arrays, thermopiles, and ferroelectric and pyroelectric detectors. 

The output of a thermal detector depends on the power absorbed by the detector. To make it absorb as much energy as possible, we blacken the surface of the detector. If the detector absorbs all of the incident energy, then the output of the bolometer is proportional to the arriving power, and the responsivity (in volts/watts) is independent of wavelength. This is very different from photon detectors, whose responsivity (in volts/watts) increases linearly with wavelength. 

---