Understanding Infrared Thermography Reading 3

Spectrall y selective instrumems use band pass filters to allow only a very
specific broad or narrow band of wavelengths to reach the detector. (A
combination of a spectrally selective detector and a filter can also be used.)
This can make the instrument highl y selective to a specific material whose
temperature is to be measured in the presence of an intervening medium or
an interfering background. Solving measurement problems by means of
spectrally selective instruments is discussed in greater detail in Chapter 4.
For general purpose use and for measuring cooler targets cooler than about
500 °C (932 °F). most manufacturers of radiation thermometers offer
instruments operating in the 8 to 14 μm atmospheric window. For dedicated
use on hotter targets. shorter operating wavelengths are selected. usually
shorter than 3 μm. One reason for choosing shorter wavelengths is that this
enables manufacturers to use commonly available and less expensive quartz
and glass optics, which have the added benefit of being visibly transparent for
more convenient aiming and sighting. Another reason is that estimating
emissivity incorrectly will result in smaller temperature errors when
measurements are made at shorter wavelengths.
Charlie Chong/ Fion Zhang