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Understanding Infrared Thermography Reading 3

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The mercury cadmium telluride (HgCdTe) detectors shown in Figure 3.2 are<br />

photon detectors cooled to 77 K (-321° F) for operation from 8 to 12 μm and<br />

to 195 K (-109 ° F) for operation from 3 to 5 μm. Because of their fast<br />

response, these detectors are used extensively in high speed scanning and<br />

imaging applications. In contrast to the mercury cadmium telluride detector,<br />

the radiation thermopile shown in Figure 3.2, is a broad band thermal detector<br />

operating uncooled. It is used extensively for spot measurements. Because it<br />

generates a direct current electromotive force proportional to the radiant<br />

energy reaching its surface. it is ideal for use in portable, battery powered<br />

instruments. The lead sulfide (PbS) detector is typical of those used in<br />

radiation thermometers that measure and control the temperature of very hot<br />

targets. Its peak sensitivity at 3μm matches the peak energy emitted by a<br />

1000K (727 °C = 1340 ° F) graybody.<br />

Because of the atmospheric absorption considerations previously discussed.<br />

most infrared thermal imagers operate in either the 3 to 5 μm or the 8 to 12<br />

μm spectral region.<br />

Note: 195K = [(-273+195) x 9/5] + 32 = -108 ° F<br />

Charlie Chong/ Fion Zhang

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