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STICKING A THERMOMETER into an armpit, mouth, ear or other body cavity is the most accurate way to take someone’s temperature. Understandably, though, this cannot be done at airports or checkpoints set up elsewhere to screen the masses for feverish victims of covid-19. So, in a bid to detect the warmth produced by a fever without touching any bodies, officials have opted for alternatives.
The hand-held “thermometer guns” now ubiquitous in China, among other places, are one option. These instruments, known technically as spot pyrometers, use a device called a bolometer to estimate an object’s temperature. A bolometer’s electrical resistance depends on how hot it is. That, in turn, depends on the amount of infrared radiation falling on it from whatever it is pointing at.
Spot pyrometers are used widely in industry to check equipment for signs of overheating, but the infrared signals they rely on can be muddled by dust, moisture, smoke, a...