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A device used for measuring high temperatures. Platinum resistance thermometers and pyrometers operating on the principle of the thermocouple have the disadvantage that they must be in contact with the hot body, but optical and radiation pyrometers can be used at a distance. Optical pyrometers estimate temperature from the light intensity in a narrow band of the visible spectrum by optical comparison of a glowing filament with an image of the hot body. Radiation pyrometers focus the body's heat radiation on a responsive thermal element such as a thermocouple.

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