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Explain why, an optical pyrometer (for...

Explain why,
an optical pyrometer (for measuring high temperatures) calibrated for an ideal black body radiation gives too low a value for the temperature of a red hot iron piece in the open, but gives a correct value for the temperature when the same piece is in the temperature when the same piece is in the furnace.

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Optical pyrometer is developed on the principle of lightening of surface depends on temperature.
When the temperature is less than `600^(@)C` (iron piece is in open) then it is not bright, hence temperature in optical pyrometer is low. But when that piece is kept in furnace, then it is bright and optical pyrometer gives correct value of temperature.
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