An optical pyrometer (for measuring high temperatures) calibrated for an ideal black body radiation gives

Question

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 furnace.

Answer 1

Let the temperature of the hot iron in the furnace be‘T’ and that of the external environment be ‘T₀’. According to Stefan’s Law, heat energy radiated per second E = σT⁴ in the open. Clearly E > E’. Hence the low readings of the optical pyrometer when the iron piece is in the open.