Explain why:
An optical pyrometer (for measuring high temperature) calibrated for an ideal black body radiation gives too low 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.

Explanation

The temperature of red hot iron in the oven is given by `E=sigmaT^(4)` . When iron is taken out in the open temperature `(T_(o))` , its radiation energy is given by , `E_(5)=sigma(T^(4)-T_(o)^(4))` . Thus , the pyrometer measures low values for the red hot iron in the open .