this disclosure describes systems , methods , and apparatuses for making a non - contact measurement of a substrate in a plasma processing chamber that accounts for stray blackbody radiation . in particular , a photocurrent is calculated that can be attributed to the stray blackbody radiation based upon a temperature of the stray blackbody radiation , a reflectance of a target substrate , and a temperature of the substrate . knowing the photocurrent attributable to the stray blackbody radiation , a non - contact temperature measurement can be made , and the photocurrent attributable to the stray blackbody radiation can be subtracted out to arrive at a more accurate non - contact substrate temperature measurement .