Abstract: Remote measurement of temperature of a process chamber provided with a substrate exhibiting a temperature dependent band gap which substrate is illuminated by a source of continuous spectrum light with a spectrum which overlaps the band gap edge of the substrate. The light which exits the substrate is focused by a lens and is picked up by a fiber optic which takes it to a spectrometer where the spectrum is analyzed to ascertain the intensity versus wavelength histograph for exit rays in the wavelength region of the equivalent band gap of the substrate. The wavelength at the point of infection in the region of band gap edge of the substrate in the histograph of the wavelength versus intensity characteristic of exit rays determines the real time substrate temperature from predetermined calibration data which correlates the wavelength at the point of inflection to actual temperature of the substrates.