The ITER radiological environment will not allow operators to calibrate the optics near the vacuum vessel. An in-situ calibration of spectral transmission from the collection optics to the detector by analyzing two Thomson scattering spectra resulting from two laser beams having different wavelengths was investigated experimentally in the large helical device (LHD). We can measure electron temperature accurately and calibrate spectral transmission at the same time even when spectral transmission degrades during the experimental campaign
