In a spectrographic workpiece metrology system having an optical viewing window, the viewing window is calibrated against a reference sample of a known absolute reflectance spectrum to produce a normalized reflectance spectrum of the reference sample, which is combined with the absolute reflectance spectrum to produce a correction factor. Successive production workpieces are measured through the window and calibrated against the viewing window reflectance, and transformed to absolute reflectance spectra using the same correction factor without having to re-load the reference sample.