I think he would need some kind of absolutely radiometrically source. Spectroradiometers are calibrated using mercury and other lines sources so that the wavelength scale of the instrument could be tuned or corrected. In Germany, Jeti uses Optronix lamps to calibrated their specbos spectroradiometers. A fine instrument indeed ;-) > >> Since I don't >> have access to a suitable reference instrument, I'm not sure >> though, whether it really improves it's accuracy or not. > > What would be the instrument of your choice? Roger Breton