The CellaCrystal PX 44 has been developed for optical temperature measurement in the production of Si and SiC crystals like Physical Vapot Transport. The calibration is specially adapted to the growing process. Thanks to the hybrid signal evaluation with a constant high resolution of < 0.1 K over the entire measuring range and the very high long-term stability thanks to the constant light sensor technology, the device meets the high requirements for the necessary measuring accuracy.