A multi-wavelength expert-system pyrometer based on a grating spectrophotometer has been able to overcome many well-known difficulties of pyrometry. These include unknown, changing, and/or spectral dependence of emissivity as well as environmental absorption of radiation. In addition to a spectrophotometer and the usual optics, the instrument includes a computer which analyzes each measurement and then returns the temperature, the tolerance (a real-time measure of accuracy), and the signal strength (a quantity directly related to the emissivity at a chosen wavelength). The instrument can save the input data, the thermal spectrum, for each temperature measurement. Accuracy to 0.10% is routinely achieved.