A method and a device to measure temperature using bicolor optical pyrometry are presented. The device is equipped with an optical fiber probe designed forinsitumeasurements in severe conditions (high temperatures, difficult accessibility, and electromagnetic fields). The directional monochromatic emissivities, at two wavelengths 1.3 and 1.55 &mgr;m, were deduced from reflection measurements. The device can be used for opaque Lambertian surfaces in the temperature range from 500 to 2500 °C. The method was put in practice on rare‐earth metal oxide powders at temperatures up to 1000 °C. ©1995 American Institute of Physics.