Pyrometry of silicon wafers under 700°C at wavelengths over 1μm is difficult because lightly doped wafers become partially transparent. In this work, a modified commercial RTCVD reactor with 8” wafer capability was used to study the temperature measurement of Si wafers over the range of 400–700°C using top and bottom pyrometric detectors. We present initial results on measurements of both reflection and transmission in-silu to determine emissivity at 3.3μm. For heavily doped wafers emissivity was independent of temperature and the measured temperature by pyrometry agreed well with that measured by thermocouple for 400–700°C. For lightly doped wafers, emissivity was temperature dependent due to the increased transparency of the wafer at low temperatures. Using fixed emissivity, the measured temperature severely underestimates the actual temperature below 550°C. By calculating emissivity from the measured reflection and transmission, accurate temperature measurement was achieved from 400–700°C without any a priori knowledge of the wafer.