What is emissivity correction and why does it matter for semiconductor wafers?

Silicon wafers have low emissivity -typically 0.1–0.7 depending on temperature, doping, and surface film -and that value changes during processing as films deposit or etch. A 1-color pyrometer requires a known, stable emissivity value. If the emissivity is wrong, the temperature reading is wrong. A 2-color ratio pyrometer measures at two wavelengths and calculates temperature from the ratio, which is largely independent of emissivity. For most CVD, epitaxy, and RTP applications on silicon wafers, 2-color pyrometry is the technically correct approach.