An infrared (IR) camera was used to measure the temperature of polymer filaments held in a stream of hot air. Filament diameters of 0.1-4 mm were tested. The air stream velocity and temperature fields were independently measured with a Pitot tube and a thermocouple. Under certain conditions, the air temperatures determined by the thermocouple closely approximate the temperatures of the polymer filaments. These air-temperature measurements were compared to the temperature readings given by the IR camera. This procedure allowed the development of calibration curves for the infrared camera. The calibration curves were found to depend on (a) the diameter of the filament and (b) the image size of the filament upon the pixel array in the IR camera. These calibration curves can be used with the IR camera to allow online measurement of fiber temperature during melt spinning, melt blowing, and other fiber production processes.