An Infrared (IR) imaging system can be constructed with a spectral range that includes at least a portion of an IR band of CO2 which firstly, enables the system to measure flow rates of flare stack gases that contain hydrocarbons, and secondly, is radiometrically calibrated, thereby assuring that the output for each pixel in images captured by the camera has apparent temperature consistent with the temperature of a blackbody used to calibrate the camera.