Abstract: Prediction of the operational performance of a forward-looking infrared (FLIR) system requires a computer model accounting for the principal variables affecting classification range. The range performance is degraded by many factors, including sensor resolution, atmospheric attenuation, platform vibration and visual display information. The Tenix FLIR_P Performance Model was developed to estimate the classification range of an electro-optic sensor, and provides comparative performance estimates for different system configurations and operational conditions. Results are presented for classification range prediction and its variation with absolute humidity in the 3-5 mm infrared band as used in some airborne thermal imaging systems.