This paper describes the modeling of multispectral infrared sensors. The current NVESD infrared sensor model, NVTherm, models single spectral band sensors. The current NVTherm model is being updated to model third generation multispectral infrared sensors. A simple model for the target and its background radiance is presented here and typical results are reported for common materials. The proposed target radiance model supports band selection studies. Spectral atmospheric propagation modeling is accomplished using MODTRAN. Example radiance calculations are presented and compared to data collected for validation. The data supports rejecting the null hypothesis that the model is invalid.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.