Fluoride-type IR windows have good transmittance in the mid-wave IR band (3 to 5 micrometers). But, their transmittance drops rapidly in the long wave IR band (8 to 12 micrometers). Many low-cost plastic IR windows are also spectral in nature. Thermographers who measure the bandpass transmittance of these windows with their IR cameras may be in for a surprise or erroneous results when the IR window or the target temperature changes. Previous work, Proc. InfraMation 2004, indicates this is not a serious problem for certain target temperature ranges. This paper expands on the model to include effects of IR window thickness, IR window temperature changes and wider target temperatures. If you use low-cost IR windows, you need to know the results of this study.