Many experimental measurement goals can best be accomplished with imaging sensors which have high spatial, spectral, and/or temporal resolution. The design and construction of new types of IR imaging systems have become feasible with the availability of reliable, relatively low cost focal plane arrays (FPA's). This paper presents the design and modeling investigations of an imaging grating spectrometer utilizing a 256 X 256 InSb FPA for Earth observing measurements in the 3 - 5 micrometers region. The design includes an optically filtered, radiometric, imaging mode for increased temporal and spatial resolution. Modeling efforts verify the conceptual feasibility and identify practical limitations in sensitivity and dynamic range. The design concepts and performance were verified experimentally by building and testing a prototype imaging spectrometer using commercially available optics, FPA, electronics, and computer equipment. Data is presented which illustrate the simultaneous spectra and spatial measurement features and the versatility of the sensor system. Both the model and the measurement results show the impact of instrument self-emissions, FPA noise, and FPA non-uniformities on the sensor system. In addition, the impact of system dynamic range and FPA pixel integration timing and read-out electronics are discussed.