The design and construction of new types of imaging systems has become feasible with the availability of reliable, relatively low cost focal plane arrays (FPA's). This paper presents experimental and modeling investigations of an imaging grating spectrometer utilizing a 128 X 128 InSb FPA for measurements in the 3 - 5 micrometers region. The modeling efforts verify the conceptual feasibility and identify practical limitations in sensitivity and dynamic range by considering system throughput, significant instrument thermal self-emission, and system noise. The model includes a detailed examination of the optical parameters, the focal plane noise sources, and the electronics. 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 nonuniformities on the sensor system. It is very apparent that cryogenic optics, improved FPA non-uniformity correction, and an upgraded data acquisition system will significantly improve the performance of the prototype imaging spectrometer system and are important considerations for future designs.