The spectral characteristics of infrared radiation from aircraft body and its plume provide a basis for band selection to detect and track the aircraft. Accurate spectral characteristics improve the performances of detection and tracking. Field multispectral measurements of the characteristics are introduced. A radiation transfer model which helps to measure the aircraft plume is proposed and analyzed. Furthermore, a multispectral imager with spectroscopic filters is designed. It uses a mid-infrared detector, of which the dimension is 640 pixel×512 pixel, and the spectral response covers 3.7~4.8 μm. The noise performance of the imager is also evaluated. A civil aeroplane is selected for the measurements. Under the transfer model, the radiances of the aircraft, both the body and the plume, are obtained in 4 spectral bands. Compared to the conventional method, more accurate infrared spectral characteristics of aircraft are acquired.