A hand-held, battery-powered Fourier transform infrared spectroradiometer weighing 12.5 kg has been developed for the ﬁeld measurement of spectral radiance from the Earth’s surface and atmosphere in the 3–5-µm and 8–14-µm atmospheric windows, with a 6-cm21 spectral resolution. Other versions of this instrument measure spectral radiance between 0.4 and 20 µm, using different optical materials and detectors, with maximum spectral resolutions of 1 cm21 . The instrument tested here has a measured noise-equivalent delta To f 0.01 °C, and it measures surface emissivities, in the ﬁeld, with an accuracy of 0.02 or better in the 8–14-µm window 1depending on atmospheric conditions2, and within 0.04 in accessible regions of the 3–5-µm window. The unique, patented design of the interferometer has permitted operation in weather ranging from 0 to 45 °C and 0 to 100% relative humidity, and in vibration-intensive environments such as moving helicopters. The instrument has made ﬁeld measurements of radiance and emissivity for 3 yr without loss of optical alignment. We describe the design of the instrument and discuss methods used to calibrate spectral radiance and calculate spectral emissivity from radiance measurements. Examples of emissivity spectra are shown for both the 3–5-µm and 8–14-µm atmospheric windows.