A four-band (8.2 to 9.2, 10.5 to 11.5, 11.5 to 12.5, and 8 to 14 ?m), prototype, thermal-IR radiometer, model CE 312 [CE 312 is the company model number. In previous papers, the CE 312 was called the CLiMAT (conveyable low-noise IR radiometer for measurements of atmosphere and ground-surface targets)], with a built-in radiance reference is been fabricated by CIMEL Electronique (Paris, France) for use as a field instrument. The instrument is briefly described, laboratory characterization is detailed, and its field measurements are compared with those from three other radiometers. The CE 312's main characteristics are linearity of better than 0.8%, field of view of 9.5 deg; noise-equivalent temperature difference of 0.06 to 0.2 K (depending on the band) for brightness temperatures of 0 to 75°C; SNR greater than 1100 for the broadband and greater than 400 for the other bands for brightness temperatures between 10 and 80°C; and repeatability of the measured radiance smaller than 0.35% after four field campaigns, corresponding to 0.2 K in terms of brightness temperature. Field measurements were conducted over different periods during 1996 at Jornada Experimental Range, New Mexico, Lunar Lake and Railroad Valley, Nevada, and Lake Tahoe, California. The CE 312 compares quite favorably with the other instruments: the brightness temperature at two different sites compared to within 0.3 K with two instruments. These measurements show that the CE 312 thermal-IR radiometer is very stable for ambient temperatures varying between 15 and 60°C and that the availability of several filters in the thermal-IR region can help tremendously to improve the accuracy of the radiance determination.