Expanding interest in infrared (IR) thin-film coatings is stimulating development of IR ellipsometers which can provide the optical constant measurements necessary to correctly design these coatings. Infrared ellipsometers are patterned after their visible counterparts; however, they suffer from a component limitation that is generally not a problem for visible instruments. Polarizers for use in the IR generally have low-contrast, limited-wavelength coverage, and beam distortion problems which limit the accuracy of IR ellipsometers. However, corrections can be made if the instrument is properly modeled. The complexity of these corrections and of ellipsometric calculations requires the use of a computer and associated codes. These instrument limitations and computer codes are discussed, and a state-of-the-art automated ellipsometer for use from 1 to 12 pm is described. Examples of measurements with this instrument show the precision attainable.