It was first shown by H. H.Hopkins that the optical transfer function can be directly measured by means of a shearing interferometer. With the advent of lasers in the past ten years, and the need for very high-quality optics both in the laboratory and in field environments, the interferometric technique appears to be ideally suited to single-wavelength measurements. Several laboratory-type interferometers have been described in the literature,2-'7 although in most cases these instruments were not suitable for either routine optical testing or for field use. More recently, a cornercube shearing interferometer was constructed for making MTF measurements on board a KC-135 jet aircraft.8 However, all these past instruments were relatively slow. Very rapid real-time OTF measurements are required in field environments. A new rapid-scanning interferometer has been developed, which requires no adjustments in use and is extremely stable, even in environments where large vibrations are encountered.