The development and operation of a 300 mm aperture, high precision scanning interferometer with associated software for the semiautomatic testing of far-infrared lenses and system are described. The instrument is capable of analyzing and evaluating systems in terms of the optical transfer function (OTF), point spread function (PSF), wavefront aberration, ray intersection patterns, polynomial fitting, aberration contouring, and refractive index variation mapping and of providing a large range of representatives of data. The speed of the analysis is commensurate with established techniques with an improvement in the accuracy and flexibility of use of the instrument over alternative measuring systems. The introduction of this instrument has allowed the complete diagnosis of system defects, along with an accurate measure of any error and its effect on system performance in terms of wavefront error, modulation transfer function (MTF), etc. Results on a variety of systems which exhibit errors are presented, and their root cause and subsequent cure are discussed. The value of such an instrument for both prototype building and production runs is described.