Abstract: The optical properties of solids is of great importance in condensed matter physics, and can be characterized by a set of optical constants. The most common experimental practice is to measure the sample's reflectivity at normal incidence over a broad frequency range, and then obtain the optical constants via the Kramers-Kronig transformation. In this article we introduce the basics of Fourier transform infrared reflectivity spectroscopy, the experimental setup, and an in-situ overcoating technique which can measure the reflectivity of samples, including those of small size or irregular shape, from several meV to several eV. This method should thus play an important role in the study of various single particle excitations or collective excitations in solids.