Lloyd’s mirror experiment is applied to testing flatness of large surfaces. Because of the grazing incidence, even rough surfaces provide the characteristic interference pattern. In the case of a perfectly flat surface, that pattern consists of narrowly and equally spaced two beam interference fringes. Departures from flatness are reflected in changes of the fringe spacing. Moiré techniques are used for visualizing and measuring these changes in the deviation from straightness of the Moiré fringes. Qualitative examples are given.
P. H. Langenbeck, "Lloyd Interferometer Applied to Flatness Testing," Appl. Opt. 6, 1707-1714 (1967)