A grazing-incidence interferometer includes first and second spaced-apart diffractive optical elements with a generally planar object disposed therebetween. The first diffractive optical element forms sheared first-diffracted-order light beams that reflect from opposite first and second surfaces of the object at grazing-incidence angles, while a zero-diffracted-order light beam goes unreflected. The second diffractive optical element combines the unreflected zero-diffracted-order light beam and the sheared reflected beams to form a collimated, combined beam. A 1× double-telecentric relay system relays the combined beam to a folding optical system that forms first and second interference images on a diffusing screen located at an image plane. Digital images of the first and second interference images are obtained and processed to characterize the thickness variation of the object.