Oblique incidence interferometer with fringe scan drive

An oblique incidence interferometer capable of fringe scanning by use of a fringe scan drive association with a first or second diffraction grating member of the interferometer to drive the associated grating member step by step over a micrometric distance in a direction perpendicular to a path of light in travel in a straightforward direction. Laser light from a light source 20 is diffracted into a zero order diffraction wave L1 in travel in a straightforward direction and a +1 order diffraction wave L2 by a first diffraction grating member 26 of a grating assembly 41. The grating assembly 41 is vertically movably supported on a surface plate 42 by a pair of level support members 43 each having a stratified leaf spring structure. The grating assembly 41 sits on a piezoelectric actuator 45 which serves as a fringe scan drive for driving the first diffraction grating member 26 over a micrometric distance in the vertical direction. As the first diffraction grating member 26 is moved in a direction perpendicular to the light path of incident light, +1 order diffraction wave incident on a specimen 28 is shifted in phase, causing a displacement of a predetermined extent to interference fringes as observed through an image sensor 32.