Abstract: Phase is modulated in a grazing incidence interferometer by modulating the incidence angle and spacing the test surface far enough from the reference surface so that the incidence angle modulation changes the phase relationship between the interferring beam reflected from the test surface and the reference beam reflected from the reference surface substantially more than it changes the sensitivity of the interferometer to surface variations between the reference and test surfaces. The preferred way of accomplishing this is with tiltable mirror 15 modulating the incidence angle slightly relative to prism 11 with its reference surface 12 and test surface 13 spaced from reference surface 12 in interferometer 10. An electromagnetic element 16 such as a galvanometer is preferred for tilting mirror 15.

Abstract: A multifaceted mirror 10 is formed of individual plane mirrors 15 bonded to a support element or core 11 by a cement 14 in positions that are accurately located independently of the core. A base 30 has faces 31 accurately in the plane of the mirror faces, and locator blocks 40 having plane reference surfaces are secured to faces 31 to provide supports for accurately locating mirrors 15. The core 11 is placed inside a positioned array of mirrors 15, and the gaps between the mirrors and the core are filled with a cement such as epoxy. Trim strips 20 can be added to extend between and over the edges of the mirrors.