View article

2. Background

The instrument measures relative displacement and angular rotation using the moiré phenomenon of optical interference (Oster and Nishijima, 1963). The moiré patterns appear when two identical overlapping periodic structures undergo a small relative displacement causing the development of a series of characteristic macroscopic interference fringes. The arrangement and periodicity of these fringes does not necessarily resemble that of the overlapping structures. Its configuration, however, is deterministic and the observed patterns directly reflect the amount and direction of relative movement. The mathematical elegance and striking simplicity of the moiré phenomenon has led to it being applied in a number of branches of physics (Amidror, 2009). The gauge comprises two pairs of two identical overlapping glass plates referred to as the combined indicator (Fig. 1). The plates are each etched with …