Moire fringe

When a lattice with a spacing d₁ and another lattice with a spacing d₂, where d₂-d₁<d₁ or d₂, are superposed in parallel, an enlarged lattice with spacing D = d₁・d₂/(d₂-d₁) that is parallel to the original lattice appears. The lattice fringes are called "parallel Moire fringe." When two lattices with the same spacing d are superposed with a rotation by angle α, an enlarged lattice with spacing D = d/α emerges in a direction perpendicular (90°different) to the original lattice. The lattice fringes are called "rotation Moire fringe." An example of applications of Moire fringes is observation of an edge dislocation. When a perfect lattice is superposed on a lattice containing an edge dislocation, where the latter lattice is rotated a little with respect to the former at the dislocation, an enlarged dislocation image appears in a direction perpendicular to the original dislocation.

(1a) Bright-field image of mica showing Moiré fringes, taken at an accelerating voltage of 200 kV.
(1b) Schematic of rotation Moiré fringes. The fringes with an enlarged spacing appear when crystal lattices A and B which have the same spacing are superposed with a small orientation change.

(2) Schematic of an enlarged dislocation image. An enlarged dislocation image is formed when a perfect lattice (B) is superposed on a lattice containing an edge dislocation (A’) with a small orientation change. It should be noted that the enlarged image is perpendicular to the original dislocation.

Related Term(s)