Moiré-pattern technique for measurement of fringe spacings

Difficulties are often encountered in measuring the spacing of indistinct fringes such as those generated in speckle photography. Accuracies of 5% are currently typical. By forming a moiré pattern between the fringes and a rotatable reference grid, one can use angular measurements instead of linear measurements, with an accuracy of 1%. The technique can enable precise measurements to be made, over a limited range, of the spatial frequencies of fringe systems, projected images, or grids.