Optical Shop Testing

Chapter 4 - Lateral Shear Interferometers

4.1.   INTRODUCTION

Lateral shearing interferometry is an important field of interferometry and has been
used extensively in diverse applications such as the testing of optical components and
systems and the study of flow and diffusion phenomena in gases and liquids.
Basically, the method of lateral shearing interferometry consists of duplicating
wavefront under study, displacing it laterally by a small amount, and obtaining the
interference pattern between the original and the displaced wavefronts.

Figure 4.1 schematically illustrates the principle of shearing interferometry for (a)
an approximately planar wavefront and (b) spherical wavefront. When the wavefront
is nearly planar, the lateral shear is obtained by displacing the wavefront in its own
plane. If the wavefront is nearly spherical, the lateral shear is obtained by sliding the
wavefront along itself by rotation about an axis passing through the center of
curvature of the spherical wavefront.

There are many physical arrangements that produce lateral shear. The famous
Italian optical scientist Ronchi is the first to have introduced laterally sheared
wavefronts to test optical components in the first half of the 20th century. He
employed diffraction at a set of suitably separated lines to produce zeroth- and
first-order beams. Prior to the discovery of lasers in the 1960s, this became a popular
technique in optical testing, still bearing the inventor name, Ronchi test.

In this chapter, we discuss arrangements that can be obtained by the use of beam
dividers, which divide the amplitude of the incident wavefront but do not change the
shape of the wavefront. This means that plane surfaces coated with semireflecting
material are used as beam dividers. Several arrangements to obtain lateral shear will
be described in this chapter mainly to show that with available components, one can
easily fashion a workable lateral shearing interferometer in one’s laboratory or
optical workshop. Lateral shearing interferometry is basically a one-dimensional
action. When it is performed in two orthogonal directions, it becomes twice a one-
dimensional function.