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Optical Shop Testing

# Chapter 1 - Newton, Fizeau, and Haidinger Interferometers

1.1.   INTRODUCTION

This chapter has been updated by the second author; it includes much of the material
from the previous version of the book. Newton, Fizeau, and Haidinger interferometers
are among the simplest and most powerful tools available to a working optician.
With very little effort, these interferometers can be set up in an optical workshop for
routine testing of optical components to an accuracy of a fraction of the wavelength
of light. Even though these instruments are simple in application and interpretation,
the physical principles underlying them involve a certain appreciation and application
of physical optics. In this chapter, we examine the various aspects of these
interferometers and also consider the recent application of laser sources to them. The
absolute testing of flats will also be considered in this chapter.

1.2.   NEWTON INTERFEROMETER

We will take the liberty of calling any arrangement of two surfaces in contact
illuminated by a monochromatic source of light a Newton interferometer. Thus,
the familiar setup to obtain Newton rings in the college physical optics experiment is
also a Newton interferometer; the only difference being the large air gap as one
moves away from the point of contact, as seen in Figure 1.1. Because of this, it is
sometimes necessary to view these Newton rings through a magnifier or even a low-
power microscope. In the optical workshop, we are generally concerned that an
optical flat, one being fabricated, is matching the accurate surface of another
reference flat or that a curved spherical surface is matching the correspondingly
opposite curved spherical master surface. Under these conditions, the air gap is
seldom more than a few wavelengths of light in thickness. In the various forms of the

Newton interferometer, we are mainly interested in determining the nonuniformity of
this air gap thickness by observing and interpreting Newton fringes. A simple way to
observe these Newton fringes is illustrated in Figure 1.2. Any light source such as a
sodium vapor lamp, low-pressure mercury vapor lamp, or helium discharge lamp can
be used in the setup. Under certain situations, even an ordinary tungsten lamp can
serve this purpose.