Reverse-Shearing Interferometry

Abstract

THE introduction of the shearing principle in the examination of optical wave-fronts by interferometry¹ has led to a great simplification of the apparatus required. Recent developments^2,3 have made possible the construction of compact portable interferometers for the examination of optical systems of large size, and several instruments are now available from optical manufacturers. For some purposes, however, it is possible to use as a shearing interferometer an even simpler system which is already in wide use for another purpose. This is the square prism used in photometry for dividing a beam of light into two. It is composed of two similar right-angled prisms cemented at their hypotenuse faces after one face has been coated with a semi-reflecting film. An application of this prism to the interferometry of length was first described by Kösters⁴, but the prism is also very useful as a shearing interferometer.