Introduction

The term “phase contrast” was originally coined in allusion to the conventional practice of using contrast agents in microscopy to view details in biological samples. Biological specimens are essentially transparent, owing to minimal absorption heightened by the typically thin sample preparations. Thus, they generate poor images with insufficient contrast under a standard microscope. Instead of relying on external contrast agents to improve absorption, the Dutch physicist Frits Zernike invented a phase contrast microscope that uses the phase alterations imparted by a transparent sample onto an incident illumination as the source of contrast in the microscope image to render vivid details of the specimen. By eliminating extraneous chemical agents, Zernike's phase contrast microscope is able to show clear images of living samples, which led to a breakthrough in medicine and biology and earned him the 1953 Nobel Prize in Physics.

The study of biological specimens illustrates one among numerous possible uses of phase visualization. The imaging and visualization of optical phase, such as wavefront modulation, disturbances or aberrations, is a challenging yet often vital requirement in optics. A number of techniques can be applied in fields ranging from optical component testing through to wavefront sensing whenever a qualitative or quantitative analysis of an optical phase disturbance is required. In general, a phase disturbance cannot be directly viewed and a suitable method must therefore be sought to extract information about the wavefront from an indirect measurement. An example of this is the generation of fringe patterns in an interferometer, which gives information about the flatness of an optical component without requiring a physical measurement of the component surface. In this book, we discuss a powerful phase contrast technique coined “generalized phase contrast” [1] that we have developed for the visualization of phase disturbances, outlining the considerable improvements this method offers over previous schemes that lead to a variety of powerful applications in optics and photonics.