Roland Shack invented the device now known as the Shack–Hartmann wavefront sensor in the early 1970s. This sensor, which in recent years has been commercialized, measures the phase distribution over the cross-section of a given beam of light without relying on interference and, therefore, does not require a reference beam.

The standard method of wavefront analysis is interferometry, where one brings together on an observation plane the beam under investigation (hereinafter the test beam) and a reference beam in order to form tell-tale fringes. The trouble with interferometry is that it requires a reference beam, which is not always readily available. Moreover, the coherence length of the light used in these measurements must be long compared with the path-length difference between the reference and test beams. Thus, when the available light source happens to be broad-band, it becomes difficult (though by no means impossible) to produce high-contrast fringes. The Shack–Hartmann instrument solves these problems by eliminating altogether the need for the reference beam.

Wavefront analysis by interferometry

Before embarking on a discussion of the Shack–Hartmann wavefront sensor, it will be instructive to describe the operation of a conventional interferometer. Consider, for instance, the system of Figure 45.1, where a spherical mirror is under investigation. While grinding and polishing the glass blank, the optician frequently performs this type of test to determine departures of the surface from the desired figure. A point source reflected from a 50/50 beam-splitter is used to illuminate the test mirror.