If life were simple, we could just go to our local hardware store and there between the mousetraps and the power tools would be an optical phasemeter. We could buy it and take it back to our lab and put it into the adaptive optics system. If only wavefront sensors could be that simple. The problem occurs when we find out that at the higher frequencies of visible and infrared radiation, the phase of an electric field does not directly interact with matter in a way that we can measure. In fact, the amplitude doesn't either (its squared magnitude does).
We can observe the phase of a beam only indirectly. It was this difficulty that limited early adaptive optics engineers. For centuries it was known that two beams can interfere with each other because they are waves. If the beams were coherent, or nearly so, the time average difference in their phases would show up as constructive or destructive interference fringes, or something in-between. These would be a good indicator of the phase difference. If one of the beam's phases were known, or arbitrarily assigned a reference zero, then the intensity from the fringes indicated the phase of the other. This has been the principle used for optical testing since the time of Galileo.
To make the jump into 20th century electro-optics, we had to have a fast way to look at the fringes, particularly from a beam that passes through the atmosphere. The development of electro-optic detectors provided just that, a way to convert the short term intensity of light into an electronic signal, and therefore its wavefront.
Online access to SPIE eBooks is limited to subscribing institutions.