In adaptive / active aptics and computer aided manufacturing of large optical mirrors, Shack- Hartmann (S-H) [ 1] method which is derived from the well-known conventional Hartmann technique is well used to test wavefront in real time because it has many advantages. a) It collects and samples virtually 100% of the light entering the optical system; b) It measures the even wavefront slope over each zone even when the phase of the light from one side of the subaperture to the other side exceeds 2ir , which is well suitable for adaptive / active optical systems as well as reflectors; c) It requires only one detector array compared to four for a shearing interferometer with same optical efficiency; d) It can also detect wavefront tilt of white light because it is independent of wavelength; e) It is mechanically less complex than the shearing interferometers, replacing optical and mechanical hardware with electronic processing. Fig. 1 shows the principle of S-H wavefront sensor. The detected wavefront and plane reference wavefront are both divided into a number of zones, usually contiguous and equal size, by S-H plate which is a small lens array and imaged into separate foci by sampling lens. The distance between couple of foci corresponding to detected wavefront and reference wavefront reveals the mean wavefront slope over each zone. From the principle, we know that S-H sensor only can measure the average wavefront slope over each zone, unable to test the slope over the area smaller than the zone. Another disadvantage of S-H sensor seems to be its sensitivity to the shape of the source to be corrected in the case of low signal conditions.
"A new type of shearing SH wavefront sensor", Proc. SPIE 1230, International Conference on Optoelectronic Science and Engineering '90, 123013 (1 July 1990); doi: 10.1117/12.2294676; https://doi.org/10.1117/12.2294676