Many laser applications require not only knowledge and control of the intensity distribution of a beam, but also of its propagation characteristics which is mainly determined by the wavefront. Real-time wavefront monitoring is e.g. essential for optimizing the beam focus by precision adjustment of all optical beam line elements. Wavefront sensors based on the Hartmann or Hartmann-Shack principle provide simultaneous recording of beam profiles and wavefronts, allowing, in case of coherent radiation, also numerical propagation of the beam.

The basic principle dates back to 1900 and is based on splitting-up the incident beam into a large number of single beamlets, either via a pinhole array (Hartmann) or a microlens array (Hartmann-Shack). Each of these beamlets generates a spot on the camera detector, whose centroid relative to a reference position defines the local propagation direction. The phase distribution (wavefront) is then reconstructed by integration over the propagation directions. A real time Zernike analysis of the wavefront allows precise evaluation of beam aberrations (e.g. sperical aberration, astigmatism, coma), either of intrinsic nature or caused by external optics. In addition, by integration over all spot intensities the beam profile is determined.