Wavefront sensing with varying transmission filters: past, present, and future

The use of Wavefront Sensors (WFS) is nowadays essential in the field of instrumental optics. In this communication, I present and discuss the principle of a new, recently proposed family of WFS. Their principle consists in evaluating the slopes of the Wavefront Errors (WFE) by means of varying density filters placed into the image plane of the tested optical system (the device is sometimes called "optical differentiation WFS"). The wavefronts are reconstructed from the obtained slopes digitally. Various luminous sources of different wavelengths, spectral widths and spatial sizes can be employed. The capacities of the method are discussed from the geometrical and Fourier optics points of view, then illustrated by a few numerical simulations showing some examples of practical applications. Two different measurement configurations are also schematically described and commented.