  The use of Wavefront Sensors (WFS) is nowadays fundamental in the field of
instrumental optics. This paper discusses the principle of an original and
recently proposed new class of WFS. Their principle consists in evaluating the
slopes of the wavefront errors by means of varying density filters placed into
the image plane of the tested optical system. The device, sometimes called
'optical differentiation WFS' is completed by a digital data-processing system
reconstructing the wavefront from the obtained slopes. Various luminous sources
of different wavelengths and spectral widths can be employed. The capacities of
the method are discussed from the geometrical and Fourier optics points of
view, then by means of numerical simulations showing that the ultimate accuracy
can be well below lambda/10 and lambda/100 Peak-to-Valley (PTV) and RMS
respectively, provided that certain precautions are taken.
