A method for observing phase objects, based on the Hilbert transform is defined. This is achieved by spatial filtering.
The amplitude images obtained are characterized by their high luminosity. The objects are correctly imaged, even if the phase varies slowly (low frequency objects), which is not the case with conventional methods such as the phase contrast and central dark ground methods.
The method also permits easy measurement of geometrical aberrations, for the Hilbert transforms of aberrant wavefronts are more easily interpreted than with the Foucault method.