The assessment of phase only filters in imaging systems by the classicaloptical merit functions

摘要

Spatial light modulators (SLM) have found a wide range of applications in many fields of optical imaging and measurement systems. We implement different phase only filters in the pupil plane of an imaging system. The phase only filter is divided in five equally spaced annuli. Each annulus has a different phase transmission and inside each annulus the phase is constant. We analyse first the influence of linear decreasing or increasing phase, second we use one phase annulus with a phase shift of n in different positions over the pupil and finally an alternating phase between 0 and n over the pupil.rnMerit functions of the different filters are calculated. The radial and axial point spread function (PSF) or the 3D line spread function show that in some cases these phase only filters will shift the best image plane. The experimental results show the close correlation to the calculated shift of the best image plane.rnThe strong side lobes that appear in the merit functions lead to the conclusion that the image quality will be influenced as well. This can be confirmed by the calculation and the measurement of the image intensity. So in order to get more information about the expected image it is necessary to study the 3D modulation transfer function (MTF). With the MTF one can see that the contrast decreases for the image obtained with each filter in comparison with the image obtained with the clear pupil.rnThe conclusion of our work is, that it is necessary to study the influence of all merit functions in order to design an optimum filter for a given application.