Confocal Differential Phase Contrast In Scanning Optical Microscopy

In this paper the combination of differential phase contrast microscopy and confocal microscopy is presented. When the energy from one half of the pupil of the collector lens is subtracted from the energy of the other half we get conventional differential phase contrast detection, a simple and powerful way to measure phase structures. One of the disadvantages of the conventional case is the fact that out-of-focus amplitude structures also give a large differential phase contrast signal. By focussing the light of both pupil halves onto point detectors we are able to combine differential phase contrast and confocal microscopy. We can therefore make use of the well known advantage of confocal microscopy, namely the insensitivity to objects out of focus, to reduce the crosstalk between signals due to out-of-focus objects and the signals due to in-focus phase objects. The confocal signal for amplitude structures can also easily be obtained. The real and imaginary parts of the optical transfer function for both confocal and conventional differential phase contrast detection are calculated for different degrees of defocus. The results clearly show the advantages of the confocal option. The implementation of this detection scheme in a scanning optical microscope and experimental results are presented.