Automatic compensation for phase aberration in digital holographic microscopy

Abstract

Based on the point spread function of the off-axis Fresnel digital holographic system with pre-magnification, the phase aberration introduced by microscope objective is obtained. Using a collimated light as reconstructing wave, the phase aberration introduced by the difference between the reconstructing wave and the reference recording wave is analyzed. This method is very simple, and it is very different from the one proposed by T. Colomb <i>et al</i>. The phase mask that can be used to compensate automatically the phase aberration in the phase reconstruction is obtained. According to the principle of digital holography, it must be pointed out that for Fresnel digital holography, the recording distance must be determined accurately before compensating automatically the phase aberration. This is done by an automatic focusing procedure, which is based on image-gray-entropy-method. The simulation result for a special three-dimensional micro object, which is polluted by a random noise, is presented. The percentage error of the reconstructing distance obtained by the focusing procedure is below 0.6 for SNR = 25. Then an automatic aberration compensation procedure, which is the same as that one proposed by T. Colomb <i>et al</i>., is applied to reconstruct the phase image. The results show that for a weak noise the above method is very effective; for a stronger noise the procedure described here is applied iteratively, starting from the initial values provided by the first evaluation; while for a very strong noise the procedure fails at all. Moreover, after applying a median filter to the primary reconstructed phase image, the aberration of the phase image obtained by further iteration decreases, at the same time the noise is strengthen.