Phase Aberration Compensation In Digital Holographic Microscopy For Biological Cells

Digital holography is a powerful imaging technique which can simultaneously and independently retrieve the amplitude and phase information of the wave transmitted through the object. Quantitative phase-contrast image is of particular interest since the sample detected like biological cells is almost transparent. However, there is the phase aberration in the reconstructed image which must be compensated. Consequently, the experimental parameters need to be preknown, such as the recording distance, the angle between the object wave and the reference wave, the magnification of the microscope objective and so on. Moreover these parameters are requested to be with high precision. Unfortunately, the actual value of these parameters is always different from the theoretic value. Then, it will lead to the phase aberration for the quantitative phase-contrast image. Nowadays, many methods are presented to compensate the phase compensation.In this paper we investigate the phase aberration compensation in digital holography on biological cells. The conception, classification and causation of the phase aberration have been discussed and especially the compensation for the phase aberration has been researched. Many significative results have been proposed as follows:1. The conception, classification and causation of the phase aberration have been studied. The mathematical equation of the tilt and second phase aberration has been given for the off-axis Fresnel and pre-magnification digital holography. For the lensless Fourier transform digital holography, three reconstruction algorithms have been analyzed which are the single Fourier inverse transform algorithm, Fresnel algorithm and angular spectrum algorithm, respectively. And the mathematical equation of the phase aberration has been discussed according to the different reconstruction algorithms.2. An automatic procedure for the non-coplanar aberration compensation is proposed in the lensless Fourier transform digital holography. With the initial measurement of the distance between the reference source and the hologram, the optimum parameters corresponding to the phase-contrast image can be achieved by a single hologram, combined with the linearity fitting. Here we propose a new, to our knowledge, autofocus algorithm which is based on the phase distribution of the reconstruction image. It is worthy to point out that most of the autofocus algorithms in digital holography apply the criterion function to the amplitude distribution of the reconstructed image. While the live cells are generally transparent, the autofocus algorithm based on the phase distribution will be more reliable.3. A surface fit based on the least square method is used to compensate the phase aberration of the thin object with a single hologram. The subtraction of the fitted aberrations from the original phase distribution gives the phase contrast image of cervical carcinoma cells free of aberration. It proves that the method is simple and useful in the lensless Fourier transform digital holography on the biological cells. However, it maybe does not work when the magnification is high in the pre-magnification digital holography.4. An approach is proposed for eliminating the tilt phase aberration introduced by the tilt reference wave with a single hologram. This tilt aberration will be corrected by shifting the filtered hologram spectrum to the coordinate center of the frequency domain, without knowledge of the focal length of the imaging lens or distances in the setup. It is worthy to notice that the method is based on the argument distribution of the hologram spectrum to locate exactly the position of the carrier frequency of the virtual image. The experiment results show that there always exists a maximum among the argument distribution. And to compensate the tilt phase aberration will make the subsequent quadratic aberration compensation simple, because only a single parameter needs to be adjusted. When the quadratic pattern disappears, the proper phase contrast images will be obtained.