Comparison of optical quadrature microscopy and Shack-Hartmann wavefront sensor

Abstract

Several quantitative phase imaging techniques, such as digital holography, Hilbert-phase microscopy, and phase-shifting interferometry have applications in biological and medical imaging. Quantitative phase imaging measures the changes in the wavefront of the incident light due to refractive index variations throughout a 3-D specimen. We have developed a multimodal microscope which combines optical quadrature microscopy (OQM) and a Shack- Hartmann wavefront sensor for applications in biological imaging. OQM is an interferometric imaging modality that noninvasively measures the amplitude and phase of a signal beam that travels through a transparent specimen. The phase is obtained from interferograms with four different delayed reference wavefronts. The phase is then transformed into a quantitative image of optical path length difference. The Shack-Hartmann wavefront sensor measures the gradient of the wavefront at various points across a beam. A microlens array focuses the local wavefront onto a specific region of the CCD camera. The intensity is given by the maximum amplitude in the region and the phase is determined based on the exact pixel position within the region. We compare the amplitude and quantitative phase information of poly-methyl-meth-acrylate (PMMA) beads in oil and one-cell and two-cell mouse embryos with micrometer resolution using OQM and the Shack-Hartmann. Each pixel in OQM provides a phase measurement, whereas multiple pixels are used in Shack-Hartmann to determine the tilt. Therefore, the simple Shack-Hartmann system is limited by its resolution and field-of-view. Real-time imaging in Shack-Hartmann requires spatial averaging which smoothes the edges of the PMMA beads. The OQM has a greater field-of-view with good resolution; however, it is a complex system requiring multiple optical components and four cameras which may introduce additional artifacts in processing quantitative images. The OQM and Shack- Hartmann has certain advantages depending on the application. A combination of these two systems may provide improved quantitative phase information than either one alone.cHJl