Abstract
Light field microscopy has been proposed as a new high-speed volumetric computational imaging method that enables reconstruction of 3-D volumes from captured projections of the 4-D light field. Recently, a detailed physical optics model of the light field microscope has been derived, which led to the development of a deconvolution algorithm that reconstructs 3-D volumes with high spatial resolution. However, the spatial resolution of the reconstructions has been shown to be non-uniform across depth, with some z planes showing high resolution and others, particularly at the center of the imaged volume, showing very low resolution. In this paper, we enhance the performance of the light field microscope using wavefront coding techniques. By including phase masks in the optical path of the microscope we are able to address this non-uniform resolution limitation. We have also found that superior control over the performance of the light field microscope can be achieved by using two phase masks rather than one, placed at the objective's back focal plane and at the microscope's native image plane. We present an extended optical model for our wavefront coded light field microscope and develop a performance metric based on Fisher information, which we use to choose adequate phase masks parameters. We validate our approach using both simulated data and experimental resolution measurements of a USAF 1951 resolution target; and demonstrate the utility for biological applications with in vivo volumetric calcium imaging of larval zebrafish brain.
Highlights
Light field microscopy, first presented by Levoy et al in 2006 and 2009 [1, 2] and further improved upon by Broxton et al in 2013 [3], is a method for single-snapshot volumetric imaging that employs a microlens array in the optical path of a fluorescence microscope
We showed that unlike traditional microscopy, in which high resolution can only be achieved for a small range of z depths around the native focus plane of the microscope, the volume reconstructed from the captured light field image of the light field microscope (LFM) preserves significant portion of lateral spatial resolution at each z plane; even over a hundred microns away from the native object plane
To understand why the addition of these phase mask trades the resolution of the LFM near the native object plane and farther away from it in this manner, we analyze the point spread function (PSF) under a single microlens as a result of point source in the volume located at two different z depths
Summary
First presented by Levoy et al in 2006 and 2009 [1, 2] and further improved upon by Broxton et al in 2013 [3], is a method for single-snapshot volumetric imaging that employs a microlens array in the optical path of a fluorescence microscope. In this paper we propose a novel extension to the LFM that aims to mitigate the nonuniformity of lateral resolution across depth by placing phase masks in the optical path of the LFM, in order to produce a more uniform performance across depth This technique, called wavefront coding, shapes the point spread function of the microscope and is a natural extension to traditional light field microscopy. This method requires splitting the incoming light between two cameras or capturing the light field and widefield images sequentially, which is less suitable for imaging dynamic phenomena Another approach to improve the low resolution at the native object plane is to divide the light captured by the objective into two optical paths with a known path difference between them, and place two microlens arrays (potentially with different focal lengths) side by side in front of the detector [14].
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