The paper by Zheng et al., reports on applying opposed-view digital holographic microscopy (OV-DHM) to retrieve phase maps associated with live cells in suspension, and from those images inferring the cell refractive index. OV-DHM is a recently developed variant of digital holographic microscopy and employs a special optical geometry based on the Sagnac interferometer. As a result, the imaging is performed with two counter-propagating beams, each providing an off-axis interferogram, which are demodulated to extract two phase images. From the phase maps, the authors extract the refractive index value by assuming a spherical cell shape. This measurement can be obtained from a single measurement, but using two beams in a Sagnac geometry provides additional information that can be useful in determining the plane of focus, as well as in averaging the out-of-focus background.
In principle, in future experiments this refractive index measurement can be used to quantify cell growth during the cell cycle and to obtain correlations with disease states and various treatments. One wonders whether the Sagnac interferometer setup can potentially be used to extract the full 3D distribution of refractive index by solving a scattering inverse problem. Another interesting avenue to explore might be exploiting the sensitivity of this system to motion along the optical axis, which is otherwise difficult to assess. For example, internal vesicle transport within cells generates opposite-sign Doppler frequency shifts to the counter-propagating beams. Thus, with a differential measurement akin to the one presented here, one can potentially extract the Doppler shift and the velocity distribution along the optical axis, in a manner reminiscent of dynamic light scattering.
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