  Digital holographic microscopy is a thriving imaging modality that attracted
considerable research interest in quantitative biological cell imaging due to
its ability to not only create excellent label-free contrast, but also supply
valuable physical information regarding the density and dimensions of the
sample with nanometer-scale axial sensitivity. This technique records the
interference pattern between a sample beam and a reference beam, and by
digitally processing it, one can reconstruct the optical path delay between
these beams. Per each spatial point, the optical path delay map is proportional
to the product of the sample physical thickness and the integral refractive
index of the sample. Since the refractive index of the cell indicates its
contents without the need for labeling, it is highly beneficial to decouple
cell physical thickness from its refractive index profile. This manuscript
reviews various approaches of extracting the refractive index from digital
holographic microscopy measurements of cells. As soon as the refractive index
of the cell is available, it can be used for either biological assays or
medical diagnosis, as reviewed in this manuscript.
