Tomographic Phase Microscopy Quantitative 3D Imaging of Living Cells Refractive Index as an Intrinsic Source of Contrast in Light Microscopy

Most biological cells and tissues exhibit negligible absorption under visible light illumination while there are still distinctive refractive index differences among organelles, which amount to several percents relative to the mean refractive index. For this reason, the refractive index has been served as a much better source of intrinsic contrast than absorption in biological studies. Local variations of refractive index in the specimen induce different phase delays from point to point in the field of view. Phase microscopy techniques such as phase contrast microscopy, differential interference contrast (DIC) microscopy and quantitative phase microscopy are used to image such specimen-induced phase delay to visualise the biological structures [1–3]. These techniques are quite sensitive enough to easily resolve the phase delay induced by a single cell, which is typically around half a radian. However, the phase delay is proportional to the product of refractive index and path length or, more generally, the convolution of the refractive index with the point spread function of the optical system. Thus, phase microscopy techniques provide neither a 3-D image of the cell nor a 3-D map of the refractive index distribution. In visualising transparent biological