Stromal area of interest and SD-OCT depth of focus illustrated on a clinical image of a normal cornea.

The stromal area of interest used in our analysis is highlighted in purple. The horizontal (bold) dashed lines illustrate the extent of the depth of field (i.e., b = αzR with zR being the Rayleigh range of a Gaussian beam, i.e. the confocal function extent, and α = 2 characterizing diffuse backscattering) with a possible location of the focal plane zf of the system (solid line; arbitrarily chosen for illustration). The value b = 850 μm was obtained after calculation based on the known device specifications ( considering the waist ω0 as the 15-μm lateral resolution at λ0 = 840±10 nm). The horizontal (non-bold) dashed lines represent the doubling of the depth of field in scattering media, which applies for OCT imaging under the assumption of simple scattering [24]. Note that the peripheral zones of the cornea are out of focus, which may partly explain the lower signal-to-noise ratio and darkening in those areas; the impact of the SD-OCT’s confocal parameters may not be negligible outside the 6-mm-wide region of interest. (TIF)