  During In Vitro Fertilisation (IVF), combining time-lapse contrast microscopy
with suitable image processing techniques could facilitate non-invasive
classification of oocyte health, driving oocyte selection to maximise success
rates. One image processing technique, Differential Dynamic Microscopy (DDM),
has been used in a variety of microscopy settings including bright field, dark
field and fluorescence. However, in some applications, effects stemming from
the choice of microscopy may invalidate underlying assumptions of DDM. Here, we
study the DDM analysis of differential interference contrast (DIC) microscopy
movies. DIC exhibits a characteristic shadowing effect that gives the illusion
of a 3D appearance, which we show causes deformation of the output DDM matrix.
We present a mathematical description of this deformation, and conclude that,
when studying isotropic motion, no account of the DIC shadow needs to be
considered during DDM analysis. We validate our conclusions with simulated
particle data, and with DIC images of colloidal dispersions. Although this
conclusion does not generally extend to anisotropic motion, we demonstrate that
for directed advection and diffusion behaviour, parameter fitting invariance
still holds. These results validate the current practice of applying DDM to
DIC, and are a foundation for further exploration of DDM in other
phase-contrast image datasets.

Statistical Mechanics