  Interferometric scattering microscopy (iSCAT), as an ultrasensitive
fluorescence-free imaging modality, has recently gain enormous attention and
been rapidly developing from demonstration of principle to quantitative
sensing. Here we report on a theoretical and experimental study for iSCAT with
samples having structural dimensions that differ by 4-5 orders of magnitude. In
particular, we demonstrate and intuitively explain the profound effects of
sub-nanometer surface roughness of a glass coverslip and of a mica surface on
the absolute signal and the shape of the point spread function of a gold
nanoparticle. These quantities significantly affect the accuracies for
determining the target size and position in all three dimensions. Moreover, we
investigate a sample system mimicking a gold nanoparticle in a simplified cell
environment and show position-dependent and even asymmetric point spread
function of the nanoparticle. The multiscale study will facilitate the
development of high fidelity iSCAT in real applications.
