  An antireflective coating presenting a continuous refractive index gradient
is theoretically better than its discrete counterpart because it can give rise
to a perfect broadband transparency. This kind of surface treatment can be
obtained with nanostructures like moth-eye. Despite the light scattering
behavior must be accounted as it can lead to a significant transmittance drop,
no methods are actually available to anticipate scattering losses in such
nanostructured antireflective coatings. To overcome this current limitation, we
present here an original way to simulate the scattering losses in these systems
and routes to optimize the transparency. This method, which was validated by a
comparative study of coatings presenting refractive indices with either
discrete or continuous gradient, shows that a discrete antireflective coating
bilayer made by oblique angle deposition allows reaching ultra-high mean
transmittance of 98.97% over the broadband [400-1800] nm. Such simple surface
treatment outperforms moth-eye architectures thanks to both interference effect
and small dimensions nanostructures that minimize the scattering losses as
confirmed by finite-difference time domain simulations performed on
reconstructed volumes obtained from electron tomography experiments.
