  Glass-ceramics with nanocrystals present a transparency higher than that
expected from the theory of Rayleigh scattering. This ultra-transparency is
attributed to the spatial correlation of the nanoparticles. The structure
factor is calculated for a simple model system, the random sequential addition
of equal spheres, at different volume filling factor. The spatial correlation
given by the constraint that particles cannot superimpose produces a
diffraction peak with a low $S(q)$ in its low-$q$ tail, which is relevant for
light scattering. The physical mechanism producing high transparency in
glass-ceramics is demonstrated to be the low density fluctuation in the number
of scatterers.
