Equations for filling factor estimation in opal matrix. Addendum to “Deep level emission of ZnO nanoparticles deposited inside UV opal”, [Opt. Commun. 259 (1) (2006) 378–384]

We consider two equations for the filling factor estimation of infiltrated zinc oxide (ZnO) in silica (SiO2) opal and gallium nitride (GaN) in ZnO opal. The first equation is based on the effective medium approximation, while the second one—on Maxwell–Garnett approximation. The comparison between two filling factors shows that both equations can be equally used for the estimation of the quantity of infiltrated nanoparticles inside opal photonic crystal.