Development of antireflective coatings realized by thin film systems requires their characterization and optimization of their properties. Functional properties of such interference devices are determined by optical constants and thicknesses of the individual films and various defects taking place in these systems. In optics industry the characterization of the films is mostly performed in a relatively narrow spectral range using simple dispersion models and, moreover, the defects are not taken into account at all. This manner of characterization fails if applied to real-world non-ideal thin film systems because the measured data do not contain sufficient information about all the parameters describing the system including imperfections. Reliable characterization requires the following changes: extension of spectral range of measurements, combination of spectrophotometry and ellipsometry, utilization of physically correct dispersion models (Kramers-Kronig consistency, sum rules), inclusion of structural defects instrument imperfection into the models and simultaneous processing of all experimental data. This enables us to remove or reduce a correlation among the parameters searched so that correct and sufficiently precise determination of parameter values is achieved. Since the presence and properties of the defects are difficult to control independently by tuning of the deposition conditions, the optimization does not in general involve the elimination of defects. Instead they are taken into account in the design of the film systems. The outlined approach is demonstrated on the characterization and optimization of ultraviolet antireflective coating formed by double layer of Al2O3 and MgF2 deposited on fused silica.