Abstract: A process for forming a two-layer coating on glass or another substrate involves applying first a layer of light metal fluoride on the substrate and subsequently a layer of a porous metal oxide e.g. silica. The metal fluoride layer may be applied by using the sol-gel technique or by other methods e.g. vacuum deposition. The metal oxide layer is preferably applied by a sol-gel technique, by either a dip coating or spin coating. Each layer is preferably about one quarter-wavelength thick. The refractive index of the first layer is lower than that of the substrate, and the second layer has lower refractive index than the first layer. This dependence minimizes reflection of the coating over a broad wavelength range.