A method of and a solution for making a
highly porous optical
antireflection coating of a selectively designed index of
refraction, by applying a colloidal dispersion derived from hydrolytically condensing, in the presence of water and a catalyst, one or more
silicon compounds of the general formula RaSiX4-a, or precondensates derived therefrom, to a substrate. In the formula, R is an
organic group having from 1 to 10 carbon atoms which may be interrupted by
oxygen atoms and / or
sulfur atoms and / or amino groups, X is
hydrogen,
halogen, hydroxy, alkoxy, acyloxy, alkylcarbonyl, alkoxycarbonyl or NR'2, R' being
hydrogen,
alkyl or
aryl and a being 0, 1 or 2. The solution also contains colloidally dispersed organic polymers at a
molar ratio, relative to the
silane, between 0.1 mmol / mol
silane and 100 mmol / mol
silane, the median molecular
mass of the
polymer being between 200 and 500,000.
Sol-vents, preferably
alcohol, may also be present in the solution. After being applied to an optical substrate, the solution is dried and organic components are removed from it to leave a
porous coating of predetermined index of
refraction.