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.