A method of illuminating a layer of a material, in particular a photosensitive resin, using a light source, in order to expose an area of that material to a useful dose of light for subsequent etching of that material in that area, consisting in effecting a first exposure through a pattern of a first mask made up of a central hole and peripheral holes with a first dose of light less than said useful dose, and a second exposure through a pattern of a second mask made up of a single hole with a second dose of light such that the cumulative total of said first dose induced through the central hole of the first mask and the second dose induced through the single hole of said second mask produces at least said useful dose over said area.