Optically effective surface relief microstructures and method of making them

The present invention relates to an element comprising a surface area with a specific, optically effective surface relief microstructure (12). The surface relief microstructure has a surface modulation of top regions (13) and bottom regions (14), wherein in a first lateral direction of the surface area there is in average at least one transition from a top to a bottom region or vice versa within every 20 micrometer, and in a second lateral direction of the mask, which is perpendicular to the first direction, there is in average at least one transition from a first to a second zone or vice versa within every 200 micrometer. In the microstructure, (i) in the first direction the lateral arrangement of the transitions is non-periodic, and (ii) the top regions substantially lie in the same top relief plateau (15) and the bottom regions substantially lie in the same bottom relief plateau (16). Through scattering effects, the surface relief microstructures are suitable to display images with a positive-negative image flip, which advantageously have a distinct and saturated color appearance but at the same time do not show any rainbow colors. The invention also relates to methods for making such elements as well as to security devices comprising such elements.