An optical substrate possesses a structured surface that enhances luminance or brightness and reduces the effects of structural defects on perceived image quality. User perceivable image cosmetic defects caused by manufacturing or handling, can be masked by introducing structural irregularities in the optical substrate, which may be non-facet flat sections or in-kind to the defects. Optical defects caused by non-facet flat sections in the prism structure of the optical substrate (e.g., flat-bottom valleys with a certain valley bottom thickness above the base layer, and/or flat-top peaks, and/or openings in the optical substrates that expose flat sections of underlying base layer) can be masked by providing distributed in-kind non-facet flat sections (e.g., flat-bottom valleys, and/or flat-top peaks, and/or openings exposing sections of underlying base layer), to diffuse the prominence of the original defects with the introduced irregularities.