There is provided an optical substrate. The optical substrate includes at least one prism structure. Each of the at least one prism structures has a first surface characterized by a first surface structure function modulated by a second surface structure function, the first surface structure function having characteristics to provide that each of the at least one prism structures has a cross section with at least one curved side to provide defocusing diffusion to light incident on the substrate. The second surface structure function has characteristics to provide additional diffusion to the light incident on the substrate.