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.