A composite lens structure is disclosed. The composite lens structure includes a diffusion plate and an optical structure formed on the diffusion plate. The optical structure includes a plurality of convex portions arranged in a 2-D matrix along a first direction and a second direction and a plurality of concave portions arranged in a 2-D matrix also along the first direction and the second direction. Each convex portion adjacent to a plurality of concave portions includes a low curvature portion and a high curvature portion higher than the low curvature portion. Each concave portion adjacent to a plurality of convex portions includes a low curvature portion and a high curvature portion lower than the low curvature portion. The convex portions, the concave portions and each junction of the convex portion and the concave portion have a curvature different from 0.