Diffuser plate

A transmissive or reflective diffuser plate having a microlens array constituted by a plurality of microlenses, the microlens array being formed on at least either of a light incident surface or a light exit surface, wherein:

• the microlenses have the same effective diameter and substantially the same curvature and have a structure that creates an optical path length difference for transmitted light or reflected light;
• each of the microlenses is arranged at intervals based on the effective diameter so as to constitute a two-dimensional basic periodic structure;
• the microlenses constitute a basic block constructed by an array of N×N lenses, where N is an integer greater than or equal to 2, having the structure that creates the optical path length difference;
• the basic blocks constitute, by repetitive arrangement thereof, a two-dimensional second periodic structure having a period that is N times as long as a period of the microlenses in the basic periodic structure;
• the basic block is either
• an N×N array C in which elements in p rows and q columns, where p and q are integers that satisfy 1≤(p and q)≤N, are defined according to expression (1):
• C(p,q)=(p−1)(q−1)mod N (1), or
• an N×N array D obtained by an arbitrary row permutation or column permutation performed on the array C;
• and
• the optical path length difference created by the lens positioned in the p-th row and the q-th column with respect to the lens in the first row and the first column in accordance with the array C or D is set to C(p, q)/N times or D(p, q)/N times as long as a wavelength of incident light.