Patent ID: 7362487

Claim:
An optical element comprising a plurality of subwave structures which have an anti-reflection function for a light beam incident on an optical surface and are two-dimensionally arranged, wherein when directions orthogonal to each other within a plane including a tangent line of a vertex of the optical surface are set as a first direction and a second direction, distances between each of the subwave structures changes in at least one of the first direction and the second direction, and an arbitrary subwave structure of the subwave structures is located with a state in which a distance between the arbitrary subwave structure and one of the subwave structures which is closest to the arbitrary subwave structure is shorter than ½ of an average wavelength of the light beam incident on the optical surface, wherein the following conditional expressions are satisfied, L ave<λ/( Ns+Ni ·sin θ i ), Lσ≦80 (nm), where Li (i=1 to n) represent distances between the arbitrary subwave structure and n subwave structures selected in an increasing order of the distance from the arbitrary subwave structure to each of the n subwave structures, λ represents a minimum wavelength of the light beam incident on the optical surface, Ni represents a refractive index of a medium of a subwave structure located on an incident side of the optical surface, Ns represents a refractive index of a medium of a subwave structure located on an exit side of the optical surface, θi represents an incident angle of the light beam incident on the optical surface, Lσ represents a standard deviation of the distances Li (i=1 to n), and a relationship Lave = ∑ i = 1 n ⁢ Li / n is used.