Patent ID: 7466642

Claim:
An optical element, comprising: a substrate in which grooves are formed; wherein the expression: 380 nm≦( n− 1)× d≦ 420 nm is satisfied where n is a refractive index of the substrate at a wavelength of 400 nm, and d (nm) is a depth of one step of the grooves; wherein the grooves are formed in four steps of depth d, depth 2 d, depth 3 d and depth 4 d; wherein the depth of the grooves is lined up in the order: depth 2 d, depth 4 d, depth d, depth 3 d, or in the order: depth 3 d, depth d, depth 4 d, depth 2 d; and wherein when light of a wavelength λ 2 in a range of 630 nm to 680 nm passes through the grooves, with respect to each of the grooves, a fractional value smaller than a decimal point of a value obtained by dividing a light path difference attributable to the grooves by the wavelength λ 2 is calculated, and when the calculated fractional values corresponding respectively to the grooves are arranged in an order in which the grooves are lined up, the fractional values are in an order ascending or descending in a stepwise manner.