Patent ID: 7864427

Claim:
A diffractive optical element comprising: a substrate made of a first resin, the substrate having a surface formed into a diffraction grating pattern; and a coating film made of a composite material comprising a second resin and inorganic particles, the coating film being disposed in contact with the diffraction grating pattern portion of the surface, wherein the first resin is at least one selected from a polycarbonate-based resin, a polystyrene-based resin, and a fluorene-based polyester resin, a refractive index of the composite material determined by Formula 3 below is greater than a refractive index of the first resin, an Abbe's number of the composite material determined by calculating refractive indices with respect to a d-line wavelength (587.6 nm), a F-line wavelength (486.1 nm), and a C-line wavelength (656.3 nm) by using Formula 3 below is greater than an Abbe's number of the first resin, the diffraction grating pattern has a groove depth of not more than 20 Î¼m, and a first-order diffraction efficiency of not less than 80% in a wavelength of 400 nm to 700 nm: n av 2 = n m 2 â¢ { n p 2 + 2 â¢ n m + 2 â¢ p â¡ ( n p 2 - n m 2 ) } n p 2 + 2 â¢ n m - p â¡ ( n p 2 - n m 2 ) [ Formula â¢ â¢ 3 ] where n av represents the refractive index of the composite material, n p represents a refractive index of the inorganic particles, n m represents a refractive index of the second resin, and p represents a content by volume of the inorganic particles in the composite material.