US Patent 7031078 Laminated diffractive optical element

An optical element of which diffraction efficiency hardly varies with wavelength is provided by using an optical material satisfying the conditions that nd>−6.667×10−3νd+1.70 and θg,F≦−2×10−3νd+0.59 where nd is a refractive index at d-line, νd is an Abbe number at the d-line, and θg,F is a second order dispersion at d-line, whereby diffraction efficiency is improved in any working visible wavelength region and more precise chromatic aberration correction is obtained.