Patent ID: 6275343
Filing Date: 2001-08-14
Classification: G02B

Abstract:
A projection optical system comprising, from an enlargement side to a reduction side:a first lens unit having a negative optical power and including only a negative lens element, the first lens unit having at least one aspherical surface;a second lens unit having a positive optical power and including at least one positive lens element;a third lens unit having a positive optical power and including a lens element; andan aperture diaphragm disposed at a front focal point of the third lens unit,wherein the lens element of the third lens unit fulfills the following conditions:0.015<.THETA.-(0.644-0.00168.multidot..nu.d)<0.0665<.nu.d<100where,.THETA.=(ng-nF)/(nF-nC).nu.d=(nd-1)/(nF-nC)ng represents refractive index for g-line (wavelength is 435.84 nm);nF represents refractive index for F-line (wavelength is 486.13 nm);nd represents refractive index for d-line (wavelength is 587.56 nm); andnC represents refractive index for C-line (wavelength is 656.28 nm).