Patent ID: 8009365

Claim:
A projection optical system with a double telecentricity comprising, in order from a enlargement side: a first lens group with a positive power, including a first lens arranged at a closest position to the enlargement side in the projection optical system; a second lens group with a negative power, including a lens or lenses which start with a negative lens that is a first negative lens to satisfy |Hn/Y|<0.75 from the enlargement side in the projection optical system, and which end with a lens that is the first to satisfy Ln/L 1 a> 0.15 from the enlargement side among the negative lens and lenses at the rear of the negative lens in the projection optical system; and a third lens group with a positive power, including a stop and lenses at the rear of the second lens group, where the lenses in the third lens group include at least two positive lenses satisfying 0.662<0.00055×νh+P, wherein the projection optical system uses a plurality of wavelengths or a wavelength band covering a predetermined range each of which is within a wavelength range from a visible region to a near ultraviolet region, and the projection optical system satisfies the following expression: 1<| f 12 /f 3|<5, where Hn is a height (mm) of an outermost off-axis principal ray, measured from an optical axis when the outermost off-axis principal ray passes through an enlargement-side surface of an n-th lens, Y is a maximum image height (mm) at the enlargement side, Ln is a distance of an air space between an n-th lens and an (n+1)-th lens, L 1 a is a distance from an apex of an enlargement-side surface of the first lens to a surface of the stop, νh is defined as (Nh−1)/(Ni−Ng), P is defined as P=(Ni−Nh)/(Ni−Ng), Nh is a refractive index at h-line, Ni is a refractive index at i-line, Ng is a refractive index at g-line, f 12 is a composite focal length (mm) of the first lens group and the second lens group, and f 3 is a focal length (mm) of the third lens group, where each of the n-th lens for Hn and Ln is an n-th lens to be arranged from the enlargement side in the projection optical system and each n is independently an integer of one or more.