Lens system for use with an optical disk

Claim 2

A lens system consisting of a single lens unit comprising a positive lens element and a negative lens element cemented to said positive lens element, said lens elements satisfying the following conditions: (1) ν + -ν - >20; and (2) ΔN + /ΔN - <0.1 and ΔN - >0 wherein ν + is the Abbe number of the positive lens element, ν - is the Abbe number of the negative lens element, ΔN + is the temperature coefficient of the refractive index of the positive lens element, and ΔN - is the temperature coefficient of the refractive index of the negative lens element.

Claim 3

A lens system as recited in claim 1, wherein said lens unit consists of said positive lens element and said negative lens element.

Claim 4

A lens system as recited in claim 2, wherein said positive lens element is a 1-st lens element having radii of curvature r 1 and r 2 , said negative lens element is a 2-nd lens element having radii of curvature r 2 and r 3 , said i-th lens elements having said radii, a thickness d i , said refractive index N i , said Abbe number ν i and said temperature coefficient of the refractive index ΔN i as in the following table: ______________________________________ i r.sub.i d.sub.i N.sub.i ν.sub.i ΔN.sub.i ______________________________________ 1 0.6779 0.6336 1.61800 63.4 -3.9 2 -0.3836 0.6300 1.80518 25.4 7.6 3 -0.8633 ______________________________________

Claim 5

A lens system as recited in claim 2, wherein said positive lens element is a 1-st lens element having radii of curvature r 1 and r 2 , said negative lens element is a 2-nd lens element having radii of curvature r 2 and r 3 , said i-th lens elements having said radii, a thickness d i , said refractive index N i , said Abbe number ν i and said temperature coefficient of the refractive index ΔN i as in the following table: ______________________________________ i r.sub.i d.sub.i N.sub.i ν.sub.i ΔN.sub.i ______________________________________ 1 0.7315 0.6320 1.67790 55.3 0.6 2 -0.3772 0.6317 1.80518 25.4 7.6 3 -1.0998 ______________________________________

Claim 6

A lens system as recited in claim 2, wherein said positive lens element is a 1-st lens element having radii of curvature r 1 and r 2 , said negative lens element is a 2-nd lens element having radii of curvature r 2 and r 3 , said i-th lens elements having said radii, a thickness d i , said refractive index N i , said Abbe number ν i and said temperature coefficient of the refractive index ΔN i as in the following table: ______________________________________ i r.sub.i d.sub.i N.sub.i ν.sub.i ΔN.sub.i ______________________________________ 1 0.5638 0.6448 1.48749 70.1 -1.1 2 -0.3178 0.5477 1.78472 25.7 7.2 3 -1.0998 ______________________________________

Claim 7

A lens system as recited in claim 2, wherein said positive lens element is a 1-st lens element having radii of curvature r 1 and r 2 , said negative lens element is a 2-nd lens element having radii of curvature r 2 and r 3 , said i-th lens elements having said radii, a thickness d i , said refractive index N i , said Abbe number ν i and said temperature coefficient of the refractive index ΔN i as in the following table: ______________________________________ i r.sub.i d.sub.i N.sub.i ν.sub.i ΔN.sub.i ______________________________________ 1 0.7575 0.6337 1.71700 47.9 -0.4 2 -0.3929 0.6334 1.84666 23.9 8.9 3 -1.0998 ______________________________________

Claim 8

A lens system as recited in claim 2, wherein said positive lens element is a 1-st lens element having radii of curvature r 1 and r 2 , said negative lens element is a 2-nd lens element having radii of curvature r 2 and r 3 , said i-th lens elements having said radii, a thickness d i , said refractive index N i , said Abbe number ν i and said temperature coefficient of the refractive index ΔN i as in the following table: ______________________________________ i r.sub.i d.sub.i N.sub.i ν.sub.i ΔN.sub.i ______________________________________ 1 0.7255 0.6323 1.67790 55.3 0.6 2 -0.4035 0.6311 1.84666 23.9 8.9 3 -1.0507 ______________________________________