Patent ID: 8649094
Filing Date: 2014-02-11
Classification: G02B

Abstract:
1. A lens system having reduced susceptibility to thermally-induced stress birefringence for imaging an object plane to an image plane; comprising: an aperture stop positioned between the object plane and the image plane; a first group of lens elements located on the object plane side of the aperture stop; and a second group of lens elements located on the image plane side of the aperture stop; wherein the lens elements immediately adjacent to the aperture stop, on both the object plane side and the image plane side, are fabricated using glasses having a negligible susceptibility to thermal stress birefringence as characterized by a thermal stress birefringence metric; and wherein the other lens elements in the first or second groups of lens elements, that are not the lens elements immediately adjacent to the aperture stop, are fabricated using glasses having a negligible or a moderate susceptibility to thermal stress birefringence as characterized by the thermal stress birefringence metric, at least one of the other lens elements in the first or second groups of lens elements, that are not the lens elements immediately adjacent to the aperture stop, being fabricated using a glass having a moderate susceptibility to thermal stress birefringence; wherein the thermal stress birefringence metric has the form: where ρ is a coefficient of thermal expansion of the glass, κ is a stress optical coefficient of the glass, α is a light absorption coefficient of the glass, E is a modulus of elasticity of the glass, K is a thermal conductivity of the glass, and μ is a Poisson's ratio of the glass, and wherein the glasses having the negligible susceptibility to thermal stress birefringence satisfy the condition that M 1 ≦0.1×10 −6 W −1 and the glasses having the moderate susceptibility to thermal stress birefringence satisfy the condition that 0.1×10 −6 W −1 ≦M 1 <1.60×10 −6 W −1 .