Low thermal stress birefringence imaging lens

1. A lens system having reduced susceptibility to thermally-induced stress birefringence for imaging an object plane onto an image plane comprising;

a plurality of lens elements arranged to image the object plane onto the image plane,

wherein the lens element that experiences a highest optical power density from transiting light when the lens system is used to image the object plane onto the image plane is fabricated using a glass having a negligible susceptibility to thermal stress birefringence as characterized by a thermal stress birefringence metric;

wherein the other lens elements 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 being fabricated using a glass having a moderate susceptibility to thermal stress birefringence;

wherein the thermal stress birefringence metric has the form:

M 1 =ρκαE /( K ·(1−μ).

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 <1.6×10 −6 W −1 .

2. The lens system of claim 1 further including an aperture stop positioned between the object plane and the image plane, the lens elements being arranged in two lens groups, with one lens group on each side of the aperture stop; wherein the lens elements in the two lens group that are proximate to the aperture stop are fabricated using glasses having the negligible susceptibility to thermal stress birefringence.

3. The lens system of claim 1 wherein one or more of the lens elements are assembled into at least one lens element housing, and where at least one of the lens elements or lens element housings is cooled.

4. The lens system of claim 1 wherein the thermal stress birefringence metric further includes at least one additional factor relating to the optical power density in the lens element, a thickness of the lens element or a clear aperture size of the lens element.

5. The lens system of claim 1 wherein the glasses used to fabricate the lens elements are selected based, at least in part, on evaluating an intensity-weighted thermal stress birefringence metric:

M 2 =I 0 LM 1

where I 0 is an optical power density in the glass, and L is a thickness of the lens element.

6. The lens system of claim 1 wherein all of the other lens elements are fabricated using glasses that satisfy the condition that M 1 ≦0.8×10 −6 W −1 .

7. The lens system of claim 1 wherein at least one of the other lens elements is fabricated using a glass that satisfies the condition that 0.8×10 −6 W −1 ≦M 1 ≦1.6×10 −6 W −1 .

8. The lens system of claim 1 where the glasses used to fabricate the lens elements are also selected for properties that relate to achieving adequate image quality performance, including refractive index and chromatic dispersion properties.

9. The lens system of claim 1 wherein one or more surfaces of one or more of the lens elements have an aspheric surface profile.

10. The lens system of claim 1 wherein the lens system is used to image polarized light.

11. The lens system of claim 1 wherein the lens system is a relay lens or a projection lens for a digital projection system.

12. The lens system of claim 1 wherein the lens system is an imaging lens system or an illumination lens system.

13. The lens system of claim 1 wherein the lens system is a combined reflective-refractive optical system that includes at least one reflective optical element.

14. A lens system having reduced susceptibility to thermally-induced stress birefringence for imaging an object plane onto an image plane comprising;

a plurality of lens elements arranged to image the object plane onto the image plane;

wherein the lens element that experiences a highest optical power density from transiting light when the lens system is used to image the object plane onto the image plane is fabricated using a glass having a negligible susceptibility to thermal stress birefringence as characterized by a thermal stress birefringence metric;

wherein the other lens elements 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 being fabricated using a glass having a moderate susceptibility to thermal stress birefringence;

wherein the thermal stress birefringence metric is given by:

M 1 ′=ρκα.

where ρ is the coefficient of thermal expansion, κ is the stress optical coefficient, and α is the light absorption coefficient; and

wherein the glasses having the negligible susceptibility to thermal stress birefringence satisfy the condition that M 1 ′≦0.0008×10 −12 mm/(N K) and the glasses having the moderate susceptibility to thermal stress birefringence satisfy the condition that 0.0008×10 −12 mm/(N K)≦M 1 ′≦0.01×10 −12 mm/(N K).

15. The lens system of claim 14 where all of the other lens elements are fabricated using glasses that satisfy the condition that M 1 ′0.005×10 −12 mm/(N K).

16. The lens system of claim 14 where at least one of the other lens elements is fabricated using a glass that satisfies the condition that 0.005×10 −12 mm/(N K)≦M 1 ′0.01×10 −12 mm/(N K).

17. The lens system of claim 14 where the glasses used to fabricate the lens elements are also selected for properties that relate to achieving adequate image quality performance, including refractive index and chromatic dispersion properties.