Low thermal stress birefringence imaging lens
View Patent ↗An imaging lens 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 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 are fabricated using glasses having at most a moderate susceptibility to thermal stress birefringence as characterized by the thermal stress birefringence metric.
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:
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 <1.60×10 −6 W −1 .
2. The lens system of claim 1 wherein the thermal stress birefringence metric further includes at least one additional factor relating to an optical power density in the glass, a thickness of the lens element or a clear aperture size of the lens element.
3. The lens system of claim 1 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 that satisfy the condition that M 1 ≦0.80×10 −6 W −1 .
4. The lens system of claim 1 wherein 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 is fabricated using a glass where 0.8×10 −6 W −1 ≦M 1 ≦1.60×10 −6 W −1 .
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 L M 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 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.
7. The lens system of claim 1 wherein one or more surfaces of one or more of the lens elements have an aspheric surface profile.
8. The lens system of claim 1 wherein the lens system further includes a diffractive optical element or a polarization compensator.
9. The lens system of claim 1 wherein the lens system further includes mirror with a multi-layer dielectric coating which differentially alters a path length for different wavelength bands, thereby reducing chromatic aberrations.
10. The lens system of claim 1 wherein the lens system has the form of a classical double Gauss lens.
11. The lens system of claim 1 wherein the lens system has the form of a classical double Gauss lens, but where one of the lens elements adjacent to the aperture stop is fabricated using a crown glass.
12. The lens system of claim 1 wherein the lens system is used to image polarized light.
13. The lens system of claim 1 wherein the lens system is a relay lens or a projection lens for a digital projection system.
14. The lens system of claim 1 wherein the lens system is an imaging lens system or an illumination lens system.