Low thermal stress birefringence imaging system
View Patent ↗An imaging system having reduced susceptibility to thermally induced stress birefringence, comprising; a relay lens, which images the object plane onto an intermediate image plane; a projection lens, which images the intermediate image plane onto the display surface. The lens elements that are immediately adjacent to a relay lens aperture stop and a projection lens aperture stop are fabricated using glasses having a negligible susceptibility to thermal stress birefringence, and the other 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. An imaging system having reduced susceptibility to thermally induced stress birefringence, for projecting an image of an object plane onto a display surface, comprising:
a relay lens, which images the object plane onto an intermediate image plane, the relay lens having a relay lens aperture stop, a first group of lens elements located between the object plane and the relay lens aperture stop, and a second group of lens elements located between the relay lens aperture stop and the intermediate image plane; and
a projection lens, which images the intermediate image plane onto the display surface, the imaging lens having a projection lens aperture stop, a third group of lens elements located between the intermediate image plane and the projection lens aperture stop, and a fourth group of lens elements located between the projection lens aperture stop and the display surface;
wherein the lens elements in the first and second groups of lens elements that are immediately adjacent to the relay lens aperture stop and the lens elements in the third and fourth groups of lens elements that are immediately adjacent to the projection lens 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, second, third, and fourth groups of lens elements, that are not the lens elements immediately adjacent to the relay lens aperture stop and the projection lens aperture stop, are fabricated using glasses having at most a moderate susceptibility to thermal stress birefringence as characterized by the thermal stress birefringence metric.
2. The imaging system of claim 1 wherein the thermal stress birefringence metric includes factors relating to a coefficient of thermal expansion of the glass, a stress optical coefficient of the glass and a light absorption coefficient of the glass.
3. The imaging system of claim 2 wherein the thermal stress birefringence metric further includes at least one additional factor relating to a modulus of elasticity of the glass, a thermal conductivity of the glass, a Poisson's ratio of the glass, an optical power density in the glass, a thickness of the lens element or a clear aperture size of the lens element.
4. The imaging system of claim 2 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.
5. The imaging system of claim 2 wherein the thermal stress birefringence metric is given by:
M 1 =ρκαE /( K ·(1−μ)),
where ρ is the coefficient of thermal expansion, κ is the stress optical coefficient, α is the light absorption coefficient, E is the modulus of elasticity, K is the thermal conductivity, and μ is Poisson's ratio.
6. The imaging system of claim 5 wherein glasses having a negligible susceptibility to thermal stress birefringence satisfy the condition that M 1 ≦0.1×10 −6 W −1 and glasses having at most a moderate susceptibility to thermal stress birefringence satisfy the condition that M 1 ≦1.60×10 −6 W −1 .
7. The imaging system of claim 6 where all of the lens elements satisfy the condition that M 1 ≦0.80×10 −6 W −1 .
8. The imaging system of claim 6 where at least one of the lens elements is fabricated using a glass where 0.8×10 −6 W −1 ≦M 1 ≦1.60×10 −6 W −1 .
9. The imaging system of claim 2 wherein the thermal stress birefringence metric is given by:
M 2 =I 0 LρκαE /( K ·(1−μ),
where I 0 is the optical power density in the glass, L is the thickness of the lens element, ρ is the coefficient of thermal expansion, κ is the stress optical coefficient, α is the light absorption coefficient, E is the modulus of elasticity, K is the thermal conductivity, and μ is Poisson's ratio.
10. The imaging 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.
11. The imaging system of claim 1 wherein one or more surfaces of one or more of the lens elements have an aspheric surface profile.
12. The imaging system of claim 1 wherein the imaging lens further includes a diffractive optical element.
13. The imaging system of claim 1 wherein the imaging lens further includes one or more polarization compensators that compensate for the polarization behavior of the relay lens, the projection lens, a dichroic combiner preceding the relay lens, or for a combination of these components.
14. The imaging system of claim 1 wherein the imaging lens further includes mirror with a multi-layer dielectric coating which differentially alters the path length for different wavelength bands, thereby reducing chromatic aberrations.
15. The imaging system of claim 1 wherein at least one of the relay lens or the projection lens has the form of a classical double Gauss lens.
16. The imaging system of claim 1 wherein at least one of the relay lens or the projection lens 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.
17. The imaging system of claim 1 wherein the imaging system is used to image polarized light.
18. The imaging system of claim 1 further including a despeckler device located proximate to the intermediate image plane.
19. The imaging system of claim 17 wherein the despeckler device is a moving diffuser or a lenslet array.
20. An imaging system having reduced susceptibility to thermally induced stress birefringence, for projecting an image of an object plane onto a display surface, comprising;
a relay lens, which images the object plane onto an intermediate image plane, the relay lens having a relay lens aperture stop, a first group of lens elements located between the object plane and the relay lens aperture stop, and a second group of lens elements located between the relay lens aperture stop and the intermediate image plane; and
a projection lens, which images the intermediate image plane onto the display surface, the imaging lens having a projection lens aperture stop, a third group of lens elements located between the intermediate image plane and the projection lens aperture stop, and a fourth group of lens elements located between the projection lens aperture stop and the display surface;
wherein the lens elements in the first and second groups of lens elements experiencing the highest optical power densities are fabricated using glasses having a negligible susceptibility to thermal stress birefringence as characterized by a thermal stress birefringence metric; and
wherein all of the lens elements in the first, second, third, and fourth 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.
21. A laser-based projection system having reduced susceptibility to thermally induced stress birefringence which projects an image onto a display surface, comprising:
at least one laser-based illumination system that provides polarized illumination light to at least one spatial light modulator device that modulated the polarized illumination light responsive to image data;
a relay lens assembly that images light collected from at least one spatial light modulator to an intermediate image plane, the relay lens having a first group of lens elements and a second group of lens elements located to either side of a relay lens aperture stop; and
a projection lens assembly that images light collected from the intermediate image plane to the display surface, the imaging lens having a third group of lens elements and a fourth group of lens elements located to either side of a projection lens aperture stop; and
wherein the lens elements immediately adjacent to the relay lens aperture stop and the projection lens aperture stop, are fabricated using glasses having a negligible susceptibility to thermal stress birefringence induced by light absorbed while transiting these lens elements as characterized by a thermal stress birefringence metric; and
wherein the other lens elements in the first, second, third, and fourth group of elements, that are not the lens elements immediately adjacent to the relay lens aperture stop and the projection lens aperture stop, are fabricated using glasses that have at most a moderate susceptibility to thermal stress birefringence induced by light absorbed while transiting these lens elements as characterized by a thermal stress birefringence metric.
22. The laser-based projection system of claim 21 wherein the thermal stress birefringence metric is given by:
M 1 =ρκαE ( K· (1−μ)),
where ρ is the coefficient of thermal expansion, κ is the stress optical coefficient, α is the light absorption coefficient, E is the modulus of elasticity, K is the thermal conductivity, and μ is Poisson's ratio.
23. The laser-based projection system of claim 22 wherein glasses having a negligible susceptibility to thermal stress birefringence satisfy the condition that M 1 ≦0.1×10 −6 W −1 and glasses having at most a moderate susceptibility to thermal stress birefringence satisfy the condition that M 1 <1.60×10 −6 W −1 .
24. The laser-based projection system of claim 21 further including an optical despeckler device, located proximate to the intermediate image plane.
25. The laser-based projection system of claim 21 wherein the laser-based projection system is used to provide stereoscopic images, and wherein the illumination system includes a polarization switching device which alternately provides polarized light of two orthogonal polarization states, the polarized light in one polarization state being used to project a left-eye stereoscopic image and the polarized light in the other polarization state being used to project a right-eye stereoscopic image.