Low thermal stress catadioptric imaging optics
Imaging optics 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 optical elements located on the object plane side of the aperture stop; and a second group of optical elements located on the image plane side of the aperture stop. The optical elements in the first and second groups that are immediately adjacent to the aperture stop are refractive lens elements fabricated using optical materials having a negligible susceptibility to thermal stress birefringence, and the other optical elements are a combination of reflective optical elements and refractive lens elements fabricated using optical materials having at most a moderate susceptibility to thermal stress birefringence.
1. Imaging optics 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 optical elements located on the object plane side of the aperture stop; and
a second group of optical elements located on the image plane side of the aperture stop;
wherein the optical elements in the first and second groups of optical elements that are immediately adjacent to the aperture stop, on both the object plane side and the image plane side, are refractive lens elements fabricated using optical materials having a negligible susceptibility to thermal stress birefringence as characterized by a thermal stress birefringence metric; and
wherein the other optical elements in the first and second groups of optical elements, that are not the refractive lens elements immediately adjacent to the aperture stop, are a combination of reflective optical elements and refractive lens elements, the reflective optical elements including non-planar reflective optical elements having optical power, and the refractive lens elements being fabricated using optical materials having a negligible or a moderate susceptibility to thermal stress birefringence as characterized by the thermal stress birefringence metric, at least one of the refractive lens elements being fabricated using a glass having a moderate susceptibility to thermal stress birefringence;
wherein the thermal stress birefringence metric includes a factor having 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 a negligible susceptibility to thermal stress birefringence satisfy the condition that M 1 ≦0.1×10 −6 W −1 and the glasses having a 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 imaging optics of claim 1 where all of the refractive lens elements satisfy the condition that M 1 ≦0.80×10 −6 W −1 .
3. The imaging optics of claim 1 where at least one of the refractive lens elements is fabricated using an optical material where 0.8×10 −6 W −1 ≦M 1 ≦1.60×10 −6 W −1 .
4. The imaging optics of claim 1 wherein the thermal stress birefringence metric is given by:
M 2 =I 0 LM 1
where I 0 is the optical power density in the optical material, and L is the thickness of the refractive lens element.
5. The imaging optics of claim 1 where the optical materials used to fabricate the refractive lens elements are also selected for properties that relate to achieving adequate image quality performance, including refractive index and chromatic dispersion properties.
6. The imaging optics of claim 1 wherein at least one surface of one or more of the refractive lens elements have an aspheric surface profile.
7. The imaging optics of claim 6 wherein at least one of the aspheric surface profile is an elliptical surface profile, a hyperbolic surface profile, a parabolic surface profile or a free-form surface profile.
8. The imaging system of claim 1 wherein at least one of the reflective optical elements is an off-axis reflective optical element.
9. The imaging optics of claim 1 wherein the imaging optics include relay optics which image a spatial light modulator onto an intermediate image plane, and projection optics which image the intermediate image plane onto a display surface.
10. The imaging optics of claim 9 wherein the relay optics include at least two concentric reflective optical elements arranged in an Offner configuration.
11. The imaging optics of claim 9 wherein the projection optics include at least two reflective optical elements arranged in a telescopic imaging configuration.
12. The imaging optics of claim 1 wherein the imaging optics perform the function of projection optics in a projection system which image an intermediate image plane onto a display surface, and wherein the projection system further includes relay optics which image a spatial light modulator onto the intermediate image plane.
13. The imaging optics of claim 1 wherein the at least some of the optical materials used to fabricate the refractive lens elements are glasses or polymers.
14. The imaging optics of claim 1 wherein both the first and second groups of optical elements include at least one reflective optical element.
15. The imaging optics of claim 1 wherein one of the first and second groups of optical elements includes at least one reflective optical element, and the other of the first and second groups of optical elements includes only refractive lens elements.
16. The imaging optics of claim 1 wherein at least one of the reflective optical elements includes optical materials having a negligible or a moderate susceptibility to thermal stress birefringence.
17. The imaging optics of claim 1 wherein at least one of the reflective optical elements includes a mirrored surface on a back side of the reflective optical element.