# US 8,786,943 B2 · Low thermal stress catadioptric imaging system > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 8,786,943 B2 - Document ID: US8786943B2 - Status: Granted - Kind: B2 - Application number: 13/282,503 - Filed: 2011-10-27 - Published: 2013-05-02 - Granted: 2014-07-22 - Earliest claimed: 2011-10-27 - Assignees: Eastman Kodak Company - Applicants: Barry David Silverstein; Joseph Raymond Bietry; Andrew F. Kurtz; Robert J. Metzger - Inventors: Barry David Silverstein; Joseph Raymond Bietry; Andrew F. Kurtz; Robert J. Metzger - Web page: https://patentgrep.com/patent/US8786943B2 ## CPC - G02B 7/028: {with means for compensating for changes in temperature or for controlling the temperature; thermal stabilisation} - G02B 13/16: for use in conjunction with image converters or intensifiers {, or for use with projectors, e.g. objectives for projection TV} - G02B 17/008: {Systems specially adapted to form image relays or chained systems} - G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS ## Abstract An imaging system having reduced susceptibility to thermally-induced stress birefringence comprising relay optics and projection optics. One of either the relay optics or the projection optics is a reflective optical system that includes reflective optical elements, and the other is a refractive optical system having a negligible or low susceptibility to thermal stress birefringence. The refractive optical system includes: a first group of refractive lens elements located upstream from an aperture stop, and a second group of refractive lens elements located downstream from the aperture stop. The refractive lens elements in the first and second groups that are immediately adjacent to the aperture stop are fabricated using optical materials having a negligible susceptibility to thermal stress birefringence, and the other refractive lens elements in the first and second groups are fabricated using optical materials having at most a moderate susceptibility to thermal stress birefringence. ## Claims ### Claim 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: imaging optics including relay optics that image the object plane onto an intermediate image plane and projection optics that image the intermediate image plane onto the display surface; wherein one of either the relay optics or the projection optics is a reflective optical system that includes reflective optical elements, and the other of the relay optics or the projection optics is a refractive optical system having a negligible or low susceptibility to thermal stress birefringence, the refractive optical system including: a first group of refractive lens elements located upstream from an aperture stop; and a second group of refractive lens elements located downstream from the aperture stop; wherein the refractive lens elements in the first and second groups of refractive lens elements that are immediately adjacent to the aperture stop are fabricated using optical materials having a negligible susceptibility to thermal stress birefringence as characterized by a thermal stress birefringence metric, and the other refractive lens elements in the first and second groups of refractive lens elements, that are not the refractive lens elements immediately adjacent to the aperture stop, are fabricated using optical materials having at most a moderate susceptibility to thermal stress birefringence as characterized by the thermal stress birefringence metric. ### Claim 2 (depends on 1) The imaging system of claim 1 wherein the thermal stress birefringence metric includes factors relating to a coefficient of thermal expansion of the optical material, a stress optical coefficient of the optical material and a light absorption coefficient of the optical material. ### Claim 3 (depends on 2) The imaging system of claim 2 wherein the thermal stress birefringence metric associated with a particular refractive lens element in the first and second groups of refractive lens elements further includes at least one additional factor relating to a modulus of elasticity of the optical material, a thermal conductivity of the optical material, a Poisson's ratio of the optical material, an optical power density in the optical material, a thickness of the particular refractive lens element or a clear aperture size of the particular refractive lens element. ### Claim 4 (depends on 2) The imaging system of claim 2 wherein the thermal stress birefringence metric is given by: M1′=ρκα. where M1′ is the thermal stress birefringence metric, ρ is the coefficient of thermal expansion, κ is the stress optical coefficient, and α is the light absorption coefficient. ### Claim 5 (depends on 2) The imaging system of claim 2 wherein the thermal stress birefringence metric is given by: M1=ρκαE/(K·(1−μ)). where M1 is the thermal stress birefringence metric, 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. ### Claim 6 (depends on 5) The imaging system of claim 5 wherein the optical materials having a negligible susceptibility to thermal stress birefringence satisfy the condition that M1≦0.1×106 W−1 and the optical materials having at most a moderate susceptibility to thermal stress birefringence satisfy the condition that M1≦1.60×10−6 W−1. ### Claim 7 (depends on 6) The imaging system of claim 6 where all of the refractive lens elements in the first and second groups of refractive lens elements satisfy the condition that M1≦0.80×10−6 W−1. ### Claim 8 (depends on 6) The imaging system of claim 6 where at least one of the refractive lens elements in the first and second groups of refractive lens elements is fabricated using an optical material where 0.8×10−6 W−1≦M1≦1.60×10−6 W−1. ### Claim 9 (depends on 2) The imaging system of claim 2 wherein the thermal stress birefringence metric associated with a particular refractive lens element in the first and second groups of refractive lens elements is given by: M2=I0LρκαE/(K·(1−μ)). where M2 is the thermal stress birefringence metric, I0 is the optical power density in the optical material, L is the thickness of the particular refractive 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. ### Claim 10 (depends on 1) The imaging system of claim 1 where the optical materials used to fabricate the refractive lens elements in the first and second groups of refractive lens elements are also selected in accordance with their refractive index and chromatic dispersion properties. ### Claim 11 (depends on 1) The imaging system of claim 1 wherein one or more surfaces of one or more of the refractive lens elements in the first and second groups of refractive lens elements or the reflective optical elements have an aspheric surface profile. ### Claim 12 (depends on 1) The imaging system of claim 1 wherein the reflective optical system includes at least two concentrically positioned reflective optical elements arranged in an Offner configuration. ### Claim 13 (depends on 1) The imaging optics of claim 1 wherein the reflective optical system include at least two reflective optical elements arranged in a telescopic imaging configuration. ### Claim 14 (depends on 1) The imaging system of claim 1 wherein the reflective optical system includes one or more off-axis reflective optical elements. ### Claim 15 (depends on 1) The imaging system of claim 1 wherein the reflective optical system has no central obscuration. ### Claim 16 (depends on 1) The imaging system of claim 1 wherein the reflective optical system includes at least one reflective optical element having an aspheric shape profile. ### Claim 17 (depends on 16) The imaging system of claim 16 wherein the aspheric shape profile is an elliptical shape profile, a hyperbolic shape profile, a parabolic shape profile, or a free-form shape profile. ### Claim 18 (depends on 1) The imaging system of claim 1 wherein the at least some of the optical materials used to fabricate the refractive lens elements are glasses or polymers.