Optical system

1. An optical system, comprising: a partial reflector having an average optical reflectance of at least 30% in a desired plurality of wavelengths; and a multilayer reflective polarizer substantially transmitting light having a first polarization state and substantially reflecting light having an orthogonal second polarization state, the multilayer reflective polarizer convex along orthogonal first and second axes, at least one first location on the multilayer reflective polarizer having a radial distance r1 from an optical axis of the multilayer reflective polarizer and a displacement s1 from a plane perpendicular to the optical axis at an apex of the multilayer reflective polarizer, s1/r1 being at least 0.1; and a first quarter wave retarder disposed between the partial reflector and the multilayer reflective polarizer, wherein substantially all chief light rays of the optical system that are incident on the multilayer reflective polarizer have an angle of incidence less than about 25 degrees, wherein the multilayer reflective polarizer comprises at least one layer substantially optically biaxial at at least one first location on the at least one layer away from the optical axis and substantially optically uniaxial at at least one second location on the at least one layer away from the optical axis, the at least one layer at the at least one second location having a first refractive index in a thickness direction, a second refractive index in a second direction orthogonal to the thickness direction, and a third refractive index in a third direction orthogonal to the thickness direction and to the second direction, an absolute value of a difference in the first and second refractive indices being less than 0.02, an absolute value of a difference in the second and third refractive indices being greater than 0.05; and wherein for an area of the multilayer reflective polarizer defined by s1 and r1 a maximum variation of a transmission axis of the multilayer reflective polarizer is less than about 2 degrees.

2. The optical system of claim 1 , wherein the absolute value of the difference in the first and second refractive indices is less than 0.01, and the absolute value of the difference in the second and third refractive indices is greater than 0.1.

3. The optical system of claim 1 , further comprising a first optical stack, the first optical stack comprising a first optical lens and the partial reflector; and a second optical stack, the second optical stack comprising a second optical lens and the multilayer reflective polarizer, wherein the second optical stack is spaced apart from the first optical stack.

4. The optical system of claim 1 , wherein s1/r1 is at least 0.2.

5. The optical system of claim 1 , wherein s1/r1 is in a range of about 0.3 to about 0.6.

6. The optical system of claim 1 , further comprising an image surface and a stop surface, the partial reflector disposed between the image surface and the stop surface, the multilayer reflective polarizer disposed between the partial reflector and the stop surface.

7. The optical system of claim 6 , wherein substantially all chief light rays that pass through the image surface and the stop surface are incident on the multilayer reflective polarizer with an angle of incidence less than about 20 degrees.

8. The optical system of claim 6 , wherein an image source comprises the image surface and the stop surface is an exit pupil.

9. The optical system of claim 6 , wherein an image recorder comprises the image surface and the stop surface is an entrance pupil.

10. The optical system of claim 1 , wherein the at least one layer at the at least one second location has a first refractive index in a thickness direction, a second refractive index in a second direction orthogonal to the thickness direction, and a third refractive index in a third direction orthogonal to the thickness direction and to the second direction, an absolute value of a difference in the first and second refractive indices being less than 0.01, an absolute value of a difference in the second and third refractive indices being greater than 0.1.

11. The optical system of claim 1 , wherein the desired plurality of wavelengths is a wavelength range from 400 nm to 700 nm.

12. The optical system of claim 1 , wherein an azimuthal variation in s1/r1 is less than 2 percent.

13. The optical system of claim 1 , wherein an azimuthal variation in s1/r1 is less than 1 percent.

14. The optical system of claim 1 , wherein for the area of the multilayer reflective polarizer defined by s1 and r1, the maximum variation of the transmission axis of the multilayer reflective polarizer is less than about 1.5 degrees.

15. The optical system of claim 14 , wherein s1/r1 is in a range of about 0.2 to about 0.8.

16. The optical system of claim 1 having a contrast ratio of at least 50 over a field of view of the optical system.

17. An optical system, comprising: a partial reflector having an average optical reflectance of at least 30% in a desired plurality of wavelengths; and a multilayer reflective polarizer substantially transmitting light having a first polarization state and substantially reflecting light having an orthogonal second polarization state, the multilayer reflective polarizer convex along orthogonal first and second axes, at least one first location on the multilayer reflective polarizer having a radial distance r1 from an optical axis of the multilayer reflective polarizer and a displacement s1 from a plane perpendicular to the optical axis at an apex of the multilayer reflective polarizer, s1/r1 being at least 0.1; and a first quarter wave retarder disposed between the partial reflector and the multilayer reflective polarizer, wherein substantially all chief light rays of the optical system that are incident on the multilayer reflective polarizer have an angle of incidence less than about 30 degrees, wherein the multilayer reflective polarizer comprises at least one layer substantially optically biaxial at at least one first location on the at least one layer away from the optical axis and substantially optically uniaxial at at least one second location on the at least one layer away from the optical axis, the at least one layer at the at least one second location having a first refractive index in a thickness direction, a second refractive index in a second direction orthogonal to the thickness direction, and a third refractive index in a third direction orthogonal to the thickness direction and to the second direction, an absolute value of a difference in the first and second refractive indices being less than 0.02, an absolute value of a difference in the second and third refractive indices being greater than 0.05; and wherein the optical system has a contrast ratio of at least 50 over a field of view of the optical system.

18. The optical system of claim 17 , wherein the absolute value of the difference in the first and second refractive indices is less than 0.01, and the absolute value of the difference in the second and third refractive indices is greater than 0.1.

19. The optical system of claim 18 , wherein s1/r1 is in a range of about 0.2 to about 0.8, and wherein for an area of the multilayer reflective polarizer defined by s1 and r1, a maximum variation of a transmission axis of the multilayer reflective polarizer is less than about 2 degrees.

20. The optical system of claim 19 , further comprising an image surface and a stop surface, the partial reflector disposed between the image surface and the stop surface, the multilayer reflective polarizer disposed between the partial reflector and the stop surface, wherein substantially all chief light rays that pass through the image surface and the stop surface are incident on the multilayer reflective polarizer with an angle of incidence less than about 25 degrees.