Optical system

1. An optical system, comprising:

an image surface;

a stop surface;

a first optical stack disposed between the image surface and the stop surface and convex toward the image surface along orthogonal first and second axes, the first optical stack comprising:

a first optical lens; and

a partial reflector having an average optical reflectance of at least 30% in a desired plurality of wavelengths; and

a second optical stack disposed between the first optical stack and the stop surface and convex toward the image surface along the first and second axes, the second optical stack comprising:

a second optical lens;

a multilayer reflective polarizer substantially transmitting light having a first polarization state and substantially reflecting light having an orthogonal second polarization state; and

a first quarter wave retarder disposed between the reflective polarizer and the first optical stack, wherein the multilayer reflective polarizer comprises at least one layer that is substantially optically biaxial at at least one first location on the at least one layer away from an optical axis of the second optical stack and substantially optically uniaxial at at least one second location 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,

wherein at least 90 percent of all chief light rays that are emitted by the image surface and that is transmitted through the stop surface is incident on each of the reflective polarizer and the partial reflector with an angle of incidence with respect to a surface normal less than 30 degrees.

2. The optical system of claim 1 , wherein an image source comprises the image surface and the stop surface is an exit pupil.

3. The optical system of claim 1 , wherein an image recorder comprises the image surface and the stop surface is an entrance pupil.

4. The optical system of claim 1 , wherein the optical system is centered on a folded optical axis defined by an optical path of a central light ray transmitted through the image surface.

5. The optical system of claim 1 , wherein the first optical stack further comprises a second quarter wave retarder disposed between the partial reflector and the image surface.

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

7. The optical system of claim 1 , wherein the desired plurality of wavelengths comprises one or more continuous wavelength ranges, and wherein at least one of the continuous wavelength ranges has a full width at half maximum of no more than 100 nm.

8. The optical system of claim 1 , wherein the optical axis passes through an apex of the multilayer reflective polarizer, and wherein the multilayer reflective polarizer has at least one location having a radial distance, r 1 , from the optical axis and a displacement, s 1 , from a plane perpendicular to the optical axis at the apex, s 1 /r 1 being at least 0.2, and wherein for an area of the reflective polarizer defined as including all locations having a displacement of no more than s 1 and a radial distance of no more than r 1 , a maximum variation of a transmission axis of the reflective polarizer is less than 3 degrees.