Front surface mirror for providing white color uniformity for polarized systems with a large range of incidence angles
A fold mirror for a projection display system is designed to lessen a perceived variation in white color balance of light reflected across the mirror wherein the white light is comprised of sub-beams of orthogonal polarization states. The mirror comprises a metal reflective layer and a plurality of optically transparent layers, wherein thicknesses and materials of the optically transparent layers are such that wavelength-averaged reflectance for p-polarized incident green light varies by less than 1%, wavelength-averaged reflectance for s-polarized blue light varies by less than 1.5%, and wavelength-averaged reflectance for s-polarized red light varies by about 4% over a range of angles of incidence between 0 and 65 degrees.
1. A mirror for a projection system comprising: a metal reflective layer; and, a plurality of optically transparent layers disposed over the metal reflective layer; wherein thicknesses and materials of the plurality of optically transparent layers are such that a wavelength-averaged reflectance of the mirror for p-polarized first color light varies by less than 2% over a range of angles of incidence between 0 and 65 degrees.
2. A mirror according to claim 1 wherein the p-polarized first color light is green.
3. A mirror according to claim 1 wherein the thicknesses and materials of
said optically transparent layers are such that a wavelength-averaged reflectance for s-polarized second color light varies by less than 2% over the range of angles of incidence, and a wavelength-averaged reflectance for s-polarized third color light varies by less than 5% over the range of angles of incidence.
4. A mirror according to claim 3 herein the p-polarized incident light is green, and the s-polarized second color light is one of red and blue and the s-polarized third color light is the other of red and blue.
5. A mirror according to claim 4 , wherein over the range of angles of incidence the wavelength-averaged reflectance of the mirror for p-polarized first color light varies by less than 1%, the wavelength-averaged reflectance for s-polarized second color light varies by less than 1.5%, and the wavelength-averaged reflectance for s-polarized third color light varies by about 4%.
6. A mirror according to claim 1 , wherein the metal reflective layer is one of aluminum, silver, and gold.
7. A mirror according to claim 1 , wherein one of the materials of the plurality of optically transparent layers is one of SiO 2 , TiO 2 , Ta 2 O 5 , Nb 2 O 5 , Al 2 O 3 , MgF 2 , and HfO 2 .
8. A mirror according to claim 7 wherein the plurality of optically transparent layers comprises: a first layer disposed over the metal reflective layer, a second layer disposed over the first layer, and a third layer disposed over the second layer; wherein an index of refraction of the second layer is higher than each of indices of refraction of the first and third layers.
9. A mirror according to claim 8 , wherein the first layer is a layer of SiO 2 , between 80 nm and 100 nm thick, the second layer is a layer of TiO 2 , between 40 nm and 60 nm thick, and the third layer is a layer of SiO 2 , between 70 nm and 120 nm thick.
10. A mirror according to claim 9 , wherein the first layer is substantially a 92.5 nm-thick layer of SiO 2 , the second layer is substantially a 52.1 nm-thick layer of TiO 2 , and the third layer is substantially a 99.5 nm-thick layer of SiO 2 .
11. A projection system comprising: a beam-forming means for forming a light beam; the light beam comprising p-polarized light of a first color and s-polarized light of a second color; and, a mirror according to claim 1 for reflecting the light beam received from the beam-forming means.
12. A projection system comprising: a beam-forming means for forming a light beam; the light beam comprising p-polarized light of a first color and s-polarized light of a second color; and, a mirror according to claim 10 for reflecting the light beam received from the beam-forming means.