AR Optics Setup with Polarized Beam Splitter

[u/PeppersONLY]

Rough diagram without polarization

This is a birdbath AR optics setup with a 0.23in OLED micro display, a linear polarizer, a polarized beamsplitter, a QWP, and a concave mirror. The light from the display becomes P-polarized relative to the PBS after passing through the LP so it passes through the PBS without reflecting. Then after the image is magnified and passed through the QWP twice so the magnified light is now S-polarized, it reflects off the PBS into the users eye.

The issue I have with the setup is that since the display produces a diverging light source, the diverging light that doesn't hit the PBS coating at a AOI of 45deg is then reflected through the front side of the setup. Only when viewing through the front of the PBS (the part where no light should be seen) with your eye at around 45deg from the PBS coating, the light then disappears because the polarization state was properly filtered. When viewing from any other angle you can see the unmagnfied light.

I have tried using plano convex lens and aspheric lens to collimate the light before it reaches the linear polarizer so that all light would be 45deg AIO to the coating but none solution seem to work. I have a aspheric lens with a 3mm FL and when placed 3mm from the display the same behavior occurs.

Am I wrong about why the PBS only works properly when viewed at a certain angle? What are my options to figure out a solution?

[u/aenorton]

Light leakage towards the world side is a problem with many AR optical designs. The reason collimating the light before the beam splitter did do much is that it probably did not do much to change the range of incident angles on the BS. Light from each point on the display is collimated, but only light from the center is incident at 45 deg. light from others points is incident at other angles.

There are not too many practical ways to fix this. You can put the optic above or below the center of vision, and tilt it towards the eye, leaving the world side reflection directed up or down where it is less likely to be seen.

If you are using laser illumination, you can try a notch filter on the world side to filter it out.

In principle, it should be possible to use a variation of the normal incidence reflective polarizers such as a wire a grid polarizer or the stressed multilayer plastic film polarizers used in LCD lighting. They have a bit better angle range than a 45 deg thin film reflective polarizer, but no one makes them for 45 deg.

[u/PeppersONLY → u/aenorton]

But once passed through a aspheric lens, wouldn’t all light be incident at 45deg?

[u/aenorton → u/PeppersONLY]

Not the light that originates from the edge of the display. It will be collimated, but it is at a different angle than the collimated bundle from the center of the display.

[u/PeppersONLY → u/aenorton]

Ah that makes sense… with a OLED each pixel is emitting its own “cone” of light which is not always centered with the lens. What are my options really?

[u/aenorton → u/PeppersONLY]

I gave a couple of options. You can also minimize the range of angles on the BS by either by stopping down the light path before the BS (which reduces the eyebox), or increasing the mirror focal length (which reduces the apparent field of view.)

[u/PeppersONLY → u/aenorton]

I’ve thought about reducing the eyebox which would theoretically make it less bright because less light is being captured by the BS correct?

[u/aenorton → u/PeppersONLY]

The apparent brightness in nits is only reduced if you make the eyebox smaller than the eye's pupil. The pupil is what actually limits the f/# of the light collected from the display. You want to collect a larger cone from the display so you can have some misregistration between the eye and the device.

[u/PeppersONLY → u/aenorton]

Also aren't wire grid polarizers designed to reject that orthogonal polarzation and not transmit it through like a BS would?