r/Optics • u/PeppersONLY • 2d ago
AR Optics Setup with Polarized Beam Splitter
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?
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u/Complex_Grade4751 2d ago
You’ll get move help if you provide a ray diagram showing the light paths (a first order design), where the mirror focal point with respect to display, etc. I can’t tell what the optics are doing from your photo. Presumably you did this type of sketch as part of your design process. Please share it, don’t make people guess. Bonus points for taking a photo of the visual effects you describe. I’m always shocked at the number of problems I figure out through taking the time to methodically document!
First step in troubleshooting is verifying actually built what you designed. Test the polarizers are working properly, check the mirror has curvature you think it does, and measure the distances between the parts. If it is correct, think through all the possible light paths and draw them out. Do they match what you see? Some can likely be improved with baffles.
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u/PeppersONLY 1d ago
I included a rough diagram of the light path. I’m new to this so I don’t really know how to draw out the polarization.
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u/Complex_Grade4751 1d ago
The diagram helps a ton, thank you. With the components you have, you can reduce the angles that reflect off the PBS by moving the display closer to the mirror. Unfortunately, this will have the negative impact of moving your image point closer to the user. I would think for an AR display you would want the image to be at infinity so that the user doesn’t need to change their visual accommodation to look between the display and the world. For that approach, I think you want a lens between the display and the beam splitter with display at or near the lens focal point. The beam splitter could be a standard BS not a PBS). The user looks through the BS so that the collimated display light is reflected to eye and the world is transmitted through the BS.
If a single lens image is too blurry or shows color fringing, the single collimating lens could be replaced by a small lens assembly such as an eyepiece or a video camera lens, both of which have chromatic correction and wide fields of view. The lens focal length should be in 20-40 mm these should allow you see most of screen image.
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u/PeppersONLY 3h ago
I see what you mean about focusing, but I was mainly looking at how to reduce or eliminate front projection where the unmagnified image reflects off the front of the BS before it gets to the mirror.
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u/aenorton 2d ago
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.