Panamorph Demos Exit Pupil Modulation for VR

Exit Pupil Modulation allows an LCD-based VR headset using a pancake lens to achieve the visual performance of an OLED display with up to a 90% power reduction. The demo at DisplayWeek 2024 showed this result in a modified MetaQuest 3 pancake optic.

Exit Pupil Modulation creates two imaging systems. The first is a modified LED array that is imaged in the exit pupil of the pancake lens. The second creates a virtual image of the LCD panel, just as in the unmodified MetaQuest3. The modified backlight unit has a microlens array designed so that each LED in the array fills the entire LCD panel. While this may look like local dimming, it is not. In an LCD display with local dimming, each LED modifies the luminance in a small part of the LCD panel – a zone. With Exit Pupil Modulation, each LED floods the LCD panel so modifications to the LEDs in the backlight alter the size of the eyebox or exit pupil. This allows the designer to trade off power & contrast vs. eyebox size. This can be a very useful tradeoff, especially if eye tracking is employed to mitigate the smaller eyebox concern.

Video and Transcript below.

Hi, Chris Chinnock for Insight Media at DisplayWeek 24. I’m in the i-Zone with a company called Panamorph. What Panamorph has done is create a new optical design for VR headsets. For example, if you have a MetaQuest 3 headset, which is what this demo is going to be based on, it’s an LCD-based solution. It has a backlight that is edge lit. That light fills the LCD panel, goes through the pancake lens and also floods lights all around the eye, not just the pupil of the eye. It floods all around the eye to create a nice eye box. That’s great, but the consequence of this whole architecture, and this is generic, is that by filling that LCD panel with the light from the backlight, you create a lot of scatter. And that scatter reduces the contrast. You can improve the contrast if you want to go to a microOLED panel, for example, but that’s a much more expensive solution.

If you want to stay with an LCD solution with a pancake lens, you should consider this solution here. They call it exit pupil modulation. What they’ve done here is modify this MetaQuest 3 optic. This is the pancake lens directly out of the MetaQuest 3. This part back here is the modification, and like I said before, the normal one has an edge lit backlight. This is now a direct back light. Here is the modified direct backlight which sits on top of the optic right here. This just has a printed micro lens array on top of it and it has a fold mirror to bring that light down through the pancake lens and to focus where the exit pupil of pancake lens is.

What’s happening here is this is actually a second imaging system. You’ve got two imaging systems going on here. It is actually imaging the backlight through the pancake lens at the exit pupil and at the same time they’re also forming an image of the LCD panel at the exit pupil as well. That sounds a little strange because it kind of is. It’s a new concept. The advantage here is that each one of these LEDs, because it’s kind of it has a little bit of space here, each one of these LEDs is designed to fill the whole panel behind this pancake lens and fill the exit pupil.

Now it’s kind of like a direct backlight in a normal LCD panel. When you do direct dimming or local dimming in an LCD panel, you are turning on LEDs at various luminance levels. But the whole panel is not being illuminated by those LEDs. With exit pupil modulation, you’re modulating the light in a different way. What you’re doing with this one is you’re actually modulating the exit pupil light from this backlight.

In the demo, you first have like four images. In the first image there’s some calibration rings to get yourself positioned in the exit pupil. They put all those LED lights on for the second image, which simulates exactly the way the MetaQuest 3 is designed today. It’s filling that whole LCD panel and filling the whole exit pupil and filling light all around your eyes with a lower contrast and blah blah blah. In the second part, they only use four LEDs. Those four LEDs fill the whole panel but now are only a part of the exit pupil is filled. What you see is that they can now turn up the power for those four LEDs, but the image quality, the image luminance to the eye remains about the same but the contrast goes up because you’ve reduced all that stray light, and because you’ve reduced the size of the exit pupil. The power has gone down to only 15% of what it was when all of those LEDs were illuminated. The third, actually the fourth image then is when they go to actually one LED. Again, that one LED can fill the whole LCD panel with light, but now it goes down to 10% of the power.

There is a trade-off. The trade-off is the exit pupil gets smaller as you reduce the number of LEDs, so it becomes a little bit harder to find the sweet spot – to find that image. That may or may not be an issue depending on your design. If you have eye tracking then that problem is solvable – you can keep moving the exit pupil to track the eye. So that is  most of the benefit I see here: lower power, higher contrast, the ability to use LCD panels to be able to produce a lower cost headset with a very new and unique  concept here: exit pupil modulation. So that’s the story. Chris Chinnock for Insight Media.

share this post