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.
The Problem
In a typical LCD VR headset:
- The backlight fills the LCD panel with light.
- Light passes through the pancake lens and floods the entire eye box—not just the pupil.
- While this creates a generous eye box, it also introduces light scatter, reducing contrast.
Switching to microOLED panels can improve contrast, but that’s a much more expensive solution. Panamorph’s approach keeps the LCD architecture while improving performance.
The Solution: Exit Pupil Modulation
Panamorph modifies the Meta Quest 3 optics:
- Replaces the edge-lit backlight with a direct backlight.
- Adds a printed microlens array and a fold mirror to direct light through the pancake lens and focus it at the exit pupil.
This creates two imaging systems:
- Imaging the backlight at the exit pupil.
- Imaging the LCD panel at the same exit pupil.
Each LED in the backlight is designed to fill the entire LCD panel and the exit pupil—similar to local dimming in TVs, but applied differently.
Demo Results
- Full LED array on: Simulates the standard Meta Quest 3 design—wide eye box, lower contrast.
- Four LEDs on:
- Same image luminance, but contrast improves because stray light is reduced.
- Power drops to 15% of full-array mode.
- One LED on:
- Still fills the LCD panel, but power drops to 10%.
- Trade-off: Smaller exit pupil makes finding the sweet spot harder.
With eye tracking, this limitation can be solved by dynamically moving the exit pupil.
Benefits
- Lower power consumption
- Higher contrast
- Enables LCD-based headsets to remain cost-effective while improving image quality.
This innovative concept—exit pupil modulation—could reshape VR headset design.