Pancake lens design only pass through about 15% of the total light trasmitted to them from the panels

[Heliosurge]

These 2 Artickes may help. The 2nd one on a skim does discuss needing to increase Brightness quite a bit.

https://www.pcopticalengineering.com/design-examples/#:~:text=Pancake%20Lens%20for%20VR&text=A%20second%20quarter%20waveplate%20converts,then%20transmitted%20to%20the%20eye.

Commentary on All-Plastic Pancake Optics from Kopin - Insight Media

Excerpt

“One disadvantage of the Pancake optics is low efficiency which is typically around 10%. Figure 2 shows the assembly schematic and pictures of the working all-plastic Pancake optical lenses.”