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

[Heliosurge]

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

Proof of Concept Optical Engineering – Zemax OpticStudio Consultant.

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.”