>>379298492
>>379298817
Some elaboration: the idea behind pancake lenses is that instead of letting the light propagate in the air, they use a layer to reflect light back into the lens, and then back in the direction of your eyes. So the light travels the same distance except it bounces around inside of the lens rather than going that distance simply through air. The problem is that you can't easily reflect light using a material but at the same time let light pass through it. So what they have to use are known as half mirrors, like in police interrogation scenes. That means half of the light is lost through a single one of those layers. 

That's why pancake lenses need extremely bright displays. If previous consumer displays were not able to sustain such high brightness, then that explains why we're seeing pancake lens headsets come out now. That also explains why Sony can do HDR for PSVR2, since they're using the usual fresnel stuff.