That's one of the problems with glasses-based 3D. You're effectively wearing sunglasses, and getting probably somewhere around 50% of the light that's on the screen to your eyes. Think about it. If each of your eyes is seeing half the image, then unless they're using two bulbs, you're seeing half the light. There's obviously overlap in some cases, so there's probably a good bit more than 50% actually making it to your eyes, but that's the basic issue.