[ksaj]

You can't get enough vitamin D from light through a glass window since it will block far too much of the UV rays.

Considering how much light is blocked from a screen, I suspect without the glass, you'd have to be in front of it for quite a while to get enough to have any effect. But this is just a guess. I haven't ever heard of anyone getting a tan through one. It would be an interesting experiment with a UV meter.