The battery works on its own, it doesn't need to be effected by anything.
It's affected however, only as long as the display is on. Battery drain would be the same if you just set the display to stay on.
Tens points for sarcasm, always a winner.
I would think that the device using a camera or sensor to "see" your face the entire time the screen is on would cause more battery drain than not having that sensor/camera running at all.