FaceID is like having the camera’s flash flashing in your eyes every 5 seconds. See the youtube video below. Notice how although I have the iPhone brightness at lowest level to accommodate for the dark environment, the "flood illuminator" lights up in full intensity, blasting the retina with IR light. This like having the camera flash striking your eyes every 5 seconds. Would you be comfortable with that? Edit: People are saying it's the proximity sensor. There are two kind of blinks. The first is the proximity sensor, every 2 seconds and its intensity if normal. Then second, is the flood illuminator, it's every 5 seconds or so.