When I was 16 (many years ago) I stood in the direct central Texas sun, in the summer, on a sidewalk, in front of a brick building, for over 30 mins waiting to buy movie tickets. Before we went inside I completely lost my vision in both eyes--totally black. I do not remember feeling weak or a rapid heart rate or dizzy. About 10 mins after I got inside the air conditioned theater and sat down my vision returned. I have never lost my vision like that again although I do not tolerate heat well at all--heat exhaustion symptoms come on really fast. I have always assumed the vision loss was heat related and am curious about what might of brought it on.