Way back when, tiny CRT radar screens were very dim. To look at them, there was a dark, tubular screen that was shaped at the end to more or less fit the face.<p>When you put your face there, you cut external light out from that end and the tube part stopped light from coming in at the sides. The bottom end of the tube was blocked off except for the screen itself. So you could look at the screen inside a tiny 'room' of complete darkness.