Answer: An interactive touchscreen.
According to its developers at Carnegie Mellon University, the LumiWatch prototype is “the first functional projection smartwatch system.” It works by displaying a slide bar on the user’s arm that they swipe left in order to unlock the watch. It then projects apps along their forearm within a 40-square-centimeter space, roughly five times the size of your average smartwatch display.
The watch is capable of Bluetooth and Wi-Fi connectivity, and the projector can display three colors: red, blue and green. Within the metal enclosure are a logic board, 15 lumen scanned-laser projector, and continuous 2-D finger tracking. The battery can power more than one hour of continuous projection or one day of intermittent use. It measures 2 inches by 1.6 inches by 0.66 inches, much larger than most current smartwatches, but if future versions are developed, that size will likely decrease.