But one common aspect of these luminescent devices is that they're only as good as the batteries inside. If you've ever switched on a flashlight only to experience a flood of frustrated disappointment, you might appreciate owning an "eternal flashlight." Lumen is designed to be powered by body heat, never needing batteries.

The finger-sized Lumen flashlight uses a small thermoelectric generator (TEG) to power a single 5-mm ultrabright Cree LED. We've seen successful use of such technology in consumer products before, such as with Power Practical's PowerPot. Researchers have been experimenting with wearable TEGs to extend the battery life of small devices, and, quite famously, a Canadian high school student created a working prototype of a body heat-powered flashlight as a science fair project. Lumen, however, could be the first of its type to be readily available to the general public.