A thermoelectric generator uses heat to make electricity. One can be found in gadgets like the PowerPot, which lets you cook over a campfire and charge your phone at the same time. But amazingly, there’s actually enough heat in your hand to power a flashlight.
The Lumen is a little lamp that converts your body heat into light. Its thermoelectric generator uses the temperature difference between one side and the other to generate power. The bigger the difference, the more power generated. The catch or, depending on how look at it, the neat twist, is that the generator works better in cold places, because this makes for more contrast between hot and cold. The alternative is adding heat, but as humans top out at around 37˚C or 98˚F, you’ll need to run a fever to do that.
A capacitor stores any excess energy, and the light comes out of a bright 5-Watt LED.
The Lumen is lightweight, at 1.2 to 1.5 ounces depending on whether you choose aluminum or titanium, and you can specify a version with a tiny vial of phosphor and tritium gas inside, which provides a gentle orangey chemical glow that will help you locate your Lumen in the dark. And you’ll need to locate it, because if you put it down for a second, or drop the thing by mistake, the main LED light will extinguish itself.