Light Emitting Diode. A solid-state electrical element that lights up brightly when an electrical current is passed through it. Tough, energy e"cient and long-lasting.
Measurement of the total quantity of visible light emitted from a source. Theoretical lumens are the maximum possible power of a light and battery in a perfect circuit, and give an exaggerated idea of actual power.
The reflector behind the LEDs and the lens in front of them control how the light hits the trail. Flood or di!user optics spread the beam out, making it wide and shallow.
Spot optics focus the power of the light more narrowly for long-distance reach.
Batteries can be self-contained (integrated into the head unit of the light) or attached via a cable and mounted on the frame/shoved into a pocket.
Lithium-ion. The form of battery used in almost all rechargeable bike lights and other devices (phones, laptops). Lighter, more e"cient and more tolerant of random charging regimes than most battery types.