The SteriPEN is a revolutionary new technology using UV light to purify your drinking water. The steriPEN is the ultimate light weight water filter, perfect for hunting, camping and travel. The smallest, lightest SteriPEN®, the AdventurerOpti, uses a revolutionary optical eye to sense the water. The patent-pending technology ensures safe use and provides purification in even the coldest of waters. The new optical sensor doubles as a convenient LED flashlight. It’s a great backup light when camping or taking a night-time walk to refill your water bottle. The Flashlight mode is activated by pressing and holding the single button down for 3 seconds. The clear lamp cover focuses the LED light.