The SteriPEN Adventurer Opti 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. Because of its reliability and light weight, Adventurer Opti has earned 4/5 stars based on 77 customer reviews at REI.com. The AdventurerOpti retail package includes 2 non-rechargeable (disposable) CR123 batteries and a neoprene case with belt loop. Device works with containers with minimum 1.75 in. diameter opening (ie Nalgene wide-mouth bottles).