SteriPen water purification for travel is small enough to carry in your pocket and the fraction of the weight of other filters. The smallest, lightest SteriPEN, the 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. It’s a great back-up 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.