The Steripen ClassicTM 3 with 40-micron pre-filter is the latest, most versatile model in Katadyns award winning Classic line. Winner of Time Magazines Best Invention of 2001, it offers enhanced ergonomics and battery life as well as improved water resistance. An 8,000 treatment lamp delivers a lifetime of purification. The 4040 micron screen Pre-Filter fits perfectly on wide-mouth one liter water bottles and removes particulates that can get in the way of UV light in water. The Pre-Filter is designed to be stored directly on a wide mouth bottle and the Steripen Classic 3TM fits perfectly in the filter. Just insert the Steripen Classic 3TM in the filter opening, turn the bottle upside down, and gently rock to circulate light throughout bottle.