From the manufacturer

The Adventurer Opti™ UV water purifier is essential gear for thousands of adventurers and features a unique technology - an optical eye senses water to ensure safe use and effective treatments. The revolutionary optical water sensor doubles as an integrated LED flashlight for added convenience on the trail or around the campsite. Hold down the activation button for three seconds to illuminate the darkness. 

Features

• Eliminates bacteria, protozoa and viruses
• Treatment size: 1/2 or 1 liter
• Treatment time: 48 to 90 seconds
• Powered by 2x CR123 batteries
• Purifies up to 8000L
• Works in containers with a minimum 1.75" diameter opening

Specifications

• Water Source: tap water, clear water
• Group size: 1, 2-5
• Usage: Often
• Product type: UV pen
• Effective against: Bacteria, protozoa, cysts, virus
• Treatment type: Purification
• Output: 0.5 liter (48 sec), 1 liter (90 sec)
• Weight: 103g
• Batteries required: Yes (included)
• Battery type: 2x CR123
• Approx Treatments per battery set: 50
• Product life: 8000 activations