When you need the power, versatility, and ruggedness of the Apex headlamp, but your adventures require a lighter weight, you need the Princeton Tec Apex Pro Headlamp. Over 100 grams lighter than the original Apex, the lighter and faster Apex Pro was designed with the unique and ever evolving needs of professional athletes and extreme adventurers in mind. The Apex Pro is powered by 2 CR123 lithium batteries protected in a highly durable and waterproof casing. Built for professionals, perfect for all users.Regulated Output: Princeton Tec lights feature regulated LEDs with a sophisticated regulating circuit that maintains constant brightness as long as the batteries have sufficient voltage. Through the Regulated Output period constant brightness, the light will maintain a certain level of lumens depending on the setting. Following this duration, the light will continue to work, though it will function at a lower lumen output. Lights without Regulated Output will begin dimming slowly from the onset, and will continue to dim until the battery is exhausted. They will not maintain a constant lumen output like that of Regulated Lighting.