TFO gun sights use the same proven technology as Truglo TFO archery pins. These sights are the most revolutionary innovation since TRUGLO, Inc. introduced fiber optics. TFO refers to Truglo's patented combination of Tritium and Fiber Optic technologies. The Tritium maintains the brightness level of the sight in low-light (or NO light) conditions. That means you can see your sight even in complete darknesswith perfect transition through all lighting conditions. Tritium does not need batteries and does not need to be charged with lightit simply glows!