TFO gun sights use the same proven technology as our TFO archery pins. These sights are the most revolutionary innovation since TRUGLO, Inc. introduced fiber optics. TFO refers to our 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 darkness—with perfect transition through all light conditions. Tritium does not need batteries and does not need to be charged with light—it simply glows!