TFO gun sights are the revolutionary innovation since TRUGLO, introduced fiber optics. TFO refers to the patented vombination of Tritium and Fiber Optic tecnologies. The Tritium maintains the brightness level of the sight in low-light (or NO light) conditions. That means that sight can be seen even in complete darkness with perfect transition through all lighting conditions. Tritium does not need batteries and does not need to be charged with light-it simply glows!