TFO gun sights from TRUGLO are the most revolutionary innovation since TRUGLO introduced fiber optics. TFO refers to the 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 lighting conditions. Tritium does no need batteries and does not need to be charged with light- it simply glows.FeaturesCNC-machined steel constructionNo batteries requiredSnag-resistant designGlows in the darkRear Sight: 0.450" high