These sights are the most revolutionary innovation since TRUGLO Inc. introduced fiber optics. TFO refers to their 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! - Uses the patented TFO technology. - Unbelievable transition through all light conditions. - Glows in the dark. - CNC machined steel construction. - Concealed fiber cannot be seen by target. - Fits standard holsters. - Snag-resistant design. - These TruGlo sights have a green front sight and a green rear sight. They fit on all 9mm/.40/.45 GAP GLOCK models as well as the GLOCK 33.