The TFO .270/.500 Set Handgun Sight uses the same proven technology as the TFO archery pins. 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 great transition through all lighting conditions. Tritium does not need batteries and does not need to be charged with light... it simply glows.