Trijicon HD XR™ Night Sight Set — Orange Front Outline for Glock® Full and Standard Size Glocks 9mm/40How it works Photoluminescent (phosphorous) sights absorb light when exposed to bright light, and emits it for a while afterwards. Tritium is a gas that glows permanently, hence making the sights visible whenever you need it. Trijicon's HD XR range combines a tritium vial with a brightly coloured photoluminescent ring around it. The brightly coloured ring increase visibility during daytime, as well as (when charged) in the dark. The Trijicon HD and