The M21 Self-Illuminated Reflex Sight is used and trusted by militaries and law enforcement agencies worldwide. It is designed and manufactured to the highest military standards to assure years of reliable operation under all field conditions. With no switches, no battery or other external sources of power, the sight is always ready for action. Illumination of the point of aim reticle is achieved by a fiber optic collector system during the day and by a miniature self-powered tritium light source at night. A large 30mm display window allows the shooter to keep both eyes open for better battlefield situational awareness. The shooter has the option of five reticles – 4.3 MOA or 5.5 MOA Dot, Triangle, Bullseye, and Open X. Day or night, no matter the weather conditions with the Mepro M21″ it is always on and always ready.