The Mil-Spec Mepro M21 Self-Illuminated Reflex Sight is used and trusted by militaries and law enforcement agencies worldwide. It is specifically unique, in that, no batteries are needed. 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 dot or 5.5 MOA Triangle, Bullseye, and X.  Day or night, no matter the weather conditions with the M 21, it is always on and always ready. With Meprolight, Everything You Aim For.