This is a Meprolight MEPRO21 Tritium Self Illuminated Reflex Sight. The MEPRO 21 provides instant, all-light aiming capability, without batteries. Designed through close collaboration with Israeli Special Forces, the MEPRO M21 is battle proven, simple to use accurately and totally reliable. MEPRO 21 offers a large 30mm diameter lens which allows rapid target acquisition with both eyes open. This was the first reflex sight in the world designed for close quarter combat. Illumination of the aiming point is achieved by a fiber optic collector system during the day, and by a miniature self-powered tritium light source at night. Transition between the two systems is instantaneous and automatic to assure good contrast between the aiming dot and the target area. The MEPRO 21 reflex sight offers users the ability to acquire targets accurately at any range, in any situation, with both eyes open. This MEPRO 21 has the Bullseye Reticle. Always ready for action – no batteries or switches. Aim & shoot faster than ever - with both eyes open! Self-illumination means always available. Operates under all weather and temperature conditions. Large 30mm diameter lens assures large FOV. Rapid target acquisition means quicker shooting in urban areas. Quicker shooting in urban areas and rapid target acquisition. Effective use of sight under extreme conditions such as bad weather, firing after physical stress and firing under pressure.