Shortest model sight yet. This sight is smaller than the former N-cell batter sight (511/551) and runs on a single 123 battery. It also has a longer battery life than the N-cell style sights. With the single battery configuration, the XPS allows more rail space than ever, leaving more room for rear iron sights or NV mount. Smaller, lighter and always fast; the XPS series is a great addition. This Sight is night vision compatible.