With the Lumenok SL in your arrows, you'll always be able to trace your arrows in flight and see exactly where your shots fall. LED light provides up to 40 hours of illumination and is easy to see even over great distances. The light activates upon the shot and offers unmatched visibility, making it easy for you to reclaim fired arrows. This lighted arrow nock fits .245" ID carbon and aluminum shafts. Works best with full carbon shafts. The patented Polymeric battery holder allows for battery replacement and protection against failure due to hard impacts.