The Meprolight M21 Polarizer and Flash Guard Kit helps solve the problems of reflex sights and the washout that is experienced when going fo dark to light. The polarize filter creates a more vivid image in the M21 in more diverse lighting conditons. The flash guard protects the 30mm lens of the M21 from scratches while reducing the glare as well. Features: Helps Filter Light Adjustable Clickable Settings Easily Screws In Reduces Glare Can Use Separately or Screwed in Together