The new Surefire Ultra models have tremendous 500+ lumen output with a beam profile that has a fairly tight hotspot. That hotspot may be tighter than you want for some applications, especially if you are using your light indoors. The new F04-A is a beam diffuser that slips onto the 1.125″ diameter bezels of the Surefire Ultra lights. It uses a high grade optic diffuser lens to spread the beam of your light into a wall of light.
I have been using the Surefire F04 (the original 1″ version) for years. There is some light lost to the diffuser but it is minimal. This would be ideal for a Surefire Scout Ultra or Surefire X300 Ultra that is mounted on a carbine that is used for home defense.
If you want a broader, more diffused beam for your Surefire Ultra, check out the F04-A Beam Diffuser.