Purple light that screws into the sight guard on all Cooper John sights. The purple light intensifies the brightness of the fiber optic material in the pins. Does NOT come with sight bracket. Fits DN3 Mark One and Bulls Eye models the Saxon and the Grave Dancer without the bracket. All other DN3 Mark 2,3 and 4 models need to add bracket. Thread size 3/8" X 32. Will NOT fit DN2 sights without the bracket.