An internal lens spring that is placed inside the diode module and keeps the lens housing under tension against the module for more precise focusing. This spring is sized for standard glass lenses so that there is still a bit of room to compress when the lens is focused to infinity. Note: The spring may be trimmed with wire cutters in those circumstances (such as with a G-1, G-2 or red lens) when there may be insufficient room between the lens housing and the inside of the diode module. May not be suitable for all diodes. Your choice of bright or black finish.