Anti-Reflection Device
Also: ARD · Kill Flash · Honeycomb Filter
A honeycomb cap fitted over a scope's objective that breaks up glare and reflection so sunlight off the lens cannot reveal a shooter's position.
An anti-reflection device, often called a kill flash, is a honeycomb grid that fits over the objective lens of a scope. Each cell of the grid shades the glass at oblique angles, breaking up the bright flash that direct sunlight would otherwise bounce back toward an observer. The goal is to deny an enemy or a wary animal the telltale glint that can give away a concealed shooter.
The device matters most for a marksman working from a hide or under a ghillie, where staying unseen is the whole point of the position. The honeycomb does cost a small amount of light transmission and can slightly dim the sight picture, so shooters weigh that loss against the concealment it gives in bright, open conditions.