“Addlers are a gadget that makes it impossible for your eyes to focus on the wearer's face. They were popular during the Human Assistance Program since humans forget anyone they cannot recognize.”
—Mr. Forkle to Sophie, in Book 4: Neverseen, Page 101
Alvar Vacker used this device when searching for Sophie Foster. He checked Sophie off his list of possibilities because of her brown eyes. Ruy Ignis is also known to use one to mask his identity, which he consistently brags about.