The Vision Ring (ルックリング, Rukku ringu, lit. "Look Ring") is an item in killer7 and arguably the most important of the game's rings.
It seems to be the tool that allows Garcian Smith to scan the immediate area for Heaven Smiles, as he loses this ability when he loses possession of it. The killer7 begin the game with the Vision Ring already in their inventory. When Garcian Smith travels to Coburn Elementary School, he gives up the ring as a sort of security key, along with various passwords, to the institution's computer system and the principal's safe. This means that when Emir Parkreiner travels to Battleship Island three years later, he cannot scan the area for Smiles.
Extra:
The vision ring and the ability to scan heaven smiles is not a default in the game. This means the ability is added when the game begins. Earliest as the tutorial stage. When cheat codes are used in the PAL game to access beta, testing and removed stages and files, the player is unable to scan for heaven smiles, meaning scanning was not added in the beta/testing stages and is therefor not a default function of the game.