Joker's x-ray specs

Joker's x-ray specs were a Detective Mode-like special set of glasses, that were colored red and yellow with swirls which could track down anyone who was in their gaze. They were used by the Joker during his attempted takeover of Arkham Asylum. They were only seen and used by the playable Joker in the Play as The Joker DLC of Batman: Arkham Asylum.

Batman Arkham Asylum
After the Joker's escape, he eliminated any security guard that stood in his way, and used his explosive Joker Teeth, his powerful gun, and his x-ray specs with which he saw the exact position of patrolling guards.

Batman Arkham Knight
Nine months after the defeat of the Joker and his death, the Evidence Room at the GCPD Lockup held numerous confiscated items that belonged to various villains from older and current criminal cases. The Specs, alongside the handgun, dart gun, and explosive Joker Teeth could be seen in the case reserved for the Joker's items.

Trivia

 * Joker's x-ray specs only showed the skeleton of an enemy and made a wacky, yet eerie noise every time that they were donned.
 * Joker remained still while he used his glasses.