The Bat-Sonar Lenses are used by Batman in The Dark Knight to detect his position or locate things via sonar.
While tracking down the Mafia's accountant, Lau, in Hong Kong Lucius Fox created a program to be inserted into cell phones. It acts and works like a submarine's sonar and send the feed back to Batman's cowl, which Batman uses to locate Lau and deliver him to GCPD custody.
Batman later innovated the idea to an extreme, writing a virus that inserted the program into every cell phone in Gotham which was routed through a super computer in the Batcave and the feed was sent directly into Batman's cowl, all for the sole purpose of tracking down The Joker.
Batman admitted that the program gives him the ability to spy on everyone in Gotham at once, too much power for anyone including himself. Batman hence instructed Lucius to destroy the system once he defeated Joker, which he promptly did via entering the self-destruction command by typing Lucius' name
Behind the Scenes
- In the film, the Lenses had been rigged to the cowl for a clear and better view,
- When asked in an interview along with Christopher Nolan, Christian Bale said that the device was put into the cowl by Lucius Fox, who first invented the gadget, and it was part of his entire suit which helped him see better in the film.
- In the ending credits it says that the device has been used as part of Bruce Wayne's special technology, it was rigged, it was shown in the film beforehand, and it was also part of the effect as it was only rigged.
- The Lenses gave Batman, for the first time in a live-action film, the white eyes that he is often depicted with in most animated and comic adaptations.