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Even in the name of battery life, it will remain on.
President of Sony's worldwide studios Shuhei Yoshida has confirmed via Twitter that the lightbar on the DualShock 4 controller cannot be turned off.
The lightbar on the PS4 controller is used with the PlayStation camera to detect motion control in the same way as Move controllers, and can be used by developers to increase the game's experience, such as the way it works as a health bar in Killzone: Shadow Fall.
The lightbar, however, cannot be switched off, as Yoshida confirmed on Twitter. Responding to a question on whether the lightbar can be deactivated in order to conserve the controller's battery, Yoshida simply stated "no."
-Source
