iOS 7 AVCaptureTorchModeAuto似乎不会在低光照下激活手电筒

I've set an AVCaptureDevice TorchMode to AVCaptureTorchModeAuto, the torch mode is set after the AVCaptureSession has started running. I'd expected the torch mode to light-up the LED in low light conditions as per Apple's documentation: I've set an AVCaptureDevice TorchMode to AVCapt