Hawkeye uses the TrueDepth camera on newer iOS devices to perform eye tracking.

The TrueDepth camera tracks your facial movements with far more accuracy that a traditional camera. We use this to conduct accurate eye tracking without any extra hardware.

However, this does mean you need an iOS device with a TrueDepth camera to perform eye tracking. This includes the iPhone X, XS, XR, and new iPad Pro. 

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