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How the technology behind Apple's Touch ID will likely change with 'iPhone 8'

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There's a lot of talk about Apple's shift to a new screen technology with the "iPhone 8," but fewer conversations about how it will all work — AppleInsider explains what changes Apple will likely need to make to implement the technology, while retaining Touch ID on the front of the rumored edge-to-edge screen on the device.
 

A rumored shift to an OLED screen in the "iPhone 8" changes the game — not just for battery life, but for related technologies. Apple has had a great deal of success with the best-in-class fingerprint reader technology that it has used since the iPhone 5s, but unless it's moved to the back of the phone, the underlying sensing technology needs to change.

As it stands with the iPhone 5s through the iPhone 7, the Touch ID technology developed by AuthenTec senses the presence of a finger with a detection ring which switches on the sensor when it detects a digit. The sensor itself is clad in a thin sapphire crystal, and uses a complimentary metal oxide semiconductor capacitive touch sensor to detect the fingerprint's whorls and ridges with 500 pixels per inch resolution. 

The captured fingerprint is then passed to the Secure Enclave and compared to stored data local to the phone. Should the mathematical models match, the phone is unlocked.

There are two major revisions of Touch ID, with the only real difference being sensing speed in more recent version on the iPhone 6 and iPhone 7 families.
 

So where's the problem?



The problem lies with the difference in thickness between the sapphire lens and the sensor. The sensor itself is 170 microns thick, with the sapphire lens not that much thicker. 

While an OLED screen assembly is thinner than that of a comparative LCD display, the glass cover is about the same thickness between the technologies. At just less than 1 millimeter thick, the screen glass is over five times the thickness of the sapphire lens on the existing Touch ID sensor on the iPhone 7.

With fingerprint sensors used in Touch ID, as separation between the contact surface where a user places their finger and the capacitive sensing array increases, there is a corresponding blurring of the finger's electric field captured by the Touch ID sensor. In the existing Touch ID implementation, the lens is sufficiently thin that any distortion, or aberration, is minimal.

When a sensor is placed behind a thicker lens, in the case of the "iPhone 8," behind the screen glass itself, the inevitable blurring without some form of correction can lead to degraded fingerprint image resolutions and decreased recognition accuracy. 

 

Math can be used to determine exactly how much distortion would be introduced with thicker glass — but it is extremely complex and to figure it out precisely would require more data about the optics of the sapphire Touch ID cover than is currently known. However, roughly speaking, the aberrations induced between Touch ID now, and if it was embedded behind much thicker screen glass would increase between 5 and 15 times what is expected now.

If Apple compensated in software for that and allowed for more fingerprint variation between what is stored and what is sensed, it would make Touch ID much less secure. This would likely be unsatisfactory for the banking industry, and would impact Apple Pay.

However, Apple has a patent for a touch sensor using an array of electrostatic lenses. By using the lenses in between a designated touch area and the sensor behind the thicker glass, the electric field associated with a user's finger can be presented undistorted — or less distorted — to a sensor embedded in the glass. 
 

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