Apple has a track record of adding features to iPhones that are not new, but are better designed, developed and implemented than others. This is true of the iPhone 5s which was just announced: the system (called Touch ID) includes a fingerprint reader that can read your thumbprint and log you in, or authorize iTunes or App store purchases. And the details of how this works are contained in a patent that reveals some of the secrets behind this new approach to fingerprint authorization.

There have been cell phones with fingerprint scanners before. The Motorola Atrix Andriod cell phone included a fingerprint scanner on the back of the phone, and numerous laptops from manufacturers like Lenovo have offered fingerprint readers as options. However, these were often unreliable, and were awkward to use. You dragged your fingerprint over a thin sensor bar, and the sensor read your fingerprint as it passed, one line at a time. This approach kept the scanner small, but it was prone to failure: If you scanned it too quickly or too slowly, the fingerprint would be distorted, and would not pass the test.

The combined power button & fingerprint scanner of the Motorola Atrix / Edward Chai

The new Apple sensor works in a different way, according to the patent application filed by Apple earlier this year, and from information revealed at today’s announcement of the new iPhone 5s.

The scanner is located under the home button of the new iPhone, behind a sapphire glass window that hides the sensor. When you press down on the button with your thumb, the sensor is activated by the contact of your skin on the steel ring around this window.

The iPhone 5S fingerprint sensor, exploded view / Apple

Under the window is a small IR filter that blocks out light from other sources (the blue window), and under this is the sensor itself, which maps the features of your fingerprint. Under the sensor is the switch for the home button, which presses down the entire assembly as one.

Apple Senior VP of Hardware Engineering Dan Riccio explains the process in a video on Apples site:

In essence, a high resolution image of your fingertip and the sub-dermal layers of your skin. It then analyzes this information, with a remarkable degree of detail and precision. It then categorizes the details of you fingerprint into one of three basic types: arch, loop or whorl. It maps individual details of your fingerprint that are smaller than the eye can see… touch ID uses all this to produce a very accurate match, with a very high level of security. All fingerprint information is encrypted and secured in an enclave of our A7 chip. This information is accessible only by our Touch ID sensor.”

The technology in this sensor comes from a company that Apple bought earlier this year called Authentec. Headed by Dale Sentak, this company sold fingerprint security products for cell phones and other tablets, but went silent after the acquisition. The only news was Sentak’s name on an Apple Patent application which detailed a new approach to fingerprint scanning. This patent (#US2013/0181949) puts more detail into how the new scanner works. It also hints at a future where the fingerprint sensor is integrated into the display itself.

A high resolution touch screen fingerprint sensor

Most fingerprint sensors use a CCD array, a long thin strip of sensors similar to those used in document scanners that measure reflected light,creating the map of the fingerprint as the finger tip passes over the scanner. The new Apple sensor instead use a grid of capacitive sensors, effectively creating a very small, but very high resolution touch screen that scans the entire figertip at once, making the process faster and easier. For this sensor, Apple claimed a resolution of 500dpi at the iPhone 5s launch. The detailed specs of the sensor have not been released, but the size of the home button is about a third of an inch across, so the maximum resolution of this sensor is probably about 150, or more likely 100 pixels.

The sensor then uses this information to recognize the pattern of the fingerprint. Because each person has an individual fingerprint, the pattern of parts that touch the sensor (the ridges of the fingerprint) and the parts that don’t (the valleys between them) creates a unique touch pattern that the sensor reads. Within this pattern, features such as the whorls and ridges create the unique aspects of the print, and these are the details that the iPhone stores and recognizes.

One of the figures from the patent that describes the new Apple fingerprint sensor / US Patent Office

And no doubt this is where some of the secret sauce of the iPhone lies: they claim that the sensor will work on multiple fingers from any orientation, so they are obviously doing a lot of analysis of the fingerprint scan to verify it. Hopefully we will get more details on how this processing is done before the product is launched.

But does it work? So far, looks like yes

The general impression from the limited hands-on time that attendees have had with the iPhone 5s at the launch event seems to be positive. The Loop described the process as “seamless to do, just following the onscreen instructions…Unlocking the iPhone 5S was very slick—just rest your finger on the Home button and the phone unlocks immediately. You don’t have to press or move your finger around waiting for it to be recognized—it just worked.”

Fingerprint sensors have been defeated before

Of course, fingerprint sensors have proven to be less than secure, with one researcher being able to bypass a touch sensor using silly putty. The TV show Mythbusters also demonstrated that it was fairly easy to find someones fingerprint, copy it and use this to defeat a fingerprint sensor lock. It remains to be seen if the iPhone sensor will be as easy to defeat, but there will no doubt be a lot of curious people trying their best on the 20th of September, when the new iPhone is released.