Since the sensor's introduction in September, a growing number of issues have surfaced — including everything from phones that don't recognize when a finger is present to those that don't approve fingerprints they're supposed to approve.
What's going on here?
While faulty software or hardware could be to blame in a few cases, the
problem might also be you. Determining the real culprit requires a
closer look at how Apple's sensor technology really works.
Touch ID is composed of an 8 x 8 millimeter, 170-micron-thick capacitive sensor
located just beneath the home button on the 5s. This is used to capture
a 500-pixel-per-inch (ppi) resolution image of your fingerprint. The
sensor can read pores, ridges, and valleys. It can identify arches,
loops, and whorls. It can even recognize fingerprints oriented in any
direction.
When you place your
finger or thumb on the sensor, it looks at the fingerprint pattern on
the conductive sub-dermis layer of skin located underneath the dermis
layer. It also measures the differences in conductivity between the tops
of the ridges and the bottoms of the valleys in your prints in this
layer. This is more accurate than looking at the dead surface of the
skin alone, which is constantly changing and isn't conductive.
This capacitive sensor is
made of raw silicon. As such, it tends to be very fragile and
susceptible to performance problems caused by dust, moisture, and
electrostatic discharge, or ESD. To protect and insulate the sensor,
Apple layered laser-cut sapphire crystal on top of the silicon.
It chose sapphire for a
few reasons. The material is very clear, and it acts as a lens for your
fingerprint. It's also hard (it scores a 9 on the Mohs scale
of hardness), which means it's difficult to scratch. If the home button
does get scraped or scuffed, the images sent to the Touch ID sensor
will be flawed and it will cease to work properly.
What's more, a stainless
steel ring encircles the button and acts as a capacitive touch switch,
turning the actual touch sensor on and off when a finger is present so
it doesn't eat up your iPhone's battery life.
After you register your
fingerprint — a process known as enrolling — an encrypted mathematical
representation of that information is stored on the device's A7
processor in what's called the "secure enclave." When the sensor captures an image, software algorithms
determine whether the print is a match with the stored information or
not. A match allows access to the homescreen. A non-match won't.
There are obviously a
few possible points of failure in this process, but it all hinges on
first getting that robust fingerprint data. "Any good biometric has to
start with a high-quality image," Integrated Biometrics' CEO Steve Thies
told Wired. His company makes a variety of compact fingerprint sensors
that use a different method from Apple's Touch ID (electroluminescence
and a thin film transistor) to read fingerprints.
Basically, the larger
the sensor, the easier it is to pick up a more accurate representation
of your full fingerprint because it's working with more data. This makes
it easier for recognition algorithms to confirm that your fingerprint
actually belongs to you. But a larger sensor also introduces two
problems: cost, in the case of a capacitive scanner like Apple's, and
thickness, in the case of another popular fingerprint technology,
optical sensing. (You've probably used the latter at the DMV or gym.)
Based on what we've seen from Apple's patent applications, it's highly likely the company considered other implementations of a touch sensor. But ultimately, it opted for a smaller version that could more easily fit inside the home button.
Apple partially gets
around the small sensor issue using the enrollment process, which
includes rolling your finger around to try to capture every microscopic
nook and cranny on your finger. Then, at least, it has a large source to
pull from, even if it's only scanning a section of that each time you
tap your finger.
Still, the less data you
have from a fingerprint to process, the harder it is to get a match.
Precise Biometrics COO Patrik Lindeberg offers a good analogy: If you
have a picture of a face and you see only a small part of that picture —
the eyes, or part of one eye — it will be hard to recognize if it is a
friend, or someone you don't know. If you have the full face, it's easy
to process. Seeing only a portion of a fingerprint sets higher and
higher requirements on software algorithms, Lindeberg says.
Indeed, the more
sensitive the algorithm (to get a more exact match), the more
false-negatives (failed when it should have passed) are produced, which
may frustrate a valid user, according to Kevin Luowitz, CTO of biometric
identity service startup CLEAR.
"The challenge is then
to find that happy balance of acceptable false-negatives and
false-positives and user experience," Luowitz says. For security's sake,
you would want the algorithm to veer towards false-negatives rather
than false-positives.
Apple's Touch ID
algorithm is designed to learn and improve over time — with each scan,
it checks if it is a better reading than what is stored, and can update
the master data for your print this way. This algorithm could certainly
be changed or improved through iOS updates, as well.
User error, and a lack
of knowledge about biometrics and how they work, could also be causing
some people's issues with Touch ID.
"A lot of us in the
industry, we are very impressed by the job Apple has done with Touch
ID," Lindeberg said. "But on the consumer side, a lot of people have
never used biometrics at all."
There are a variety of
small things that could be going on to interrupt a successful Touch ID
experience. First, for it to work properly, your finger needs to make
contact not just with the sapphire of the home button, but also the
stainless steel ring surrounding it.
Next, the sensor itself
works by measuring electrical differences between the ridges and valleys
of your fingerprints. If your hands are too dry, it's going to be
difficult for your print to be recognized (this could be a growing
problem in the dry winter months ahead). Conversely, if your fingers are
too moist or oily, recognition can also fail, as those valleys get
filled. If the button gets dirty, as it likely will over time, you'll
also want to clean it to keep Touch ID working properly. Apple suggests
using a clean, lint-free cloth.
But what about that touch sensor itself? Some have worried
that, like traditional capacitive-based fingerprint sensors, it will
degrade over time. Thies of Integrated Biometrics thinks that as long as
the sapphire crystal and metal ring are not damaged and are properly
sealed, the sensor should last the life of the phone. Capacitive sensors
in the past were unprotected, or covered in a very thin layer of
carbon, and thus were very fragile.
For those experiencing
Touch ID issues that cause their phone to freeze, or to not work as well
over time, restarting the phone or recalibrating the sensor are your
best bets. And if you're new to Touch ID or having trouble, Apple also has a guide you can reference for help.
Fingerprint sensors may
not be a new technology. But Touch ID is certainly a new implementation
of it. It's bound to experience some bumps as Apple tweaks its
algorithms, and as users get accustomed to using biometrics on a daily
basis.
At the very least, by
understanding how it works and the inherent pitfalls of fingerprint
sensors, we can help minimize those issues ourselves.
Source: CNN
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