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In-display fingerprint sensors and cold weather could mean problems

It's the time of year in the northern hemisphere where things can go from warm and delightful to chilly and brisk and that means it's time to hear more about how fingerprint sensors on smartphones are having troubles. It might feel inconvenient, but there is a reason your fingerprint sensor might seem to work a little poorly in cold weather, and this is especially true of in-display fingerprint sensors. It's not a bad reason, though. It's by design.

Cold changes things

We've all probably heard that heat makes things expand and cold makes them contract. Cold air also holds less moisture which can cause something porous, like the tips of your fingers, to get dry and it means you'll spend more time with your hands inside gloves or a pocket where lint and dirt can live. All three of these things are tough for a fingerprint sensor to overcome.

Even a capacitive sensor can have trouble in the cold weather.

Capacitive sensors, like what we see on the Galaxy Note 9 or Pixel 3, have become a lot better than they used to be — the iPhone 5S had a great sensor for its day, but the first winter after its release saw a flood of complaints. On a capacitive sensor electrical capacitors are built in an array that lets it "see" where the whorls and ridges of your fingerprint are because all parts involved conduct electricity. The hardware and software algorithms used have been refined and using a capacitive sensor in the cold can be almost as successful as it is when it's hot, even though the ends of your fingers aren't the same. Engineers are able to compensate for dry cracked skin, shrunken fingertip ridges, and even microscopic bits of lint and dirt from your pockets clinging to those rough and cracked fingerprint whorls.

While you might find the capacitive fingerprint sensor in your Note 9 is a bit less accurate in December than it is in June, the in-display sensor in your OnePlus 6T or Mate 20 Pro has a much larger potential to fail because of the technology being used.

Read between the lines

We've seen a few phones from Vivo, Huawei, and now OnePlus use an in-display fingerprint sensor and so far they all have been optical sensors and built mostly the same way. An optical fingerprint sensor is placed under the OLED layer in the screen, bright light is shined between the individual pixels in the area you're supposed to place your finger, and everything inside that area is scanned. I hesitate to say it's taking a picture of your fingerprint because it's not collecting the data in any way that can be recreated as a drawing or photo — it's building a set of data points based on exactly what it can see whilst scanning.

This data is then placed into a piece of hardware designed to act as a trusted keyholder. An app or the lock screen on your phone can ask for the keys, the keyholder then asks you to scan your finger, and if it matches the keyholder grants access. The single most important feature of this or any other type of biometric security is that it be built so the scanner can't be fooled. That's the important part in all this, and while we like the convenience of scanning the tip of a finger, it's far more important that it only works when it is supposed to work.

Fun fact: The human body shrinks as it cools until it reaches 4°C, then expands as it freezes because it's over two-thirds water.

Now imagine you just got your new OnePlus 6T and are excited to get it set up and start using it. You probably sat down and added some critical apps, signed into a few accounts, and set up a fingerprint or two. That's exactly what we do, too. Getting a new phone is fun. Later, after an hour's jog or a day on the job site, your finger won't unlock your phone. That's not uncommon nor unexpected, and it's happening because your phone doesn't think those are your fingers. Cold fingers shrink (I'll resist the low-hanging fruit here) and get dry, which causes the edges of the raised lines of your fingerprint to change both size and shape and crack and fray.

While the tech behind capacitive fingerprint sensors has been able to predict and work around most of these issues, optical sensors haven't. Yet. And that's because of what's known as a false positive acceptance rate — the tiny fraction of a percentage that a biometric sensor is wrong and says yes when it should have said no.

No means no

Biometrics are tricky. Ideally, the crossover rate between accepting false readings as positive and true readings as negative is the most important part and that should be well below 0.05% (one in two-thousand). But where the individual biometric challenge is used is very important. Some items, like a safe or an interior door, may be protecting something of high-value and a person who wants access can be required to wait a reasonable amount of time for the data to be collected and processed. Others are the exact opposite. A police officer with a smart gun needs his or her weapon to be ready when it's needed and may not have time to wait for a lengthy challenge, so a biometric lock that has a higher false positive acceptance rate is used as a last line of defense.

Our smartphones fall into the middle. The information inside of them can be quite valuable for some (if this sounds like you, a six-digit PIN is a better option) but for most people, quick access is very important. Companies which make phones need to balance the right amount of security — false readings are not accepted — with the right amount of speed — it doesn't take 15 seconds to unlock your phone. That's tough to do, but a look at the OnePlus 6T shows us that it is possible, even with an in-glass optical sensor. Until we change the data set being entered by presenting a shrunken, dry, cracked and dirty finger that's very different from the comfortable finger we used to set things up initially.

We're impatient and can't be bothered to wait 5 seconds to unlock a phone. The companies making them know this.

OnePlus or Vivo or any other company using in-display fingerprint sensors could send out an update that makes all this go away, but then fooling the sensor with a photo or someone else's finger would be a lot easier. They could also make sure it never happens — even I could build a fingerprint scanner that can never make a match and never work. Getting the balance right is the most difficult part, and as the technology matures and companies that pioneered its use get better at writing the software that operates them, the situation will improve.

In the meantime, why not make a set of "warm, inside hands" fingerprints and a set of cold "outside hands" prints to help things along?

Jerry Hildenbrand
Senior Editor — Google Ecosystem

Jerry is an amateur woodworker and struggling shade tree mechanic. There's nothing he can't take apart, but many things he can't reassemble. You'll find him writing and speaking his loud opinion on Android Central and occasionally on Twitter.

20 Comments
  • Not sure who would ever buy the OnePlus 6T over the OnePlus 6... You get a headphone jack, notification light and one of the best in the industry fingerprint scanners with the OnePlus 6. With the 6T you lose the jack and light and the in screen fingerprint scanner is pretty much garbage compared to a capacitive scanner. They both have an ugly notch. Not much of a reason to buy the 6T over the 6 at all. You are actually getting a worse phone.
  • That's not the point of this article.
  • Because the 6t works on Verizon
  • because I can get it on t-mobile and not pay 578 upfront but pay over time
  • not everyone is stuck in the 20th century with their headphone jacks, notification lights, horse and buggies and smoke signals. get on with it dude and get the 6t...super fantastic phone
  • Bigger battery, smaller notch, works on Verizon. Your priorities might not be the same as everybody's else's.
  • Can't wait to see how in-screen ultrasonic fingerprint readers fare
  • This is one reason I'm worried about the reports that Samsung is removing their face unlock from the S10 line because they feel the in-sreen reader is faster. That's not a good reason and shrinking bezels isn't either.
  • I live in North Central Indiana, it's 25 degrees here and I can tell you that I have already noticed that the FPS on my 6T does not work as well outside as it was a week ago when it was 50 degrees. It's a bigger problem at night when face unlock doesn't work.
  • It's a dry skin issue, not a temperature issue
  • It's both. But the dryness is caused by the coldness, so we wouldn't have the dryness issue without the cold. Hence, it is a temperature issue too.
  • I doubt my skin dried out that much in 2 days. I'm sure you have a good point, but again, our temperature just dropped.
  • Interesting, I have not had any issues yet, but it is just starting to get cold where I live.
  • The simple solution is to use a skin moisturizer for your hands regularly during the day to prevent dry winter skub.... A fingerprint sensor will work fine. I've owned cell phones for over 20 years. I did not bother to use the fingerprint lock at first, then I used it a tonne, but have reverted to a four digit code. I'm not that concerned about mobile privacy. At work, which is live oil and gas plants, phones are not permitted outside of the lunchroom. Everyone has a smartphone and they only get used at breaks. A four digit code is good enough. I'd probably use the in display fingerprint sensor in a few (maybe four) years when I upgrade from my Note 8.
  • Yeah, moisturizer is always a good idea depending on your skin. Mine are always dry, and even when I was a child my grandmother would ask "What did you do to your hands?" because they were weathered and dry. My best friend is the opposite and her skin is like velvet, but I'm not sure it's just because she's 18. Her phone is always a gross smudge farm, so I don't know if an in-display FP reader would work that well for her.
  • And dry fingers is why I register my tongue... It's always wet! Eww, I just grossed myself out.
  • I've found during winters in the north, the battery dies instantly, this is the case for about a continuous 4 months of the year. In this case, the fingerprint sensor not working doesn't really matter. Even in my front pocket.
    From my experience, I don't think manufaceturs do any cold weather testing at all.
  • There's an easy and near flawless - at least for me - workaround for this. Iris scanner. It works for me, indoors or outdoors, day or night, clear lens glasses or my Ice Iridium Oakley lenses. I just hope the reports of Samsung ditching the iris scanner for an in-display FPS is just a rumor.
  • It could mean problems especially if you're wearing gloves. Luckily, the Pixel 3, the best phone you can buy, has 3D face unlock. Oh, wait...
  • Does anyone really care about a brand logo, 10gbs of ram and more wattage for charging? I understand that more usually means better (just like bigger is better) but $700 for hardware that MOST people will never truly take advantage of is kinda crazy. I realize that only true fanboys will likely buy this phone but even a lot of them won't use 10gbs or ram. The lack of IP water resistance, 3.5mm headphone jack and wireless charging make it a deal breaker for me personally but I understand some people just want to know they are carrying around the phone with the most ram or fastest possible wired charging. To each their own; but it's overpriced.