False Acceptance in Winter: Why Do Fingerprint Scanners Fail in Cold Weather?

One of the biggest benefits of biometric fingerprint recognition is speed. Just a touch of the finger and you are done. In most cases, it will take less than a second to unlock phone, verify identity, mark attendance or authenticate payment. No codes, keys, fobs or any other information / artifact to carry.Friction ridges on fingertips are more than adequate to be all of them and more. But don’t fingerprint scanners behave strangely sometimes for no good reason? At times, they would take multiple scans to recognize or will not recognize you at all. And this problem becomes more apparent in freezing cold weather.

Fingerprint recognition systems make use of many technological approaches to do things right and they have certain quality expectations when it comes to input data (the fingerprint scan), to produce desirable results. There are many things that can spoil a fingerprint scan: environmental factors, user behavior, poorly maintained systems, etc.

Very low temperature (-10 degrees or less) in winters can affect performance of fingerprint scanners negatively, leading to false acceptance and false rejections. But is it the scanner to blame or something else? Let’s find out.

False acceptance, in the context of fingerprint recognition system, is an incident in which the recognition system grants access to an unauthorized person. False rejection, on the contrary, in an incident in which the recognition system denies access to an authorized or enrolled person.

False acceptance is an undesirable event keeping it under control is very important. Allowing an unauthorized person to a secured area / information can be catastrophic. It will probably be the worst nightmare for any access control system or personal identification system. Since it is an undesirable even, it is tried to be kept at minimum. In an ideal case, it will be zero.

To make sure biometrics systems perform as expected and do not result in false positives or false negatives, they are taken through rigorous testing and performance evaluation. Due to increasing influence of fingerprint recognition systems in all sorts of identification and authentication applications, curbing false positives and false negatives in biometric systems has become very crucial.

FAR (false acceptance rate) is one of many performance metrics used for evaluation and testing of biometric recognition systems.Performance metrics are created to rate performance of a biometric system, solution or application. Different metrics can be used for the purpose.

The FAR is the probability of cases for which a biometric recognition system fallaciously authorizes an unauthorized person. It happens when a biometric system or application inaccurately matches a biometric input with a stored template, fallaciously returning a match and granting access to an unauthorized individual.

FRR or false rejection rate is the probability of cases for which a biometric system fallaciously denies access to an authorized person. It happens when a biometric system, solution or application fails to match the biometric input with a stored template, fallaciously returning a no-match and denying access to an authorized person.

You can read more about biometric performance metrics here.

If we talk in terms of performance metrics, freezing cold weather can increase the chances of both FRR and FAR.

In very low temperatures (minus 10 degree Celsius and below) fingerprint scanners start to behave in undesired ways. Most of the times, it will refuse to enroll a user or it will take multiple scans to get past the system for already enrolled users. In a very few occurrences, it will allow you even if you are not registered (false acceptance). Why does it happen?

In very cold weather, human skin loses its natural moisture and becomes severely dry. For some skin types, this problem may start to appear even early (at around minus 5 degree Celsius). Unfortunately, friction ridges on human skin also don’t stay unaffected from cold weather and loses its natural moisture. In worse cases, will start cracking and peeling off. Minute details of friction ridges pattern are also affected, at least in terms of their usability in biometric recognition.

Cold weather leads to skin dryness, which in turns results in stiffer skin and less blood perfusion. Low temperature makes things shrink and human skin is not an exception. Dryness of friction ridges can also lead to cracks on skin and skin peeling, which also leads to deteriorated performance on fingerprint recognition systems. System may completely refuse to enroll or verify a scan.

Loss of moisture also leads to reduce conductance in skin. But that is not the only reason of reduced skin conductance. To protect from freezing temperatures, human body’s protective mechanism results in reduced blood circulation in skin to retain temperature and keep warm. Due to reduced blood circulation and lost skin moisture, skin conductance is negatively affected. Skin conductance is the fundamental requirement for capacitive fingerprint sensors to work. This is the type of sensor which is widely used on smartphones for locking / unlocking as well as user authentication.

No wonder if it takes multiple attempts to unlock your phone when it is very cold (usually below minus 10 degree Celsius). While it may seem as if there is a problem with the phone or sensor, it will actually be the cold temperature to blame, which causes your friction ridges to become unusable for biometric recognition.

Fingerprint scanner systems are built using different kind of material and each type of material used in constructing the system has its own physical properties like freezing point, melting point, etc. Material is chosen carefully so that it does not affect the performance of the scanner in usual weather conditions. Unless exposed to extreme temperatures (very high or very low temperatures), fingerprint scanners work perfectly in normal weather conditions.

For example, Integrated Biometrics Kojak’s spec sheet suggests that the device can be operated within the temperature range of -10°C ~ +55°C / 14°F ~ 131°F. The manufacturer claims the device to withstand extreme temperatures. We do not normally see temperatures naturally going above or below these ranges unless you need to operate the device in otherwise extremely hot or cold conditions.

Low temperature and performance of the fingerprint recognition system is not directly related, however, few other factors induced by cold weather can affect the recognition performance negatively.

• If recognition system is not properly sealed, condensation may collect under the scanner surface, which may render them unable to work properly.
• It may cause other systems like liveness detection to fail, rendering the recognition system potentially insecure against the spoof attacks.

Since the problem lies in deteriorated skin condition in winters, there are a few things that can help maintain desired fingerprinting performance in cold weather.

• Go for scanners that come with water and dust-proof construction, for example IP rated fingerprint scanners offer sealed construction immune to external conditions affecting internals of the system.
• Gloves can keep your hands warm and moist, but they are not the absolute solution of this problem. First, gloves result in inconvenience of taking them off before scanning your finger tip. This can particularly be inconvenient when you use fingerprint recognition on your smartphone, which people tend to use several times a day.
• Apply moisturizer / lotion in adequate amount, they may help keep your finger skin moist and healthy. However, when applied in more than adequate quantity, they will work as a contaminant and result in failed finger scans or false acceptance.
• Re-enrollment of the users experiencing this problem can be a potential solution, however, enrolling fingerprints while they are not in their normal condition, is not advisable.

Some biometric modalities can be sensitive to weather conditions and can affect the recognition process when those weather conditions occur. Fingerprint is normally a very good biometric modality but it may start experiencing problems if temperature falls below minus 10 degree Celsius.

Though cold weather does not directly affect fingerprint scanners, but it can leave the friction ridges unusable for biometric identification / authentication. Exposure to cold weather results in skin dryness, affecting pattern of friction ridges and conductivity of skin, which in turn affects the ability of fingerprint scanners to capture a good quality fingerprint image.

Keeping finger skin moist with gloves, using lotion and using scanners with weather-proof construction can be the potential solutions of the problem.