For many years now, the promise of biometrics has not been fully realized in large part because performance in the lab is not representative of performance in the field. That’s the reason major locksmiths began shying away from biometrics. As one recently said, “They’re 1 percent of my sales and 10 percent of my service calls.” Failure rates were running from 3 to 20 percent. In a 50-person firm, they could work around this. However, if hundreds of people are involved, the customer has a big mess which falls back on the locksmith.
The core problem is that conventional biometric technologies rely on unobstructed and complete contact between the fingerprint and the sensor, a condition that is elusive in the real world, a world that is wet, dry, or dirty. Users are not all young office workers with great skin who are experienced at using biometrics. Bottom line – good images give good results; bad images give bad results.
Wet or Dry Fingers May Fail
Wet conditions are notoriously difficult for both semiconductor and conventional optical fingerprint sensors to handle. And, yet, moisture is a fairly common real world condition. Some environments are naturally damp, due to climate (Oregon) or setting (a spa). Some people have moist hands. It is also typical for people going through security to be nervous — and to have sweaty hands.
Conventional optical technologies are often unable to produce images in wet conditions because excess moisture obscures fingerprint ridges, resulting in images of puddles, not fingerprints.
Has any other real world condition caused so much trouble in the biometrics industry? Dry fingertips are common, caused by anything from climate conditions and natural skin characteristics to frequent hand-washing and air travel. For instance, a high desert climate causes dry fingers in an entire population.
Most optical sensors are configured to look for the presence or absence of total internal reflectance (TIR), which is the phenomenon whereby the interface between glass and air acts like a mirror at certain angles. The contact between the skin and the platen defeats the TIR, allowing those points of contact between the finger and the sensor to be imaged. Thus, those points of contact must be complete and unobscured to enable the conventional sensor to collect a fingerprint image.
With dry fingers, this is simply not the case. Establishing firm and complete contact with the sensor is very difficult with dry fingers. There is not enough moisture in the skin nor is the skin pliable enough to facilitate the contact necessary for TIR imaging.
It’s a Dirty World Out There
The real world is a rough place and most of us are showing some wear and tear on our hands. Additionally, people don’t have time to wash and lotion their hands before they use a fingerprint sensor.
A construction site is an interesting case. Construction workers work with their hands and have the cuts and calluses to prove it. Additionally, the construction site is dirty so workers may have grime on their hands when they approach a fingerprint sensor. Altogether, this real world scenario is a nightmare for system administrators whose conventional fingerprint sensors depend on quality contact between the finger and the platen.
We’re All Different
Many people, both young and adult, have small or fine fingerprint features that can be difficult to image. If the sensor cannot differentiate between these fine characteristics, system performance will suffer.
Age is another physiological characteristic that can affect the ability of a sensor. One effect of aging is the loss of collagen in the skin; elderly fingers have soft fingerprint ridges that collapse into each other when the finger touches a surface. Because many sensor technologies depend on the quality of contact between the finger and the sensor to collect a good image, soft fingerprint ridges can be difficult.
Behavioral differences across user populations can affect performance. Some people may tend to press hard and others, being more tentative, may barely touch the sensor at all. For technologies that depend on the quality of that touch, this can be a big problem.
Multispectral Technology Overcomes Common Obstacles
Multispectral imaging is a sophisticated technology developed to overcome the fingerprint capture problems conventional imaging systems have in less-than-ideal conditions. This more effective solution is based on using multiple spectrums of light and advanced polarization techniques to extract unique fingerprint characteristics from both the surface and subsurface of the skin. The nature of human skin physiology is such that this subsurface information is both relevant to fingerprint capture and unaffected by surface wear and other environmental factors.
The fingerprint ridges that we see on the surface of the finger have their foundation beneath the surface of the skin, in the capillary beds and other sub-dermal structures. The fingerprint ridges we see on our fingertips are merely an echo of the foundational “inner fingerprint”. (See Figure 1.)
Unlike the surface fingerprint characteristics that can be obscured by moisture, dirt or wear, the “inner fingerprint” lies undisturbed and unaltered beneath the surface. When surface fingerprint information is combined with subsurface fingerprint information and reassembled in an intelligent and integrated manner, the results are more consistent, more inclusive and more tamper-resistant.
Multispectral fingerprint sensors capture high-quality images because the direct imaging process does not depend on a clean finger/sensor interface. At last, biometrics can provide the same type of reliability as a card but removes all the negatives of the card, including cost of the cards themselves and the more expensive cost of managing cards. After all, nobody leaves their finger at home nor does it wear out.
Operation is simple. The user comes up to the reader, lays his/her finger on the reader, and an image is captured. The unique points are translated into information that is compared to that on file. If they match, the user sees a green light, the lock output is energized and the user walks through the door.
Importantly, now biometrics, which determines that you are you – not what you carry – can be sold by locksmiths with confidence in more places and more applications. For example, more than 40 million people are already enrolled on multispectral imaging-based systems at locales ranging from the classic door access control situation to the gates of the world’s favorite theme parks. Such readers are keeping borders secure around the world. Indeed, more than 400,000 people pass through multispectral imaging sensors every day at the Hong Kong border crossing.
Bill Spence is vice president, transaction systems, for Lumidigm.
