How Biometrics Works

At first glance, using handwriting to identify people might not seem like a good idea. After all, many people can learn to copy other people's handwriting with a little time and practice. It seems like it would be easy to get a copy of someone's signature or the required password and learn to forge it.

But biometric systems don't just look at how you shape each letter; they analyze the act of writing. They examine the pressure you use and the speed and rhythm with which you write. They also record the sequence in which you form letters, like whether you add dots and crosses as you go or after you finish the word.

Unlike the simple shapes of the letters, these traits are very difficult to forge. Even if someone else got a copy of your signature and traced it, the system probably wouldn't accept their forgery.

A handwriting recognition system's sensors can include a touch-sensitive writing surface or a pen that contains sensors that detect angle, pressure and direction. The software translates the handwriting into a graph and recognizes the small changes in a person's handwriting from day to day and over time.

People's hands and fingers are unique -- but not as unique as other traits, like fingerprints or irises. That's why businesses and schools, rather than high-security facilities, typically use hand and finger geometry readers to authenticate users, not to identify them. Disney theme parks, for example, use finger geometry readers to grant ticket holders admittance to different parts of the park. Some businesses use hand geometry readers in place of timecards.

Systems that measure hand and finger geometry use a digital camera and light. To use one, you simply place your hand on a flat surface, aligning your fingers against several pegs to ensure an accurate reading. Then, a camera takes one or more pictures of your hand and the shadow it casts. It uses this information to determine the length, width, thickness and curvature of your hand or fingers. It translates that information into a numerical template.

Hand and finger geometry systems have a few strengths and weaknesses. Since hands and fingers are less distinctive than fingerprints or irises, some people are less likely to feel that the system invades their privacy. However, many people's hands change over time due to injury, changes in weight or arthritis. Some systems update the data to reflect minor changes from day to day.

For higher-security applications, biometric systems use more unique characteristics, like voices.

Your voice is unique because of the shape of your vocal cavities and the way you move your mouth when you speak. To enroll in a voiceprint system, you either say the exact words or phrases that it requires, or you give an extended sample of your speech so that the computer can identify you no matter which words you say.

When people think of voiceprints, they often think of the wave pattern they would see on an oscilloscope. But the data used in a voiceprint is a sound spectrogram, not a wave form. A spectrogram is basically a graph that shows a sound's frequency on the vertical axis and time on the horizontal axis. Different speech sounds create different shapes within the graph. Spectrograms also use colors or shades of grey to represent the acoustical qualities of sound. This tutorial has a lot more information on spectrograms and how to read them.

Some companies use voiceprint recognition so that people can gain access to information or give authorization without being physically present. Instead of stepping up to an iris scanner or hand geometry reader, someone can give authorization by making a phone call. Unfortunately, people can bypass some systems, particularly those that work by phone, with a simple recording of an authorized person's password. That's why some systems use several randomly-chosen voice passwords or use general voiceprints instead of prints for specific words. Others use technology that detects the artifacts created in recording and playback.

Iris scanning can seem very futuristic, but at the heart of the system is a simple CCD digital camera. It uses both visible and near-infrared light to take a clear, high-contrast picture of a person's iris. With near-infrared light, a person's pupil is very black, making it easy for the computer to isolate the pupil and iris.

When you look into an iris scanner, either the camera focuses automatically or you use a mirror or audible feedback from the system to make sure that you are positioned correctly. Usually, your eye is 3 to 10 inches from the camera. When the camera takes a picture, the computer locates:

It then analyzes the patterns in the iris and translates them into a code.

Iris scanners are becoming more common in high-security applications because people's eyes are so unique (the chance of mistaking one iris code for another is 1 in 10 to the 78th power [ref]. They also allow more than 200 points of reference for comparison, as opposed to 60 or 70 points in fingerprints.

The iris is a visible but protected structure, and it does not usually change over time, making it ideal for biometric identification. Most of the time, people's eyes also remain unchanged after eye surgery, and blind people can use iris scanners as long as their eyes have irises. Eyeglasses and contact lenses typically do not interfere or cause inaccurate readings.

As with irises and fingerprints, a person's veins are completely unique. Twins don't have identical veins, and a person's veins differ between their left and right sides. Many veins are not visible through the skin, making them extremely difficult to counterfeit or tamper with. Their shape also changes very little as a person ages.

To use a vein recognition system, you simply place your finger, wrist, palm or the back of your hand on or near the scanner. A camera takes a digital picture using near-infrared light. The hemoglobin in your blood absorbs the light, so veins appear black in the picture. As with all the other biometric types, the software creates a reference template based on the shape and location of the vein structure.

Scanners that analyze vein geometry are completely different from vein scanning tests that happen in hospitals. Vein scans for medical purposes usually use radioactive particles. Biometric security scans, however, just use light that is similar to the light that comes from a remote control. NASA has lots more information on taking pictures with infrared light.

Some people object to biometrics for cultural or religious reasons. Others imagine a world in which cameras identify and track them as they walk down the street, following their activities and buying patterns without their consent. They wonder whether companies will sell biometric data the way they sell e-mail addresses and phone numbers. People may also wonder whether a huge database will exist somewhere that contains vital information about everyone in the world, and whether that information would be safe there.

At this point, however, biometric systems don't have the capability to store and catalog information about everyone in the world. Most store a minimal amount of information about a relatively small number of users. They don't generally store a recording or real-life representation of a person's traits -- they convert the data into a code. Most systems also work in only in the one specific place where they're located, like an office building or hospital. The information in one system isn't necessarily compatible with others, although several organizations are trying to standardize biometric data.

In addition to the potential for invasions of privacy, critics raise several concerns about biometrics, such as: