Physiological features
Additional biometric features could easily be added to the features described below. However, we have limited our descriptions to the most important and most commonly used features in order to avoid getting bogged down in details. Some physiological features that cannot be consciously altered also do not change much over time. For example, the characteristic patterns of fingerprints never change during a person’s lifetime, nor do the patterns of the retinal blood vessels. The face is certainly an exception, since even though it basically does not change, it can be transformed to a large degree by a different haircut, growing or shaving a beard and the like. Basically, though, it is possible to say that biometric features based on adult physiology do not require ongoing adjustment of the reference pattern, since any changes are negligibly small or non-existent.

Facial features
Based on everyday experience, we can assume that the human face is suitable for use as a biometric feature. However, transforming this assumption into a technical implementation is fraught with difficulty. Faces can change greatly within a short time, and their appearance depends strongly on external factors such as eyeglasses, beards, make-up, illumination and viewing angle. If a person’s face is photographed using visible light and the information captured by the photograph is suitably processed, is will often be possible to make a decision about the identity of the person behind the face. The technical toolkit for this process includes very powerful computers, fuzzy logic and neural networks, which indicate the amount of effort that it entails. In addition, the stored image should be three-dimensional, or the person being examined should turn his or her face, to prevent the system from being deceived by holding a photograph in front of its sensor. In general, facial features may prove to be a very interesting subject for future biometric methods, but presently they cannot yield a high enough probability of accurate identification to allow them to be widely used.

Retinal features
Every human retina has its own unique pattern of blood vessels, with their branches and nodes. This pattern can be captured using a beam of infrared light directed through the pupil. The light reflected by the retina is collected by a CCD camera, which in turn sends the recorded image data to a computer for analysis. Retinal imaging is one of the very best biometric methods, since it can be used to uniquely identify a person with a very high degree of probability. However, it is not readily accepted by users, since they must place their eyes very close to the scanner in order to be identified. This often results in a fear of infection and anxiety with regard to the infrared beam.

Iris features
The iris is a variable diaphragm that controls the amount of light reaching the retina. Like the retina, it is a biological feature that is unique to each individual. An iris scan can be performed at a greater distance than a retinal scan, since the measurement process is simpler. With this method, the iris (which is located at the front of the eye) is imaged by a CCD camera using visible light. The data evaluation is similar to that used for retinal images. Contact lenses can strongly influence the measurement results under certain conditions, and thus cause problems.

Hand geometry
Identification systems based on three-dimensional measurements of the hand, or parts or the hand, were used as early as the 1970s. These measurements can be based on features such as finger length, finger diameter and fingertip radius. Unique individual features can be determined using very few measurement points (e.g. five). The actual measurements can be made very simply using infrared LEDs and photodiodes, with the hand geometry being measured by recording which photodiodes are fully or partially blocked by the hand. Since only a few measurements are needed for identification, the procedure is sufficiently fast and uncomplicated for users. The user only has to place his or her hand in an instrument, which then performs the measurements.

Fingerprints
The best-known biometric identification method based on a physiological feature is without doubt fingerprinting. In the electronic version, it is naturally no longer necessary to coat the fingers with black ink and press them onto a piece of paper. Instead, a thumb or fingertip is placed against a transparent plate, and a camera mounted under the plate scans the skin surface without any contact. Alternatively, ultrasonic sensors or semiconductor-based capacitive sensors can be used. The comparison with the reference pattern is usually based on the primary features of the classification scheme developed by Edward Richard Henry, which are arches, loops and whorls. Information about the type, position and orientation of approximately 20 such features is stored, and this information is used to generate the reference pattern. These characteristic features are called the ‘minutiae’. Certain groups of users dislike this method, since fingerprints have been used for years as a tool for combating crime. Wounds on the fingertips can also make unambiguous personal identification difficult. It can also be difficult to identify persons who work a lot with their hands. Many systems have sensors for measuring the temperature of the finger or the pulse rate, in addition to the scanner. This is intended to prevent an amputated finger from being used for identification purposes. Nevertheless, fingerprint systems are widely used, since they present relatively fewproblems in terms of technical difficulty and user acceptance. The time needed to sense the fingerprint and perform the subsequent test also lies within reasonable limits. The sensors used have a resolution of around 400 dpi.