Ocular biometrics

Ocular biometrics is currently comprised of two techniques: iris scanning and retinal scanning. In each case, the uniqueness of the anatomical architecture of iris and retinal tissues provides the ability to achieve accurate user identification.

Since both are highly accurate, with reported error rates of less than one in one million, the optical and medical distinctions between these techniques should be considered when contemplating widespread acceptance and use.

Optical considerations

The reason one can easily see another person’s iris with the naked eye, but cannot visualize another person’s retina, is largely due to the complexity of the optics needed to visualize the retina compared to the iris.

Corneal

In contrast to retinal scanning, iris scanning is largely unaffected by the optical or refractive state of the eye. Common refractive disorders that could alter retinal scanning include high degrees of near-sightedness (myopia), far-sightedness (hyperopia), and astigmatism. Common corneal diseases that do not cause loss of tissue clarity, but do result in corneal “warping” (e.g., keratoconus), may also adversely affect retinal scanning accuracy. Consequently, unless a highly myopic, hyperopic, or astigmatic individual is wearing contact lenses, that individual will have a defocused retinal image when they take off their glasses for scanning.

On the other hand, the iris image is largely unaffected by the corneal refractive state.

Both retinal and iris images, however, will be altered dramatically by anything that results in a loss of corneal clarity, such as corneal scarring, corneal edema, and some cosmetic contact lenses.

Lenticular

The lens of the eye is anatomically behind the iris but is in front of the retina. Consequently, any opacification of the lens, called cataract formation, has the potential to alter the retinal scan, but a cataract will never affect an iris scan. This is not a small issue given the fact that cataracts are one of the most common eye diseases.

Pupillary

The iris sphincter is called the pupil. Constriction of the pupil limits the view of the retina. To avoid this, retinal scanners use a low intensity light in the infrared spectrum to achieve image capture without constricting the pupil in the process. Individuals with small pupils will give unreliable retinal scans, and it is not practical to set aside 20 to 30 minutes to pharmacologically dilate pupils when immediate identification is expected from any biometric system. Unfortunately, advancing age is one of the most common causes of papillary constriction (miosis). Bright ambient light such as outdoor daytime screening situations and certain medications also result in miosis.

Medical considerations

Optical considerations notwithstanding, retinal scanning is far more likely to be affected by common systemic and ocular medical conditions than iris scanning.

The most common retinal diseases are vascular in nature. Some are systemic in nature such as diabetic retinopathy, hypertensive retinopathy, and vascular occlusive disease. Others are local, such as macular degeneration. All can profoundly affect the anatomy of the retinal vascular pattern, in turn, altering the previously normal vascular pattern of a person’s retinal scan. Furthermore, some common techniques used in treating these common diseases like retinal laser photocoagulation alter the retinal architecture and appearance markedly.

In past decades, cataract surgery often affected the iris appearance adversely, but newer techniques rarely cause those changes. Like the retina, the iris architecture may be altered surgically. Many glaucoma interventions (laser or surgical iridectomy) actually remove iris tissue thereby changing the scan.

Unlike the retina, common systemic or ocular diseases do not commonly affect the iris architecture.

Conclusion

In the biometrics literature a significant amount of time is spent comparing iris and retinal scanning techniques in terms of cost, ease of use, ease of image capture, and accuracy. By these measures, iris scanning either compares favorably or surpasses retinal scanning.

There are also important optical and medical distinctions to consider prior to widespread adoption of these techniques, and when these considerations are evaluated, iris scanning may again be superior to retinal scanning as a biometric identification technique.

Scott Eccarius, MD