We wrote in January about various biometric scanning systems and fingerprint scanners. At the time we also mentioned retina scanners as one of the systems with potential for wider use in the future.

But it seems that interest in developing retina scanners for the retail market is diminishing. There are two reasons for this: invasiveness and inefficiency.

Retina scanners work by shooting laser light at your eyeball to scan the blood vessels embedded deep within. The veins make a pattern that is as unique to each person as a fingerprint, so they're hard, if not impossible, to duplicate. Once the authorized person has had his or her eyeballs scanned into the system, retina scanners give very few false negative readings (rejecting an approved user) or false positives (admitting an unapproved one).

The millisecond bursts of laser light are perfectly safe as well as accurate. Unfortunately, most people don't like the idea of lasers shooting beams of concentrated light energy into their eyes! (I know I don't; I've put off even having laser surgery for years now, despite the glowing reports I've received from friends who have undergone the procedure.)

Additionally, retina scanners are user-intensive and slow. For the scanner to work, one must put one's face within a few inches of the scanner and look directly into it for 10 to 15 seconds while it maps the vein patterns. Having one person after another line up and peer at the device for that long is not an inviting prospect for a company with more than a few employees.

(There's also the problem of some people's squeamishness about second-hand contact - you've probably noticed Ms. Toddler's Mom who cleans off the grocery cart handle with a sterilized wipe, or met Mr. Finicky who polishes the mouthpiece of a telephone before talking into it. Just imagine their reaction to putting their faces that close to . . . Horrors!)

Blood vessels also reflect a person's physical condition. One blogger on a favorite web site wondered if the red eyes associated with his Monday hangover - or the swollen blood vessels from a bout with a cold or a sleepless night - would interfere with accurate readings.

Retina scanners are also expensive compared to more user-friendly proximity or fingerprint scanners. That factor, coupled with employees' hesitancy to submit their eyes to repeated scanning, will probably conspire to keep retina scanners from widespread use, at least in the foreseeable future.

But there is another alternative that is being researched and put to some use in the security industry.

Iris scanners focus on a different part of the eye. The iris is superficial - that is, it's located almost at the surface, and it's covered with minuscule bumps and ridges that are easy to read and difficult to duplicate. Like fingerprints, the iris's pattern is unique to each person.

Proximity isn't such an issue with iris scanners as it is with retina scanners, either. One can stand as much as 18 inches away, and the reading is quicker - though probably not quite as accurate. One of the concerns being addressed by some researchers is that contact lenses could be used to falsify the "iris-print."

Whether or not eye scanners of any sort become the future gold standard for identification, it's likely that some sort of bio-scanning systems will. As one writer put it, coupling user-supplied information like a user-name - identifying "who you are" - with a biometric reading based on your unique physical self - "what you are" - approaches an ideal combination. Not only is it almost impossible for someone else to falsify a reading based on certain physical characteristics, but it also eliminates the need to remember yet another password.

Other scanning methods include facial dimension scanners and hand scanners, both of which measure not just individual characteristics - fingerprints, shape of the eye, etc. - but the relationship between features - the distance between the brow ridge and the chin, the amount of space between the knuckles on your fingers, the prominence of cheekbones, etc. Even the minute amount of DNA in the sweat or oil on your fingertips can be measured. In fact, DNA readers might well be the ultimate scanning and identification device as we move further into the 21st Century. We'll see!