WIRED.com writes about Apple’s Touch ID fingerprint reader.

Fingerprint reading is accomplished through a complex method.

Touch ID is composed of an 8 x 8 millimeter, 170-micron-thick capacitive sensor located just beneath the home button on the 5s. This is used to capture a 500-pixel-per-inch (ppi) resolution image of your fingerprint. The sensor can read pores, ridges, and valleys. It can identify arches, loops, and whorls. It can even recognize fingerprints oriented in any direction.

When you place your finger or thumb on the sensor, it looks at the fingerprint pattern on the conductive sub-dermis layer of skin located underneath the dermis layer. It also measures the differences in conductivity between the tops of the ridges and the bottoms of the valleys in your prints in this layer. This is more accurate than looking at the dead surface of the skin alone, which is constantly changing and isn’t conductive.

Touch ID needs a good database of fingerprint records to ensure that the fingerprint is quickly recognised.

Apple partially gets around the small sensor issue using the enrollment process, which includes rolling your finger around to try to capture every microscopic nook and cranny on your finger. Then, at least, it has a large source to pull from, even if it’s only scanning a section of that each time you tap your finger.

It doesn’t stop learning.

Apple’s Touch ID algorithm is designed to learn and improve over time — with each scan, it checks if it is a better reading than what is stored, and can update the master data for your print this way. This algorithm could certainly be changed or improved through iOS updates, as well.

So what can go wrong when you use Touch ID?

There are a variety of small things that could be going on to interrupt a successful Touch ID experience. First, for it to work properly, your finger needs to make contact not just with the sapphire of the home button, but also the stainless steel ring surrounding it. Next, the sensor itself works by measuring electrical differences between the ridges and valleys of your fingerprints. If your hands are too dry, it’s going to be difficult for your print to be recognized (this could be a growing problem in the dry winter months ahead). Conversely, if your fingers are too moist or oily, recognition can also fail, as those valleys get filled. If the button gets dirty, as it likely will over time, you’ll also want to clean it to keep Touch ID working properly. Apple suggests using a clean, lint-free cloth.

I was barely a few days into using Touch ID and I found myself wondering how I ever lived without it. Unlocking my phone is now quick and intuitive. To quote Steve Jobs, “It just works.”