Iris scanning as a form of biometric recognition doesn't get as much attention as retinal scanning or fingerprints, but it's got a lot of advantages. Irises don't change over time, like fingerprints can, there's no need for any actual physical contact to get a scan, and it's hard to fool an iris scanner with surgery or other medical alteration. Now, there's one even bigger advantage: you can carry an iris scanner around with you in one hand.
Russell Brandom over at the Verge has a nice look at AOptix, a company that has created what we can only think of as a "recognizer." It's an iPhone case that includes an iris scanner, a fingerprint scanner, and facial recognition ability--it's kind of a one-stop shop for figuring out who someone is.
Iris scanners work by essentially taking a picture of the iris, the colored part of the eye around the pupil. The specific patters are as unique as fingerprints. You take these pictures by shining a bright infrared light at the iris, and then matching the patterns with a database. The biggest problem? It can be fooled by, well, an actual photograph of an iris. But when taking a scan from 10 inches away, it'd be hard to slip in a photo in front of the scanner.