There was once a time when, upon the discovery of a chewed-on corpse in a secure underground military bunker, it was impossible to determine whether responders were dealing with a regular sociopath or a full-scale zombie invasion. Those dark times have ended. Scientists can now tell the difference between a fingerprint made by a living body or dead flesh.
This isn't just an technique meant to screen out zombie killers. It also rules out Demolition Man style security breaches in which people use dead fingers to beat fingerprint scanners. More seriously, it can end the material reward for chopping people's fingers off to get access to secure areas, or even high-value cars. There have been some gruesome reports of people having their limbs amputated to beat such scanners.
There are many differences between living and dead hands. (For instance, it's creepy to hold living hands some of the time and dead hands all of the time.) Most of those differences are difficult for a machine to quickly and reliably quantify. The most reliable difference, though, is the fact that living skin pales when it is pressed against a surface. The capillaries that rush blood around the surface of the body aren't able keep themselves open under pressure, and blood leaves that part of the skin. Although different fingertips may be different shades, the color change is constant.
Living fingers with all their blood in them absorb light of about 550 nanometers. When the skin is pressed into paleness, the color changes to absorb light at about 1450 nanometers. If there's no change, the blood has already left the body. If the sensor doesn't sense a change between first contact and an extreme press of the hand, it can surmise that either the dead walk among us, or someone is missing something important.
Via New Scientist.