Fingerprints Go the Distance

Slightly smaller than a square tissue box, AIRprint houses two 1.3 megapixel cameras and a source of polarized light. One camera receives horizontally polarized light, while the other receives vertically polarized light. When light hits a finger, the ridges of the fingerprint reflect one polarization of light, while the valleys reflect another. “That’s where the real kicker is, because if you look at an image without any polarization, you can kind of see fingerprints, but not really well,” says Burcham. By separating the vertical and the horizontal polarization, the device can overlap those images to produce an accurate fingerprint, which is fed to a computer for verification.

I’m guessing that what they mean is that the light source is polarized and the cameras have polarizing filters in front of them to see the two components. Reflectivity generally depends on the polarization and angle of the incident light; the whole reason that polarized sunglasses are effective is that light scattered off of a surface tends to be polarized parallel to that surface, and at Brewster’s angle absolutely none of the perpendicular component will be reflected. So it seems reasonable that detection of the two polarizations improves the contrast of the image you get. Light at normal incidence will reflect the polarized light with no change, but light scattered off of the valleys will mix in some of the orthogonal polarization, which gets picked up by the other camera.