A touchless sensor works similar to an optical sensor. In general there is a precision glass optic with a distance of 2-3 inches from the fingerprint while the finger is scanned. The fingerprint is put on an area with a hole. One disadvantage can be considered is that dust and dirt may fall through the hole onto the glass optic with the possible result of bad images. Another point is that the scanned fingerprints are spherical which leads to more complex matching algorithms.