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Skin markings like scars, birthmarks and tattoos are considered soft biometrics, easily measurable physical characteristics that can change. Tattoos are becoming more common with estimates that approximately 36% of people between 18 and 29 have at least one tattoo. Most are specific to the individual, though many social groups adopt tattoos of similar design to designate membership. This is very common among gang members. Tattoos can give information on social characteristics such as time in prison, number of crimes committed and ethnic affiliations.

Biometric scientists from Michigan State University developed software to aid in photographic tattoo identification. Tattoo images taken by law enforcement can be matched to existing images in photo databases which is much more efficient than a text-based search. The tattoos are matched using complex mathematical algorithms that compare similar characteristics. This allows matching when pictures may be blurred, such as those taken by a surveillance camera. Images are compared based on color, texture and shape of the tattoo.

Existing photo databases can be linked to the software, providing information from law enforcement agencies all over the United States. As of 2009, the system contained 64,000 digital tattoo images obtained from the Michigan State Police...
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