Retrospective facial recognition (RFR) differs from live facial recognition in a single defining way: it runs after an event, not during one. An image of an unknown person -- typically pulled from CCTV, a mobile phone, social media, or a crime scene -- is converted into a numerical representation of facial features and compared against a reference database of already-known people, most often custody photographs or a similar offender image gallery. The system does not return a single answer. It reorders the entire reference database from most to least likely match and returns a ranked list of candidates, commonly the top 100 to 200, for a trained operator to review. A human decides whether any candidate is actually a match; the software's role stops at generating and ranking the shortlist.

This is what UK police forces describe as the practical advantage: South Wales Police has stated that without RFR an identification can take around fourteen days, and with it, minutes. The technology depends entirely on the quality and content of whatever reference database a given deployment searches, which is also where it varies most between countries: Italy's SARI Enterprise searches the national AFIS photographic database and is described by Italian police as producing an investigative lead rather than a legally determinative identification; Toronto Police Service compares against lawfully obtained booking photographs using NEC's NeoFace Reveal; Victoria Police in Australia uses a system called iFace against an offender image database; York and Peel Regional Police in Canada share an Idemia-based system searching combined booking photographs. The underlying technique -- one-to-many search against a specific gallery, human adjudication of a ranked shortlist -- is consistent across all of these; what changes is which gallery is searched, under what legal authority, and with what oversight.

This is a distinct technology from a country's own specific implementation of retrospective search against a particular database -- for example, UK retrospective search against the Police National Database has its own dedicated page on this site, since the PND's scale, the vendor history around it, and the specific legal and oversight questions it has generated are substantial enough to warrant separate treatment. This page covers the general capability as deployed elsewhere.