There are broadly two types of facial recognition systems that are used. The first doesn't work in real time, it's static and uses individual images. If you have a photo of someone, it's possible to compare it against the database of tens of thousands or even millions of images to find a potential match.
The second type of facial recognition system is more dynamic and therefore more controversial. It works in real time. It's called automated facial recognition. It all starts with cameras that can be positioned in public places such as outside shops or on high streets. The image is captured by these cameras and then processed by artificial intelligence software. It's able to pick out humans and their faces from other objects around them, such as cars, lampposts and street signs.
The faces are analyzed in a matter of seconds and compared to images that are already held on a database. The systems that are currently being trialed can identify tens of faces in one image. They can also scan huge crowds with very little effort.