Fujitsu security system uses cameras mounted on the ceiling of the area in front of the access door to the secure area. Each of the cameras watches over an area of about one square meter and the signal is fed to a computer that spots when someone is walking up to the door. If it’s a single person then they are allowed to authenticate themselves at the door and enter as normal. If there’s someone else in the area the computer won’t let the door open until either the second person has also authenticated or until they walk away.”]
Source: https://www.csoonline.com/article/2122740/fujitsu-security-system-targets-employee-tailgating.html