Security screening systems are the X-ray machines, scanners and software that check bags, parcels and people for weapons and explosives at airports, metro stations, borders and the entrances of large buildings. They are also called security screening equipment. In India, every Delhi Metro station and every airport has them.
An X-ray scanner sends a beam of X-rays through a bag. On the other side, an X-ray detector measures how much of the beam got through. Dense things, such as metal, stop more X-rays than soft things, such as clothes. Most scanners use two X-ray energies. Comparing the two readings tells the software what each object is roughly made of. It colours the image: orange for organic material such as food, plastic and many explosives, green for mixed material, and blue for metal. The operator, the screening officer at the screen, looks at the coloured image and decides whether to open the bag.
Newer scanners at large airports use computed tomography (CT), the same method as a hospital CT scanner. The machine takes hundreds of images from different angles and builds a 3D picture of the bag. The operator can turn the picture around on screen. So passengers no longer need to take laptops and liquids out of their bags. India's Bureau of Civil Aviation Security (BCAS) sets the rules for this screening.
The software does more than show the image. It marks objects that look like a weapon or an explosive, so the operator looks at them first. It also records every image. And it can send an image to a remote screening room, where a second officer checks it, so that one officer is not alone with a difficult bag.
Say an airport has 60 cabin-bag scanners across two terminals. The maker's fleet software shows the state of every scanner on one screen. It shows which scanners are running, how many bags each one checked this hour, and how long each operator took per bag. On one scanner, the software sees that the X-ray tube's output has been falling for three weeks. It books a service visit before the image quality drops below the required level.
Screening is one part of protecting a site. Large sites also defend the airspace above them with counter-drone systems, which detect and stop drones. The image-recognition methods inside the software are described in computer vision. The rule that an officer checks each machine alert before acting is the same idea as alert triage in other security work.