Terrorists and others wishing to cause harm or disruption have long used postal and courier services to deliver hazardous items to target recipients. Some X-ray machines are in the form of cabinet units and are used for the inspection of items of mail or suspicious packages at high security locations. The X-ray tube may be positioned above or below the chamber. Packages are placed in the chamber via a drawer or through a hatch. A switch on the control panel activates the X-ray tube, which remains on for the pre-set time or until the switch is released. Usually an image of the package is formed on a fluorescent screen and is viewed via a mirror and through a leaded glass window. On some units the image is viewed on a television screen.
Portable X-ray scanners are commonly used by EOD technicians, customs, police, military and other law enforcement agents to inspect items in locations where mains electricity is not available, or where a suspicious item (such as an abandoned bag ) needs to be examined to establish if it is dangerous or not.
Very small hand-held X-ray scanners are currently being developed to allow police and customs officers to seek out illegal drugs, weapons, IEDs or hidden people. While the images produced by these highly-portable scanners are not crystal-clear, they are sharp enough to help reveal prohibited items in cargo boxes, car boots, etc.
Hand baggage screening X-ray machines are typically conveyor units usually consist of three main parts:
1. Main Body: This houses the X-ray tube; tunnel with overlapping lead impregnated curtains at the input and output ends of the tunnel; and perspex screens.
2. Conveyor: This moves the items through the X-ray tunnel and produces them for collection or searching.
3. Control Console: This features the keyboard for operating the machine and using the image enhancement features; and a television monitor - sometimes two monitors - for analysing the X-ray images. The images can usually be electronically retained and manipulated after the item has passed though the X-ray beam. Once an image has appeared on the screen, the image will remain on the screen until it is replaced by the image of the next bag, even when the bag is gone.
The use of conventional x-ray inspection to screen hand baggage has improved. The original systems were fluoroscopes with poor penetration, resolution and dynamic range, and a high radiation dose to the baggage and also possibly the human screener. Systems have improved and can now provide the screener with detailed information (via dual x-ray energy) of the composition of the contents of the bag. Most X-ray imaging systems employ methods of image enhancement to increase the visibility of the acquired image but most still depend on an operator to make the detection decision. The X-ray tube may be fitted at the top, base or side of the unit. The X-ray beam is emitted in a very thin vertical ‘slice’, to greatly reduce the radiation levels inside the unit. As an item passes through the conveyor unit, it breaks a light beam and this triggers the X-ray tube to operate. X-rays are generated by the emitter and pass through the object being x-rayed. The detector measures penetration results. These results are translated by computer into the picture, allowing the operator to discriminate between different types of matter. When the item clears a second light beam, the X-ray tube switches off.
Advanced technology X-ray machines screen bags from two angles instead of one and show the images on side-by-side screens.
 "A police station in Pakistan destroyed by a Postal Bomb")
