In addition to passenger baggage, most planes carry enormous amounts of cargo. All of this cargo has to be checked before it is loaded.
Most airports use one of three systems to do this:
Medium X-ray systems - These are fixed systems that can scan an entire pallet of cargo for suspicious items.
Mobile X-ray systems - A large truck carries a complete X-ray scanning system. The truck drives very slowly beside another, stopped truck to scan the entire contents of that truck for suspicious items.
Fixed-site systems - This is an entire building that is basically one huge X-ray scanner. A tractor-trailer is pulled into the building and the entire truck is scanned at one time.
Check Your Bags: CT Scanners
The first security check that your checked bags go through depends on the airport. In the United States, most major airports have a computer tomography (CT) scanner. A CT scanner is a hollow tube that surrounds your bag. The X-ray mechanism revolves slowly around it, bombarding it with X-rays and recording the resulting data. The CT scanner uses all of this data to create a very detailed tomogram (slice) of the bag. The scanner is able to calculate the mass and density of individual objects in your bag based on this tomogram. If an object's mass/density falls within the range of a dangerous material, the CT scanner warns the operator of a potential hazardous object.
Step Through, Please: Metal Detector
All public access to an airport is channeled through the terminal, where every person must walk through a metal detector and all items must go through an X-ray machine.
Almost all airport metal detectors are based on pulse induction (PI). Typical PI systems use a coil of wire on one side of the arch as the transmitter and receiver. This technology sends powerful, short bursts (pulses) of current through the coil of wire. Each pulse generates a brief magnetic field. When the pulse ends, the magnetic field reverses polarity and collapses very suddenly, resulting in a sharp electrical spike. This spike lasts a few microseconds (millionths of a second) and causes another current to run through the coil. This subsequent current is called the reflected pulse and lasts only about 30 microseconds. Another pulse is then sent and the process repeats. A typical PI-based metal detector sends about 100 pulses per second, but the number can vary greatly based on the manufacturer and model, ranging from about 25 pulses per second to over 1,000.
Step Through, Please: X-Ray System
While you are stepping through the metal detector, your carry-on items are going through the X-ray system. A conveyor belt carries each item past an X-ray machine. X-rays are like light in that they are electromagnetic waves, but they are more energetic, so they can penetrate many materials. The machine used in airports usually is based on a dual-energy X-ray system. This system has a single X-ray source sending out X-rays, typically in the range of 140 to 160 kilovolt peak (KVP). KVP refers to the amount of penetration an X-ray makes. The higher the KVP, the further the X-ray penetrates.