The scanner for carry-on luggage is a sophisticated shadow box. An X-ray tube,not dissimilar to the one in a dental office, throws a “spotlight” on the gadget as it travels the belt. Opposite the tube are two sheets of material that glow when exposed to x-rays. The sheets show shadows, like a finger puppet show from a bedroom lamp.
The two sheets are separated by a slab of metal that acts like a “color filter”. This provides the equivalent of two-color vision. One color is for dense objects, made of metal or ceramics. The other color is for low density: plastics, liquids, organics. The intensity of each color corresponds to how much X-ray is blocked by each item. To help things stand out, the operator has an enhanced, “false color” view.
The above scheme can’t say specifically what carry-on luggage contains. Some carry-on scanners have an additional detector that is sensitive to more “colors”, enough to identify a material. But it is imprecise. Checked luggage is examined with the much more sophisticated CAT scanner. Theoretically, a CAT scanner can perform material identification of the entire contents of a bag.
In theory! , which was worked out by Arthur Compton in 1923. For material identification, a CAT scanner has two detectors. But quoting from (pdf) Detection of Organic Materials by Spectrometric Radiography Method,
...the specified design of the dual-energy detector array do not allow detection of all potentially dangerous substances with sufficiently low error probability.
(Note: The “dual energy detector” is actually a “full color” spectrometer. Even with “full color vision”, it has a problem.)
A checked bag is a blizzard of items that camouflage each other. How well a checked baggage CAT scanner works against current and future threats is a closely held secret. With all due respect for the political process, this should be the inquiry, not the really lousy experience of modern airplane travel. How good can it be when you’re thankful just to get there?
The political drama distracts. And the above might have put you into overload. So we’ll take “baby steps.” After you’ve absorbed the above, refreshed yourself, had a cocktail,..the Solution.