X-rays are useful for looking through your luggage and finding things that you might make a weapon out of, typically out of metal. But they will not find weapons made out of things that are otherwise transparent to plastic. If you have different frequencies of light and you interplay them, you can see what the trend line is in the thing's attempt to absorb it or not. And once you do that, you're better at detecting what could be in the suitcase if you move the frequencies back and forth.