This past weekend, I went on a trip. While going through security at CLE, we all put our luggage and pocket contents into bins to go through the X-ray machines. On top of every bin of stuff, the TSA workers placed what looked like a box covered in well-worn tape. The boxes were about the shape and size of a ream of printer paper (heck, for all I can tell, maybe they were taped-up reams of paper). I was able to see the screen displaying some of the X-ray results, and they appeared to be featureless inside.
What would they have been? My first guess is that they were some sort of control, to make sure that the X-ray machine was functioning properly. My second thought was that the tomography algorithm worked better if there was a known object in the region to be imaged (there must have been some sort of tomography, because I saw the operator taking “slice” images through the 3D pile of items). But I can’t figure out any details, if either of those is the case.
When I flew recently the xray trays were now fitted with a heavy blanket attached on one side and it went over the top of the contents. Maybe there is a need to ensure that the material being x-rayed is pressed as low as possible to make for a good reading?
I wonder if there is a weight detection thing going on as well. Can they weigh the items being scanned to identify anomalous material (I can only think of handguns carved out of soap, but with the way they carry on now soap made from guns would be just as suspect)?
If weighing things down was the purpose, then they weren’t using them very well-- Most of them seemed to just be leaning up against the side of a pile of stuff. Of course, “TSA agents doing their job poorly” is hardly implausible.
Might have been something as mundane as making sure nothing falls out of a tray whilst inside the machine.
The sort of reactive process that happens after they had a bad day retrieving something lodged in the contraption as the line of angry passengers runs around the outside of the building.
Maybe they improve imaging quality by distributing the beam attenuation more symmetrically? On the sides the beam passes through two wall of the bin. From top and bottom, only one. Also, without a cover, the projection taken from the top would have more low energy photons passing through actual scanning volume (your carry on luggage), than the beam from the bottom (as it passes through the plastic bin first which attanuates low energy photons). I can imagine that this might cause artifacts in the computed image.
I flew out of Will Rogers World Airport this morning, and discovered that you’re just about on the money here. Some of the trays got a tape-wrapped brick like object placed on top of an inverted tray of a smaller size. I asked the agent what that was all about, and she told me that it was because trays that are too light would often not reliably activate the sensor that triggers the x-ray/scanner. This can result in a line of a few trays needing to go through the system again, causing back-ups and annoying delays.
That sounds like a winner, especially considering that when I was going through, there was a glitch that resulted in a mismatch (the screen was showing one tray, while saying that the tray it was showing was a different one). That one must not have weighed enough to trigger the sensor, even with the added weight.