I just got this bottle of water through the TSA and feel like I am going to put on some
kind of terror watchlist, but thanks to some new technologies this is actually
becoming the new norm, and it’s helping make airport security wait times much shorter.
Thanks to these things. Airport security scanners. Now, these may look like regular old scanners,
but seasoned travelers have known to look for the lines with these machines for a few years,
because they are much better than the old x-ray scanners. You know the ones, the ones
that you have to take practically everything out of your carefully packed bag, or you get
held up because some person who apparently hasn’t flown in 20 years left their laptop in their bag.
Well, that’s all about to change, but why? How are these new fangled
machines making airport security less miserable, and much safer.
Both these and the new scanners use basically the same technology. X-rays.
But these new scanners can somehow detect not only the shape of what is in your bag,
but are able to figure out what material it’s made out of, which allows it to differentiate
whether the liquid that’s in your water bottle is water, or some kind of explosive.
Now, this confused the hell out of me when I first heard about it because to
the best of my knowledge x-rays can’t tell materials apart that accurately.
I think most of us know how x-ray imaging works,
but just in case some people in the audience don’t.
It works by sending beams of electromagnetic radiation
through an object, and depending on the density and thickness of the material
it will absorb different amounts of the radiation. Producing a grayscale image,
with the brightness of pixels being determined by the level of absorption of the radiation.
This allows them to identify things like knives or other metallic pointy things with ease. However
these older machines needed things like laptops to be removed, not because it couldn’t identify the
laptop, but because the dense materials of the laptop could be used to hide objects below it.
This is where the first upgrade of these new scanners comes in.
They are CT scanners, which are essentially 3D versions of x-ray scanners. With the x-ray
emitter and detector rotating around the bags on a gantry, allowing it to form 3D
images. Allowing the security staff to look around your bag without having to pull your laptop out.
I have NO idea how this technology could be used to accurately differentiate between
materials. How can these things detect explosives? A bottle of water and a bottle
of hydrogen peroxide would look pretty much identical on a regular x-ray image.
To tell them apart the machine would need to be able to figure out the atomic makeup of the
material with a decent degree of confidence. And do that quick enough that it can process
thousands of bags a day, with as few false positives as possible, while also dealing
with all the other things crammed into your bag. This is far from a controlled laboratory setting.
There are SO many variables that affect absorption. Material density,
thickness, beam intensity.
My curiosity was piqued, every time I traveled through airport security
I found myself wondering how they worked, so I looked into it purely for my own pleasure,
but the answer was so interesting I had to make a video about it.
The first key to understanding this technology is understanding how x-rays get absorbed.
The primary method is through the photoelectric effect. When electromagnetic radiation
strikes an atom some of that energy is absorbed and emitted as an electron.
Denser materials have more atoms in a given volume and so there is
a great chance of collisions between photons and atoms. The same principle
applies for material thickness. With thicker materials requiring
the photons to travel further through them with more chances for collisions to occur.
Just take a look at an image of bones on an x-ray. Even though the center of the femur is
the thickest part, it appears darker, meaning less radiation is absorbed. That’s because
the inner core of bones are not terribly dense compared to the compact outer shell.
We need to eliminate thickness as a parameter,
and the rotating gantry can do just that. It allows us to accurately measure the thickness
of the material, allowing us to fill that variable into any further equation.
The next part of absorption depends on the energy of the x-ray beam. Higher energy
x-rays are less likely to be absorbed and can penetrate denser materials more
effectively than lower energy x-rays. So, if we fired two levels of x-ray intensity at a
material we could gain more information about the density of that material.
Finally, and this is obviously the important one for identifying the exact material,
is the atomic number of the material. Atoms with higher atomic numbers have
a higher chance of absorbing a photon through the photoelectric effect. Meaning an atom with
more protons in its nucleus has a higher likelihood of absorbing the x-ray beams.
If we can isolate that variable we can in theory at the very least estimate the
material composition, and that’s exactly what researchers did in this paper [REF]
They used a dual energy x-ray scanner,
which allowed them to fire high energy and low energy x-rays at materials in their scanner.
Then they could apply the absorption recorded into this equation. Remember
we said we would need to know the thickness of the material to correct for that variable
earlier. Look, there it is in this equation.
This gives a new variable called mass attenuation, and by finding the ratio
of the mass attenuation between the higher energy and low energy beams we get a new ratio
and that ratio is directly correlated to the atomic number of the material.
So we can apply it to a graph and begin classifying materials, and that’s exactly
what they did. I think I might love graphs and tables as much as Johnny Harris loves maps.
Look at this table. This is the high energy absorption and this is the low,
and this is the ratio between them. We can identify water sitting at 7.42
with the researchers estimated effective atomic number. Zed.
With this we could have a computer algorithm automatically flag dangerous materials in
bags and alert a security agent to open the bag for further inspection.
And that’s exactly what machines like these dual energy CT scanners of smiths detection can do.
But wait, there’s a problem. Look at this table of explosive compounds.
Well there it is. They all sit at around the same value as water.
Is this why we haven’t been allowed to bring water
through security this whole time? This is why isn’t it?
The error rate is just too high. Water is so close to all of these explosives
that it triggers false positives. And false positives REALLY slow things down
at airport security. Most of us have been there, having the shame of our bag
being automatically pushed to the naughty corner and having to wait for a security agent to rifle
through your belongings to look for the bottle of water you forgot was in there. We don’t like it,
and airport management certainly don’t either. It’s slows everything down and increases staffing
costs. It’s just easier to ban water and water based liquids from passing through.
So why are we now suddenly allowed to bring water through. What changed? That’s an answer
I couldn’t track down easily. So I got on a call with an engineer from Smith Detection.
I have kind of hit a deadend in our research, which is why we reached out to you, of,
I’m not quite sure how it can tell the difference between water and explosives
because in the papers I have seen water is like 7.42 in their grading, and hydrogen peroxide or
other like explosives are like 7.3 7.45, like they are too bunched together. My assumption is
that water was causing too many false positives and that’s why we haven’t
been allowed to bring water through airport security scanners. Is my assumptions right there?
“So that was the start,so, the atomic numbers, the effective zeds were collected together grouped
together, and if you fell within certain bands you were classed organic and given an orange colour,
if you were mixed materials and distincts then you were green, if you were highly absorbent, high
effective zed, then you were given blue, so that was the start of it”
“The next stage was really, more than having a single view, you could have more than one view
of a bag view. You could have a side view, and you could have a top view,
or a side view. That starts to give you more data, that is the journey towards the other
metric or the other characteristic, which is density. So when you plot density and zed
effective, you can discriminate between materials much better than previously.
Okay my research led me down the correct path, but the piece I was missing was that
by taking multiple images of the objects inside the bag, the scanner has effectively
built a 3D model of the item, and with that it knows the volume of the item. Using this,
along with the absorption data, the scanner can estimate the density of the material.
This technology has actually been in use behind the scenes in airports to scan your checked in
bags for quite some time. The issue was making it small and reliable enough to operate in a
practical security line where the getting people through as quick as possible is key.
No-one likes having to stand in a queue, and making that experience as easy as possible is
good for business. Especially as the quicker you get passengers through, the more time they have
to spend money on overpriced beer on the other side. These things need to process bags quickly,
they can’t be breaking down constantly, they need to be easy and quick to service, and any false
positives is going to force staff members to take your bag out of the system and manually check it.
Which is a pain in the ass for everyone involved, and increases staff costs for the airport.
And the next generation of security scanners are already being developed. Using scattering
and diffraction of x-rays, which is molecular specific, to analyze materials. Right now it’s
too slow for regular airport screening, but some are already in use in checked baggage halls.
So, if you have learned anything from this video. Next time you are in the airport,
look for one of these machines. You don’t have to take your laptop out
and if you are at the right airport, they may even let you take your water through with you too.
Small incremental engineering improvements like this may seem trivial in the grand scheme
of things, but lets run some numbers. My local airport in Ireland, Shannon,
is among the early adopters of this new policy. It’s a relatively small airport with 1.95 million
passengers last year, but if this technology saved everyone going through security just 5 minutes,
that 9.75 million minutes saved per year, or 18.5 years. That’s a lot of time that people
could be using for more valuable things. Small time saving measures like this make us more
productive as a society, and makes life easier for individuals. This is the power that a single
inventive engineer can have on the world. We all strive to have impactful careers.
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