Why Airport Security Suddenly Got Better

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  

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