Question: how do you plan to make the camera that can see through walls and can detect contraband?

Matthew Selwood answered on 2 Mar 2019:

Well, the starting point it to use x-rays, as we already know that they can see through skin and flesh. To see through walls and thicker materials, you need x-rays of a higher energy, or “brighter”. You can achieve this using a laser to make x-rays, instead of using a ray-cathode tube (which is what a lot of “normal” x-ray imaging cameras use). The x-rays will still be absorbed a little by any any all materials, so you can figure out what is behind the wall by what x-rays are absorbed and where.

One of the main issues is where to put your detector. When you go to the hospital for an x-ray, you have the source and detector on different sides of the object (your arm!). Trouble is, to do this you need access to the back of the room in order to put a detector there, which isn’t always possible. To get around this, I am instead looking at what x-rays are “back-scattered”: this means x-rays that hit the object and bounce backwards, instead of being absorbed or transmitting through. Very few x-rays are back-scattered, so I am trailing a technique called a “coded aperture” – you can see more info on this in my profile if you are interested. Basic premise is using lots of holes to get a lot more light, but still get a good image!

Detecting contraband is a bi-product of some chemistry. When the x-ray photon hits a substance, there is a chance it may go through “photo-fission”. The atom splits apart when it absorbs x-rays of high enough energy. When it does so, it releases a photon of light of a very specific wavelength, like a fingerprint. If you detect the energy of the photon, you can know what atom it came from. If you detect where the photon came from, you can create an image.

Combining it all, you can look at what atom types are concentrated where behind your wall, without ever having to open the door!