Passing light through bones and brain

Holography related topics.
drsagarjawale
Posts: 1
Joined: Tue Sep 04, 2018 12:45 pm

Passing light through bones and brain

Post by drsagarjawale »

Dear friends, please see the video below. The woman in the video lets light pass through bone and brain. Please explain what exactly is present in the hand held device where she puts a light source in front of which is a filter and a crystal and then light passes through the optical equivalent of brain.
https://www.youtube.com/watch?v=awADEuv5vWY&t=3s
Din
Posts: 402
Joined: Thu Mar 12, 2015 4:47 pm

Re: Passing light through bones and brain

Post by Din »

In the video, it was just a laser pointer to explain the principle. In the actual device, it'll be a coherent IR source.
lobaz
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Joined: Mon Jan 12, 2015 6:08 am
Location: Pilsen, Czech Republic

Re: Passing light through bones and brain

Post by lobaz »

I am not sure if I understood it well - the presentation lacks many technical details. (This is, of course, necessary if one has to explain a complicated stuff to non-experts in 15 minutes.)

In the video, there is a laser that gets collimated (probably). Light then passes through a hologram that modifies the shape of light wave. Light then passes through a "brain simulation", i.e. some scattering medium. Light leaving the scatteding medium is then observed on a screen. If you watch the video in the presentation carefully, you see a bright dot on the screen. If there was no hologram present, you would see a large light spot caused by scattering the collimated laser. Thus, the hologram does a sort of "inverse" to the scattering - it is able to cancel it out.

Even more impressive example based on the same principle would be taking a hologram of an image behind a ground glass. When trying to observe the image or the hologram in a conventional way, no image would be visible. The ground glass scrambles information into a visual noise. However, if you reconstruct a real image from the hologram, and put exactly the same ground glass to the original position, the original image appears again on the screen.
Din
Posts: 402
Joined: Thu Mar 12, 2015 4:47 pm

Re: Passing light through bones and brain

Post by Din »

lobaz wrote: Wed Sep 05, 2018 5:15 pm Thus, the hologram does a sort of "inverse" to the scattering - it is able to cancel it out.
It's basically phase conjugation.

Let's say you pass light through a scattering medium, say a cube of clear jelly. Let's illuminate the cube of jelly with an unexpanded laser beam and follow two rays of light through the jelly. One ray is scattered through, say, 30 degrees to the beam axis and another is scattered through -15 degrees, ie it's on the other side of the beam axis. Now, if you put a mirror behind the cube and parallel to it, the light that scattered at 30 deg, will reflect at 30 away from the cube and similarly for the -15 deg light. But now, let's again pass light through the cube, but this time place a holographic plate behind the cube. Reference the hologram from the other side of the plate, ie the side away from the cube. Now, once again, pass a laser beam through the cube. Again, one ray is scattered through 30 and one through -15. But, now, you're holographically recording the two rays of light, with a separate reference beam behind the plate, ie you're recording in transmission geometry. If you now reconstruct the hologram, the reconstructed beams will go back along the same 30 deg and -15 deg vectors. The hologram will act as a 'phase conjugate mirror', in that it will exactly reverse the direction of the light back to where it originally came from, because that's what you recorded in the first place. If you now replace the cube, the reconstructed rays of 30 and -15 re-enter the cube. If the properties of the cube have not changed, the ray entering at -15, and the ray entering at 30, will both leave the cube in the same direction as the originally illuminating laser beam.
lobaz wrote: Wed Sep 05, 2018 5:15 pm Thus, the hologram does a sort of "inverse" to the scattering - it is able to cancel it out.
It's not the hologram that inverts the scattering, the hologram just records the wavefunction of the scattered light. By passing the reconstructed wavefunction through the same medium, the medium reverses the scattering, by the Principle of Reversibility, which says that any light propagating in one direction, can be propagated in the reverse direction in exactly the same way. If you placed a different cube in the path of the reconstructed wavefront, the illuminating light would not emerge, the light would not be 'unscattered'.
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