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Author Topic: Northern Lights & Holograms  (Read 13563 times)

Offline Tann San

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Northern Lights & Holograms
« on: 24/11/2003 12:02:25 »
Hi all, I'm from a CS background.  Recently I was talking about the northern lights with somebody.  Later on I crossed my throughts with rainbows.  OK so what I was wondering was if there was any applicability of using rainbows as holograms.  

The way I see it from a technical point of view is that both the northern lights and rainbows produce a range from the color spectrum within a limited area, so why cant this be used on a much more precise and smaller scale?

Can anyone give me more information,

-Tann


 

Offline Ylide

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Re: Northern Lights & Holograms
« Reply #1 on: 24/11/2003 18:01:40 »
Well, holograms produce images differently than the way rainbows and the northern lights produce color/illumination.  

Rainbows are caused from light refraction through water vapor.  The individual droplets themselves are what cause the refraction, which is why you don't see much refraction through liquid water.  The northern lights are a completely different phenomenon.  The simple explanation is that it is caused by the earth's magnetic field trapping charged particles.  

With full spectrum light energy, you don't perceive individual color because you are getting each wavelength of visible light at the same time.  Rainbows occur because the water vapor spreads the wavelengths out slighty so that each wavelength is separated from the others slightly, causing the visible spectrum pattern.  

I don't see how it would be practical to use either of these phenomena in holography....typically a hologram is a distinct image, whereas the above two things are simply patterns of light.  The northern lights may be closer to your solution, but you'd need to shape a magnetic field into the image you wanted to display...a task that I believe is beyond our current technology.  



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Offline Tann San

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Re: Northern Lights & Holograms
« Reply #2 on: 24/11/2003 20:07:12 »
Arr too bad I really thought that was a good idea, thanks for the sharp reply.

-Tann
 

Offline Quantumcat

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Re: Northern Lights & Holograms
« Reply #3 on: 25/11/2003 09:03:50 »
I was going to say the same thing, but I decided not to because I wasn't sure if you really couldn't holograph a rainbow.

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Offline tweener

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Re: Northern Lights & Holograms
« Reply #4 on: 30/11/2003 01:32:57 »
I don't know much about holography other than lasers are used to illuminate a scene and the resultant interference patterns at the receiver are stored so that when a laser is shone through them they re-create the image.

Is it possible for a computer to "render" a 3D image and convert it to a hologram?


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Offline Tann San

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Re: Northern Lights & Holograms
« Reply #5 on: 30/11/2003 23:54:46 »
Hi, it would appear that I know even less about holography than you :) I dont see any reason why a computer couldn't render the 3D image.  It would all depend on how and what you used to generate the hologram.  I could read up on lasers used in holography but I was hoping my idea was in a "non laser" direction.  I'll do some more research into current methods.  Thanks for the reply, it got my mind working again!

-Tann
 

Offline Quantumcat

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Re: Northern Lights & Holograms
« Reply #6 on: 01/12/2003 15:37:01 »
well the thing is a rainbow isn't an object really.

A hologram need light coming from a definite surface.

A hologram happens by one of two identical lasers from the same place that are coherent (usually a beam splitter with the same laser works best) going to the object, having the object change the laser slightly (it will be out of step with the other beam because it's bounced of near and far places) that beam interferes with the beam that has had nothing done to it on the photographic paper, the two beams compare themselves to each other sort of and create a seemingly chaotic and non-sensical pattern on the photographic paper. When the beam that had nothing done to it is shone through the hologram again, you can see the object because the beam takes on how the object beam had changed when compared to that beam (it's hard to explain, I hope you understand what I'm saying) and you can see the object again. When you move, the reference beam changes (well, you see a different part of it sort of) and you see the part of the object you'd see if you were standing in that place actually looking at the object. It's really amazing. I don't think you can holograph a rainbow because it isn't an object. It's just raindrops. You have to have an actual surface of an actual object to holograph it. The beam can't bounce off different further-away-nesses because there aren't any it's just millions of tiny bits of water creating an effect by spreading the sunlight. WHat do you mean by a non-laser direction? Holographs are made with lasers and only lasers. You can't make a holograph without a laser. I love holograms, when I worked at Questacon my favourite exhibit was the hologram room, I love explaining them, though not very good at it as you can probably tell. A two D picture known as a photograph doesn't move because it only has an object beam, no reference beam to compare it to. If it could compare itself it'd change like a hologram does. But a photograph is like a holograph if you lived in a 2D space, because it's flat but if you lived there it'd move (because it's like exactly how the world would be if it was 2D if you know what I mean :-S) A holograph is like a photograph to someone in a 4D space because it's a representation of the 'D before it and 4D people can see all the things in a three D world all at ones (like we can see everything in a photograph all at once) we can't see everything in a holograph all at once though because it's the same as our world if you know what i mean :S but anyway I'm sure you get the picture. Oh, and you'd need 3 beams to make a 4D hologram too. (one less each dimension you go up)

Also, I don't know a -single thing- about the northern lights so I can't tell you why they can't be holographed.

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« Last Edit: 01/12/2003 15:43:59 by Quantumcat »
 

Offline Tann San

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Re: Northern Lights & Holograms
« Reply #7 on: 01/12/2003 22:07:35 »
Hi, hope no one minds me bouncing ideas around!

Ok so thats the holograph system explained (explained well I reckon :) how about shedding some light on rainbows.

I read the following page and think I've gotten my head round most of it:

http://www.unidata.ucar.edu/staff/blynds/rnbw.html [nofollow]

I was wondering why this couldnt be "trapped".  Say you had a large class cube.  This was constructed of lots of smaller cubes.  Each cube contains water.  What would happen to the light as it was passed through?  Would the glass interfere with the rays?  Is this dead similar to the laser method, i.e. having a surface to bounce off of?.  Maybe this would still need lasers.  One thing I did notice was that they talked about the paths of light crossing each other within a rain drop and causing either destructiveness to the bounced rays journey or in some cases actually re-inforcing the rays journey.  Would this mean you wouldnt need a light source as powerfull as a laser?
 

Offline Quantumcat

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Re: Northern Lights & Holograms
« Reply #8 on: 02/12/2003 09:03:13 »
No, you don't understand. The powerfulness isn't the issue with lasers, it's the fact that the two beams have to be coherent, that is, all their waves are completely instep. If they aren't in step they can't compare with each other and you can't get a hologram. Think of it this way: You do a question on english grammar, and you want to check if it's right, so you look at the answer but it's in french. How are you going to check it? You can't!! It's exactly the same with lasers. They have to be absolutely identical, the only difference being how the waves have changed by the object. You cannot holograph a rainbow because to make one you need normal light, you cannot use a laser (plus to see a rainbow you don't bounce light off an object, you diffract or whatever it)

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Offline tweener

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Re: Northern Lights & Holograms
« Reply #9 on: 02/12/2003 15:27:49 »
Quantum, that was a great description of how a hologram works!  Thanks!

Tann, I looked at the link you gave.  That is a good and very detailed explanation of a rainbow.  To distill it down a little, the rainbow is formed by varying wavelengths of sunlight being refracted at slightly different angles, thus "breaking" the sunlight into colors.  A glass prism does exactly the same thing.

Lasers, by definition, are a coherent light and thus consist of only one wavelength of light.  Having only one wavelength, they cannot be used to make a rainbow.

As for the northern lights, I do understand how they work and will try to explain.  They are caused by charged particles from the sun being trapped by the earth's magnetic field and then interacting with the atmosphere.  The charged particles from the sun striking the magnetic field are constrained to move along the lines of magnetic force, which are angled sharply toward the poles.  The particles actually make a spiral pattern as they move down the field toward the earth.  As they encounter molecules in the atmosphere, electrons in the outer shells are energized.  As the electrons drop back to their "normal" or resting state, they emit a photon of energy (sometimes light, sometimes invisible) which we can see as light in the sky.  It only happens at the poles (north and south) because that is where the lines of force come into the atmosphere.  In lower latitudes, the particles never make it into the atmosphere.

So, like a rainbow, the aurora are not really subject to being a hologram because they are not objects that can interfere with a laser beam.

I hope this helps.


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Offline Tann San

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Re: Northern Lights & Holograms
« Reply #10 on: 02/12/2003 16:15:03 »
Wow never hoped for so much discussion! :)

So whats the problem with holograms?  Why dont we have holographic adverts/entertainment etc around our cities?  I was always under the impression that the main problem was the amount of power needed to generate the laser beams.  That was part of the reason I was looking into alternative methods.

-Tann
 

Offline tweener

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Re: Northern Lights & Holograms
« Reply #11 on: 03/12/2003 02:22:34 »
I don't think power is the problem.  I think the main problem is expense, because of the complexity, and the strange "look" of a hologram.  Because they are generated with a laser, they don't have color other than the color of the laser, and they have a "grainy" look because of the diffraction patterns from the laser.


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Offline Quantumcat

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Re: Northern Lights & Holograms
« Reply #12 on: 03/12/2003 09:02:36 »
You can make coloured holograms, by using a red, green and a blue laser John! There were two in the hologram room in Questacon ... I miss that place =(

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Offline Tann San

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Re: Northern Lights & Holograms
« Reply #13 on: 03/12/2003 18:03:07 »
We're the ones at Questacon really good looking?
 

Offline Quantumcat

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Re: Northern Lights & Holograms
« Reply #14 on: 05/12/2003 07:05:58 »
Yes, extremely!!! They were worth thousands of dollars. Go to www.questacon.edu.au and take the virtual tour, and you might be able to see the hologram room =)

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Offline tweener

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Re: Northern Lights & Holograms
« Reply #15 on: 06/12/2003 04:59:09 »
quote:
Originally posted by Quantumcat

Yes, extremely!!! They were worth thousands of dollars. Go to www.questacon.edu.au and take the virtual tour, and you might be able to see the hologram room =)



I hadn't really thought about mixing RGB laser light, but it's the same thing your computer monitor is doing.  

See what I mean about the expense though ?


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Offline Tann San

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Re: Northern Lights & Holograms
« Reply #16 on: 06/12/2003 18:15:54 »
Hi visited the site.  That was a really nice interface for the virtual tour.  I use flash all the time and have no idea how they made that robotic arm.  Nice challenge :).  Couldnt find the holograph room though.

So am I understanding this right.  The hologram is formed where the lasers intersect?  The more lasers you have the more points you can have in the hologram i.e. more pixels (in a way).  Do the lasers have to terminate somewhere or are they open ended and just diminish with distance/power?

I think I can learn alot by reading sites and books etc.  I know this seems kind of lazy to just keep asking people but I'm enjoying talking about it more than I would just hunting and reading.  hope you dont mind.

Thanks again (dont think I've said that for a while here!)
 

Offline Quantumcat

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Re: Northern Lights & Holograms
« Reply #17 on: 06/12/2003 19:21:25 »
I'll explain what a laser is, I think your idea may be a little foggy (just in case)

A laser is a machine that produces light waves that are perfectly coherent. Coherent means they've all got the same wavelength, same speed, same frequency, everything. Imagine a pool surface, and you drop a rock in: the waves look like that but they're travelling the same distance apart, the same height, and the same speed. Lasers dissapitate because the light eventually gets bounced off bits of dust and other things, eventually there is none left but if you had it in a vacuum and a perfect laser etc it would go on forever, and you wouldn't be able to see it (because light has to enter your eyes for you to see things)

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Offline Tann San

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Re: Northern Lights & Holograms
« Reply #18 on: 07/12/2003 19:27:00 »
Hi, wow that was a really good description, see what I mean would of took hours of reading to learn the same thing.  Thanks.  You've given me alot to think about now.  The wave image you projected into my head is a lot clearer now.

I'm going to chew over this stuff for a bit but I reckon I'll be back...:)
 

Offline Tann San

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Re: Northern Lights & Holograms
« Reply #19 on: 10/12/2003 17:45:38 »
Hi again, well I havent really got a new question but I found some more information on non laser holograms.

http://amasci.com/amateur/holo1.html [nofollow]

http://www.holoworld.com/holo/diode.html [nofollow]

ok so i was reading the links of those while writing this.  i think I keep missing this (tired eyes and all that) but how come the final image cant be that big.  couldnt someone just make a honking big laser system to make a huge image?  or maybe make several smaller images and mosaic them somehow?

 

Offline Tann San

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Re: Northern Lights & Holograms
« Reply #20 on: 13/12/2003 16:24:38 »
Hi, ok I got one for you.  What if you had double the amount of lasers and instead of focusing the beam on a target plate you had two aimed at each other.  Would that then produce a dot of light in space instead?  I understand (sort of) that the laser light is invisible normally but would the two contacting make a difference?
 

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Re: Northern Lights & Holograms
« Reply #21 on: 13/12/2003 18:44:34 »
It would depend upon whether the beams were in phase or not. If you imagine 2 waves colliding, they superpose (add together) in a process called interference, producing a wave with an amplitude which is the sum of the displacement of each individual wave.

Therefore, since a wave goes up and down, with the interval between the ups and downs being referred to as the wavelength, if two waves with identical wavelengths meet, and one of the waves is 'up' and meets a wave the same size that is 'down' the 2 waves would cancel each other out at that point and you would see nothing. Conversely, if, when they met, both of the waves were 'up' the result would be a light of twice the brightness.

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Offline Tann San

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Re: Northern Lights & Holograms
« Reply #22 on: 13/12/2003 18:53:00 »
Hi, ok I think I got all of that.  Can see the images in my head.  when you say "if both were up" at the end you mean if both lasers originated and terminated at the same place dont you?  if a laser is not visible until the two beams are recombined then does that mean that visually nothing would appear to happen, you know you said the brightness would be increased...

thankyou
 

Offline chris

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Re: Northern Lights & Holograms
« Reply #23 on: 13/12/2003 19:03:39 »
Sorry, it's difficult without a diagram.

By "both waves are up" I am referring to a positive displacement rather than a negative one. If you imagine a SINE wave which alternates between positive and negative, my word "up" is referring to the maximum position in the positive wave. This will cancel out a "down" wave i.e. one with the same amplitude but negative.

So if 2 lasers are fired towards each other, if they have the same wavelength and the sources are an exact number of wavelengths apart from each other then the 2 beams will meet "in phase"; that is, the waves will exactly match each other, add together and make a bigger wave.

But if the 2 sources are separated by a distance that gives an odd half a wavelength e.g. they are 10.5 wavelenghts (just an example) apart, then the waves will meet and be the exact mirror image of each other - the positives will overlap the negatives. When they add together they will cancel out and make the wave disappear.

Is that a bit clearer ?

Draw a diagram if you are still struggling.

Chris

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Offline tweener

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Re: Northern Lights & Holograms
« Reply #24 on: 14/12/2003 00:39:28 »
Adding sine waves together so that they add or cancel their energy is called superposition.  If two laser beams intersect in free space, they will still be "invisible" because there is nothing to reflect any of the photons back toward the observer.  At some point within the volume where the beams intersect, the laser energy may cancel and at others it may be double strength due to the superposition of the waves, but the photons do not interact to cause one to go off in another direction.  Each beam will continue on in the direction it was going, and any observer that is not in the beam will not see any of the laser light, because none of it is traveling outside the beam.

Does this make sense?  The only way to see a laser beam is for it to be propagating through something that causes some of the light to be reflected or refracted in a direction back toward your eye.  Think of your car headlights - in clear air, you don't see any light until it hits some object. In fog, you can see the beams because the fog is scattering the light in all directions all along the beam.  Same with snow.




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Re: Northern Lights & Holograms
« Reply #24 on: 14/12/2003 00:39:28 »

 

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