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  #1  
Old 10-17-2009, 11:25 AM
coremelt coremelt is offline
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can you cancel out light in the same way noise cancelling headphones work?

Noise canceling headphones work by creating an audio wave with opposite phase to the noise you are hearing. Does this work for light? If so that would let you "project dark" which would be great for increasing contrast on projectors and have lots of applications in the AV world.

I suspect that if this is theoretically possible the problem is that computer chips and projection devices can't switch fast enough to match the waveforms that they sense so no canceling is possible in practice. Am I right or is this not even theoretically possible?
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  #2  
Old 10-17-2009, 11:35 AM
USCDiver USCDiver is offline
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You have to remember that light acts as both a particle and a wave. The eyes do their sensing by absorbing the particles (photons). So you can block those by closing your eyes or wearing a mask.
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Old 10-17-2009, 11:47 AM
ZenBeam ZenBeam is offline
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It is theoretically possible, but in practice, not even close. It could be done with radio frequencies.

But remember that the noise cancellation effect is beyond the headphones. If there were a material which was a perfect reflector for audio sound waves, they could just use that. For light, there is; you could just put aluminum foil to block out the light you don't want.
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  #4  
Old 10-17-2009, 12:08 PM
coremelt coremelt is offline
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Originally Posted by USCDiver View Post
You have to remember that light acts as both a particle and a wave. The eyes do their sensing by absorbing the particles (photons). So you can block those by closing your eyes or wearing a mask.
You're missing the point. You can create noise canceling projectors which work the same way as the Bose headphones but block noise within a cone of projection, not just for the person wearing them.

What I'm asking is can we create a light canceling projector that works by detecting the photons in a space and projecting light with same frequency and opposite phase.
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  #5  
Old 10-17-2009, 12:20 PM
The Hamster King The Hamster King is online now
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You can certainly create interference patterns with light where the opposing peaks and valleys of the waveforms cancel each other out. But I can't think of how you could create a light-canceling device. It would take time to detect the waveform and calculate it's opposite and since the signal is moving at the speed of light any opposing waveform your projected would never be able to catch up.

Last edited by The Hamster King; 10-17-2009 at 12:20 PM..
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  #6  
Old 10-17-2009, 12:30 PM
coremelt coremelt is offline
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Originally Posted by The Hamster King View Post
You can certainly create interference patterns with light where the opposing peaks and valleys of the waveforms cancel each other out. But I can't think of how you could create a light-canceling device. It would take time to detect the waveform and calculate it's opposite and since the signal is moving at the speed of light any opposing waveform your projected would never be able to catch up.
Fairpoint, I thought of this, but then perceptually is there a threshold within which the human visual system might not notice the delay in which a light canceling system can work?
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  #7  
Old 10-17-2009, 01:43 PM
Chronos Chronos is offline
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It's not the reaction time of the human visual system you have to worry about; it's the reaction time of the light itself. The period of visible light is about 10-15 seconds, so you'd have to react faster than that to be able to achieve any cancellation at all. You could, in principle, anticipate what the light wave is going to do before it does it, and cancel it out that way, but that would only work with an extremely consistent source like a laser, not something like the Sun or a light bulb.
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  #8  
Old 10-17-2009, 02:20 PM
Joey P Joey P is offline
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Originally Posted by The Hamster King View Post
You can certainly create interference patterns with light where the opposing peaks and valleys of the waveforms cancel each other out. But I can't think of how you could create a light-canceling device. It would take time to detect the waveform and calculate it's opposite and since the signal is moving at the speed of light any opposing waveform your projected would never be able to catch up.
The sensor would have to be far enough away to give the machine time to do it's work. Depending on how far away that is (feet, miles, 100's of miles? I have no idea), the machine might even have to interpolate what distortions happens to the light between the sensor and itself and adjust for that as well.
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  #9  
Old 10-17-2009, 03:34 PM
panamajack panamajack is offline
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The sensor would have to be far enough away to give the machine time to do it's work. Depending on how far away that is (feet, miles, 100's of miles? I have no idea), the machine might even have to interpolate what distortions happens to the light between the sensor and itself and adjust for that as well.
No amount of distance would give the machine any extra time; you can't transmit the detected signal any faster than light. I don't think 'slowing down' the light from the source by inserting something in-between would work either (and an opaque wall would accomplish the desired result anyway).
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  #10  
Old 10-17-2009, 04:54 PM
Chronos Chronos is offline
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Actually, I should point out that there are systems which can and do react to light fast enough to cancel it out, but such systems are precisely what an opaque wall is. The incoming light excites the electrons in the wall in just such a way that the light wave on the far side of the wall is canceled out, and there's a reflected light wave on the near side of the wall.
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  #11  
Old 10-17-2009, 05:04 PM
Joey P Joey P is offline
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Originally Posted by panamajack View Post
No amount of distance would give the machine any extra time; you can't transmit the detected signal any faster than light.
You're right, I didn't think about that part of it.

But such a machine could, I think, still be effective if the light source wasn't changing very much.
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  #12  
Old 10-17-2009, 08:39 PM
JWT Kottekoe JWT Kottekoe is offline
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The other major obstacle is the short wavelength of visible light.

Even with sound it is difficult to do cancellation unless you have one of two situations:

1) The source of the noise is highly localized and you can put a cancelling transducer within a fraction of a wavelength of the source.

2) The place where you want to recieve the audio is very localized and you can cancel the noise within a fraction of a wavelength of the reciever.

Case 1 is the basis for some commercial machinery which is made quieter by active noise calculation. This is usually working at low frequencies with large wavelengths.

The one situation where case 2 works really well is headphones. The wavelegth of middle C is about 4 feet. The highest frequency you can hear has a wavelength of about an inch. The headphones are placed precisely with respect to the reciever (your ear drum) and within a fraction of a wavelength over the audible band.

Light has a wavelength of a fraction of a micron.
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  #13  
Old 10-17-2009, 09:53 PM
WarmNPrickly WarmNPrickly is offline
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I suspect coherence would be an obstacle too.

What if the device split the light in two, then let one path travel 1/2 wavelength further than the other before it recombined? Of course it would only work at one wavelength.
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  #14  
Old 10-17-2009, 10:21 PM
Alex_Dubinsky Alex_Dubinsky is offline
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You don't need to catch up with light. You can expect your light source to be emitting light with the same properties into the next nanosecond.

With coherent, monotonal light (ie, a laser), it is trivial. But actual light has an enormous mix of frequencies, and each source has its own phase (and i'm not sure, does polarity count too?) I think it could be done, but the amount of information ('bandwidth' in the technical sense) involved is huge. Can someone calculate it for us? It would be terabits, no? Audible sound is kilobits, maybe megabits.

And that is just for one point! For sound, you've got 2 point receivers. For light, you'd have to do it at each pixel.
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  #15  
Old 10-17-2009, 11:28 PM
Chronos Chronos is offline
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I don't think it's possible even in principle for an incandescent light source, like an old-fashioned light bulb or the Sun, and it might not even be possible in principle for spectral-line sources like fluorescents or LEDs.

Oh, and the words you were using should be "monochromatic", not "monotonal", and "polarization", not "polarity".
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  #16  
Old 10-17-2009, 11:42 PM
Alex_Dubinsky Alex_Dubinsky is offline
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I don't think it's possible even in principle for an incandescent light source, like an old-fashioned light bulb or the Sun, and it might not even be possible in principle for spectral-line sources like fluorescents or LEDs.
Care to explain why?

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Oh, and the words you were using should be "monochromatic", not "monotonal", and "polarization", not "polarity".
You shush >=O
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  #17  
Old 10-17-2009, 11:50 PM
WarmNPrickly WarmNPrickly is offline
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I don't think it's possible even in principle for an incandescent light source, like an old-fashioned light bulb or the Sun, and it might not even be possible in principle for spectral-line sources like fluorescents or LEDs.

Oh, and the words you were using should be "monochromatic", not "monotonal", and "polarization", not "polarity".
You don't think you could split the beam in two, run it through a prism, reflected through specially designed mirrors such that every path is 1/2 wavelength greater than it's pair and then recombine them?
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  #18  
Old 10-18-2009, 03:29 PM
Chronos Chronos is offline
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For a laser, sure, you could do that, and in fact something like that is involved in most practical applications for lasers. For a non-laser, though, 1/2 of which wavelength? A light bulb produces light at all wavelengths. At best, you'll filter out a few specific wavelengths, but everything else will get through.

On thinking about it some more, you could pull it off for any monochromatic source, but it'd be a lot harder for a non-coherent source.
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  #19  
Old 10-18-2009, 09:19 PM
Kevbo Kevbo is offline
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Optical interference is readily observed in oil films and insect wings. The colors you see are partly due to selective cancellation of specific wavelengths of white light, and partly due to selective reinforcement.
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  #20  
Old 10-19-2009, 04:00 AM
si_blakely si_blakely is offline
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The trick would be to use metamaterials - engineered materials with a negative refractive index. You could create a mirror where light reflects on the back along the incident path, with a shifted phase. Current metamaterials are far too frequency/wavelength specific to allow general solutions, but in the future...

Metamaterials may help with the response time issue raised earlier, too. Remember that the limit of information propagation is c, the speed of light in a vacuum, and that the speed of light in air is somewhat lower (0.9997c). Metamaterials may have a speed of light faster than that of air, so you can get information ahead of the in-air wavefront, but given the small percentage differences in velocity, you could only do this over a long distance.

The better solution (for the OPs posited projection) is to engineer better screens, using metamaterials that have high reflection over a narrow incident/wider viewing angle, and zero reflectivity for other incident light. Work has begun on metamaterial traps that have zero incident reflectivity (for microwaves currently) that direct all incident radiation into the center. This will be ideal for energy capture (ie solar) but super-black materials are a possibility over a wide range of angles.

Si
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  #21  
Old 10-19-2009, 07:30 AM
CalMeacham CalMeacham is offline
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It's tough to produce interference effects between two independent light sources -- as others have noted, you need to have two sources coherent, which has in the past meant that both rays have generally come from the same source, with the path length difference relatively short. Paul Dirac was convinced that you wouldn't see interference between two light sources, and said so in his book on Quantum Mechanics.

Then in 1963 Mandel and another guy demonstrated that you could, in fact, get interference between two different light sources, and published the result in Natuyre (vol. 198, pp. 255-6). Lots of people have done it since, usually with lasers (Am. J. Phys. vol. 61 242-5 1993, for example)


I still don't think it's "easy", though (I've never seen it myself). I don't know that it's been demonstrated with anything except highly coherent sources and under very strict circumstances, and (as been pointed out above) the sort of device the OP asks for would like it to work over a broad range of colors and with "natural", low-coherence light. I seriously doubt that you're ever going to get this to happen. Even for a limited viewing area and range with ambient light I doubt you'll ever see it.


Lippmann photography is a useful effect that uses a broad range of colors with natural ambient light, and it requires very special equipment and treatment because the ranges over which the interference effects occur is so damned small. In a way, it's the closest analogue to what you're asking about

You see, sound waves are easy to generate to produce coherent effects, and have a long coherence range, and can work over a broad spectrum of wavelengths -- all exactly the opposite of light.
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  #22  
Old 10-19-2009, 08:22 AM
billfish678 billfish678 is offline
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Originally Posted by CalMeacham View Post
It's tough to produce interference effects between two independent light sources -- as others have noted, you need to have two sources coherent, which has in the past meant that both rays have generally come from the same source, with the path length difference relatively short. Paul Dirac was convinced that you wouldn't see interference between two light sources, and said so in his book on Quantum Mechanics.

Then in 1963 Mandel and another guy demonstrated that you could, in fact, get interference between two different light sources, and published the result in Natuyre (vol. 198, pp. 255-6). Lots of people have done it since, usually with lasers (Am. J. Phys. vol. 61 242-5 1993, for example)


I still don't think it's "easy", though (I've never seen it myself). I.
Technically speaking, you've never NOT seen it your yourself

Since your an optics kinda guy, I thought this might interest you. Back in the 60s, early 70s?, some astronomers used IIRC what was called intensity? interferometry to measure apparent size of the largest stars.

Optically, its nothing like you expect when you talk about interferometry. Nothing precise about it. They had big moveable "telescope" mirrors mounted on a railroad track. The mirrors were crap precision wise, just giant light buckets. At the focus were just photometers, no imaging was involved.

They did some kind of correlation thing with the photon noise to measure apparent angular size of the sources.

They wrote a short book on the story, how they did, and the results. When they proposed it, there were a fair number of optical experts that were sure it could not work even in theory, much less practice. The non math, non theory parts of the book make for an interesting read of "science drama".

Its a neat little book, and if our modest library had it, it cant be too hard to dig up.
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  #23  
Old 10-19-2009, 08:42 AM
Cardinal Cardinal is offline
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The trick would be to use metamaterials - engineered materials with a negative refractive index. You could create a mirror where light reflects on the back along the incident path, with a shifted phase.
I've heard two things about the anti-reflective coating on my glasses: that there was a destructive interference set up by the reflection off the coating and then the lens itself, and that it let more light through. The second was from opticians, not usually my most trusted source for physics tutoring, plus they were trying to sell me the coating.

On the other hand, my dad walked in when I first got the coating on a pair of glasses, and said, "magnesium fluoride". So it does seem to be the same stuff used on telescopes. Wiki backs up the destructive interference idea, http://en.wikipedia.org/wiki/Anti-reflective_coating. Why then do telescopes use it? Wiki mentions getting better contrast through not getting "stray light", but I can't picture this. I couldn't explain why or how this works to a bright high school student of mine.
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Old 10-19-2009, 08:48 AM
CalMeacham CalMeacham is offline
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Cardinal and billfish -- no time now; busy day. I'll answer, if no one else does, but not until much later, maybe tonight.
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Old 10-19-2009, 08:55 AM
billfish678 billfish678 is offline
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Why then do telescopes use it?
The two cent answer is two part.

First, many optical surfaces need to be coated with something "tough" because they are "delicate".

Second, when you make a simple coating that causes destructive interference at one wavelenght, it cause constructive interference at another.

So, for a telescope/optics system, you make the coating so that it causes more of the light with a certain wavelength you want to get through. Like say the wavelength of light your sensor is most sensitive to. And sometimes you design a coating to NOT pass a specific wavelength.
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  #26  
Old 10-19-2009, 09:25 AM
WarmNPrickly WarmNPrickly is offline
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For a laser, sure, you could do that, and in fact something like that is involved in most practical applications for lasers. For a non-laser, though, 1/2 of which wavelength? A light bulb produces light at all wavelengths. At best, you'll filter out a few specific wavelengths, but everything else will get through.

On thinking about it some more, you could pull it off for any monochromatic source, but it'd be a lot harder for a non-coherent source.
I was thinking in terms of a collimated point source. When the light is refracted through the prism, you get a linear band where the location is related to wavelength. The mirror is sloped with the wavelength of the light so that the path length varies by half a wavelength.

Of course you would need a very smooth mirror and very sensitive mechanics to get a few hundred nanometer slope in the mirror. Peizoelectrics can get that type of sensitivity in AFM and the mirror could be as simple as gold deposited on mica.
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Old 10-19-2009, 08:41 PM
CalMeacham CalMeacham is offline
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On the other hand, my dad walked in when I first got the coating on a pair of glasses, and said, "magnesium fluoride". So it does seem to be the same stuff used on telescopes. Wiki backs up the destructive interference idea, http://en.wikipedia.org/wiki/Anti-reflective_coating. Why then do telescopes use it? Wiki mentions getting better contrast through not getting "stray light", but I can't picture this. I couldn't explain why or how this works to a bright high school student of mine.
Any time you pass from a material with one index refraction to another with a different index of refraction you will get a reflection, as when you go from air to water, or from air to glass. The bigger the difference in index of refraction, the bigger the reflection coefficient.

Most glasses have a refractive index of about 1.5. If you put down a layer of material for which the thickness multiplied by the refractive index of that substance is a quarter of a wacelength, you will get destructive interference at normal incidence. (The ray travels twice through the material, so it travels a half a wavelength between the time it enters the layer and the time it leaves after being reflected) The ray that travels through the layer is half a wave out of phase with a ray that simply reflects from the surface, so the amplitdes add, and cancel out. Bingo -- you have reduced the strength of the overall reflected beam by making a beam that bounces off the surface of the Mag Fluoride and the ray that passes into the mag fluoride and reflects from the glass underneath very nearl cancel out.

For the most efficient cancellation, you want the refractive index of your coating material to be about the square root of the initial medium (air, with an index of 1, for practical purposes) times the square root of the underlying glass (about 1.5, as i say). The refractive index should this be about 1.225. For Mag Fluoride it's 1.38 in the middle of the spectrum, which is close enough. Mag Fluoride is also tough, which is worth using even if the refractive index isn't a perfect match.


Of course, this will only work for one wavelength, but you choose the thickness so that the wavelength is close to the middle of the visible spectrum, and live with the reflections from other wavelengths.


This is about the easiest anti-reflection (AR) coating you can do. It only requires a single thickness of material. Typically, coating engineers design "multi0layer stacks" that use multiple coatings of different materials, usually alternating a high and low refractve index. The more layers you use, the better you can extinguish reflections and the broader the spectral region you can cover, or the sharper you can make the cutoff.


As for Stray Light, using such an AR coating will reduce this only because it reduces reflected light. Stray Light is generally handled by using apertures and special coatings on telescope walls, which has nothing directly to do with coating.


Quote:
Optically, its nothing like you expect when you talk about interferometry. Nothing precise about it. They had big moveable "telescope" mirrors mounted on a railroad track. The mirrors were crap precision wise, just giant light buckets. At the focus were just photometers, no imaging was involved.

They did some kind of correlation thing with the photon noise to measure apparent angular size of the sources.

They wrote a short book on the story, how they did, and the results. When they proposed it, there were a fair number of optical experts that were sure it could not work even in theory, much less practice. The non math, non theory parts of the book make for an interesting read of "science drama".

Its a neat little book, and if our modest library had it, it cant be too hard to dig up.
Such astronomy using synthetic apertures to increase the effective size of the telescope is a common technique that has long been used to measure, for instance, the diameters of distant stars:

Quote:
Astronomical interferometry
Main article: Astronomical interferometer

The VLA interferometryThe angular resolution that a telescope can achieve is determined by its diffraction limit (which is proportional to its diameter). The larger the telescope, the better its resolution. However, the cost of building a telescope also scales with its size. The purpose of astronomical interferometry is to achieve high-resolution observations using a cost-effective cluster of comparatively small telescopes rather than a single very expensive monolithic telescope. The basic unit of an astronomical interferometry is a pair of telescopes. Each pair of telescopes is a basic interferometer. Their position in u,v space is referred to as a baseline.

Early astronomical interferometry was involved with a single baseline being used to measure the amount of power on a particular small angular scale. Later astronomical interferometers were telescope arrays consisting of a set of telescopes, usually identical, arranged in a pattern on the ground. A limited number of baselines will result in insufficient coverage in u,v space. This can be alleviated by using the rotation of the Earth to rotate the array relative to the sky. This causes the points in u,v space that each baseline points at to change with time. Thus, a single baseline can measure information along a track in u,v space just by taking repeated measurements. This technique is called Earth-rotation synthesis. It is even possible to have a baseline of tens, hundreds, or even thousands of kilometers by using a technique called very long baseline interferometry.

The longer the wavelength of incoming radiation, the easier it is to measure its phase information. For this reason, early imaging interferometry was almost exclusively done with long wavelength radio telescopes. Examples of radio interferometers include the VLA and MERLIN. As the speed of correlators and associated technologies have improved, the minimum radiation wavelength observable by interferometry has decreased. There have been several submillimeter interferometers, with the largest, the Atacama Large Millimeter Array, currently under construction. Optical astronomical interferometers have traditionally been specialized instruments, but recent developments have broadened their capabilities.[citation needed]
http://en.wikipedia.org/wiki/Interferometry



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When the light is refracted through the prism, you get a linear band where the location is related to wavelength. The mirror is sloped with the wavelength of the light so that the path length varies by half a wavelength.
Actually, one problem with prisms is that the dispersion is NOT linear. It's usually pretty far from it (except of a VERY tiny wavelength range). Optics folks tend to prefer diffraction gratings, for which the dispersion rule is a lot simpler, and IS usually pretty close to linear over a larger range than for prisms.
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Old 10-19-2009, 09:00 PM
billfish678 billfish678 is offline
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Originally Posted by CalMeacham View Post

Such astronomy using synthetic apertures to increase the effective size of the telescope is a common technique that has long been used to measure, for instance, the diameters of distant stars:




Cal. I know that. I know that you know about that too.

The interferometry I was referring to WAS NOT phase based, at least in the commonly understood sense of the term. It used basically giant shaving mirrors on railroad tracks for pete's sake. If it was that crude how COULD it posssibly be phase based when working in the visible/near visible range ?

IIRC it was intensity based. I could be loosing my mind, but I don't think its something most people think of when they think of interferometry, particularly these days.
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Old 10-19-2009, 09:09 PM
WarmNPrickly WarmNPrickly is offline
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Originally Posted by CalMeacham View Post

Actually, one problem with prisms is that the dispersion is NOT linear. It's usually pretty far from it (except of a VERY tiny wavelength range). Optics folks tend to prefer diffraction gratings, for which the dispersion rule is a lot simpler, and IS usually pretty close to linear over a larger range than for prisms.
Of course I know this, but a curved mirror is also possible. You just won't be able to use the deposited metal on mica trick for the curved mirror. Although you could place it on a flexible surfaced then bend it with piezoelectrics. Since your talking about nm it wouldn't have to be very flexible. Aberrations are inevitable.
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  #30  
Old 10-20-2009, 01:09 AM
Cardinal Cardinal is offline
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As for Stray Light, using such an AR coating will reduce this only because it reduces reflected light. Stray Light is generally handled by using apertures and special coatings on telescope walls, which has nothing directly to do with coating.
First off, Bwuhh? The stray light is handled with coatings with have nothing to do with coating.

Do you mean that the coatings on the walls are unrelated to those on the lens?

Second, is the AR coating is reducing stray light by reducing reflections inside the scope, that might be reflecting off the lens after having first passed through it?
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Old 10-20-2009, 01:55 PM
CalMeacham CalMeacham is offline
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First off, Bwuhh? The stray light is handled with coatings with have nothing to do with coating.

Do you mean that the coatings on the walls are unrelated to those on the lens?

Second, is the AR coating is reducing stray light by reducing reflections inside the scope, that might be reflecting off the lens after having first passed through it?
"Stray Light" generally refers to unwanted light that's rattling around inside your telescope or whatever -- even light on the wavelengths you want that's going where it shouldn't is "stray light". Most of this is handled, as I say, by coating the walls of your telescope with stuff to prevent it's reflecting cleanly off, and making sure it gets absorbed by good blacking, or by using apertures to block it.

Coatings are generally used to prevent transmission of wavelengths you don't want and maximize transmission of wavelengths you DO want. In addition, such coatings should be "clean" and free of scattering, which will contribute to Stray Light.

I'm not sure what sense you're using Stray Light in.



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Cal. I know that. I know that you know about that too.

The interferometry I was referring to WAS NOT phase based, at least in the commonly understood sense of the term. It used basically giant shaving mirrors on railroad tracks for pete's sake. If it was that crude how COULD it posssibly be phase based when working in the visible/near visible range ?
Sorry -- I assumed you were referring to the usual case. I had no idea from your post that you were referring to anything different. I don't know what you are, in fact, referring to. Your description doesn't coincide with anything I'm familiar with.

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Of course I know this, but a curved mirror is also possible.
Sorry to you, too, but that seemed to be what you were saying. I'm at a loss to understand what you are talking about here.
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  #32  
Old 10-20-2009, 02:07 PM
WarmNPrickly WarmNPrickly is offline
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Sorry to you, too, but that seemed to be what you were saying. I'm at a loss to understand what you are talking about here.
Nothing to be sorry about. I was brushing over the obvious non-linearity to make the principles of the device understandable. You would probably have to have some sort of physics degree to figure out what curvatures were necessary to make it work, but nanometer adjustments are comonplace with AFM and nearly atomicly flat surfaces are created daily (also for AFM). I'm just saying that I think it might be plausible if difficult to do. I have no idea if there are other unforseen issues.
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Old 10-20-2009, 02:20 PM
billfish678 billfish678 is offline
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Originally Posted by CalMeacham View Post
"

Sorry -- I assumed you were referring to the usual case. I had no idea from your post that you were referring to anything different. I don't know what you are, in fact, referring to. Your description doesn't coincide with anything I'm familiar with.


No need to be sorry about it. But that IS why I brought it up. Because when I first ran across it, my reaction was "no frackin way can that work, it aint interferometry as I have ever understood it". But there was book on it. In the non-fiction astronomy section. With lots of math that at first glance appeared solid. Apparently funded by the NSF or something like that. With published results.

If you are really interested in this, I can browse the library next week and see if I can find it.
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  #34  
Old 10-20-2009, 03:47 PM
Cardinal Cardinal is offline
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Quote:
Originally Posted by CalMeacham View Post
Most of this is handled, as I say, by coating the walls of your telescope with stuff to prevent it's reflecting cleanly off, and making sure it gets absorbed by good blacking, or by using apertures to block it.
My point is that your previous point included this:
Quote:
Stray Light is generally handled by using apertures and special coatings on telescope walls, which has nothing directly to do with coating.
Maybe you meant that the wall coatings have nothing to do with AR coatings on the lens.

As for the other thing I said, I meant, "are AR coatings used to interfere with light that might have bounced back to the bottom of the lens, to eliminate it from reaching the sensor out of phase with the good light, or out of place?
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Old 10-20-2009, 03:54 PM
Chronos Chronos is offline
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To be blunt about it, the "special coating" you use on the inside of the telescope walls is black paint. Really good black paint, maybe, but it's a fundamentally simpler problem than making something that doesn't produce excessive reflections but still allows light to pass through it, like you need on the lenses.
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  #36  
Old 10-20-2009, 04:26 PM
CalMeacham CalMeacham is offline
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Quote:
To be blunt about it, the "special coating" you use on the inside of the telescope walls is black paint. Really good black paint, maybe, but it's a fundamentally simpler problem than making something that doesn't produce excessive reflections but still allows light to pass through it, like you need on the lenses.
Actually, it's often really good black paint with lumps of stuff added to make the walls bumpy and unlikely to allow grazing-angle reflection.
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Old 10-20-2009, 04:38 PM
billfish678 billfish678 is offline
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Quote:
Originally Posted by CalMeacham View Post
Actually, it's often really good black paint with lumps of stuff added to make the walls bumpy and unlikely to allow grazing-angle reflection.
Amatuer telescope makers like some baffles and certain kinds of felt. Or finely crushed walnut shells mixed in with the black paint.

Before the really fancy black paints, you know what made a good black surface if done right (if I understand correctly) ? A block of razor blades clamped together.
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Old 10-20-2009, 04:57 PM
Happy Fun Ball Happy Fun Ball is offline
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Originally Posted by billfish678 View Post
Cal. I know that. I know that you know about that too.

The interferometry I was referring to WAS NOT phase based, at least in the commonly understood sense of the term. It used basically giant shaving mirrors on railroad tracks for pete's sake. If it was that crude how COULD it posssibly be phase based when working in the visible/near visible range ?

IIRC it was intensity based. I could be loosing my mind, but I don't think its something most people think of when they think of interferometry, particularly these days.
The telescopes you refer to did use phase (there is no such thing as intensity based interference). I don't remember the name of the telescope you speak of, but basically it measured the coherence length of the light. While the light from a hot object, like a star, is completely uncorrelated (as it is the product of a very large number of atoms radiating independently); light gains coherence as it propagates through space. What you need to understand this is a good book on statistical optics. Wikipedia has nothing to say about it... Neither does hyperphysics or Wolfram's physics site; I am really surprised by this.

Anyway, since I cannot find a site online, you have only my word for it. If we define a parameter, say alpha, as the separation of two apertures (mirrors for the case of your telescope) times the radius of an incoherent source divided by the distance to the source, then the mutual intenisity (and the visibility of the interference function) will fall as J1(a)/a where J is a Bessel function of the first kind. By increasing the distance between mirrors it is easy to find the nulls of the Bessel function and thus determine the radius of the star (as long as you know the distance and have a good wavelength filter - the light needs to be relatively monochromatic). For the sun, the first null of the Bessel function occurs for a mirror separation of ~8 microns IIRC (this is the coherence length of the sun at 1 AU).

This effect can be used to create nulling interferometric telescopes.

Regarding anti-reflection coatings, all of these rely on interference and the coherence length of the source. For incoherent sources like the sun or light scattered by objects, this length is very small so the coatings need to be very thin.

Last edited by Happy Fun Ball; 10-20-2009 at 05:02 PM..
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  #39  
Old 10-20-2009, 05:10 PM
billfish678 billfish678 is offline
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[QUOTE=L. G. Butts, Ph.D.;11685017]The telescopes you refer to did use phase (there is no such thing as intensity based interference). [QUOTE]

But like I said, phase not in the "normal" sense of the word when one thinks of interferometry. You mention measuring coherence length by measuring intensity. That is probably what I am recalling.

When I think of astromonical interferometry, I think of two telescopes collecting light. You bring that light together in one spot. You optically adjust the optical path differences so that you can control it to a fraction of wavelength of light. It all very optically precise and rather difficult to do.

Obviously YOU understand this set up that I recall. But rather crude optics and no high tech correction of optical path differences, much less not EVEN bringing the two light sources together (IIRC) is your grandfathers interferometry, not what most folks think of these days.
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Old 10-20-2009, 09:28 PM
Happy Fun Ball Happy Fun Ball is offline
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Originally Posted by billfish678 View Post
But like I said, phase not in the "normal" sense of the word when one thinks of interferometry. You mention measuring coherence length by measuring intensity. That is probably what I am recalling.

When I think of astromonical interferometry, I think of two telescopes collecting light. You bring that light together in one spot. You optically adjust the optical path differences so that you can control it to a fraction of wavelength of light. It all very optically precise and rather difficult to do.

Obviously YOU understand this set up that I recall. But rather crude optics and no high tech correction of optical path differences, much less not EVEN bringing the two light sources together (IIRC) is your grandfathers interferometry, not what most folks think of these days.
Looks like you were right. Here is a good paper on the subject:

The Hanbury Brown-Twiss and related Experiments (warning pdf).

What I was remembering was the Michelson stellar interferometer while what you were remembering sounds like the more advanced Hanbury Brown-Twiss interferometer. Both make use of the complex degree of spatial coherence, but the HBT relies only on intensity measurements without interfering the beams. Anyway, good stuff...

On another note, I read from your reply that I might have been coming off as an asshole (what with the bolded I and you); if this is the case, sorry, wasn't my intention...
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Old 10-20-2009, 10:04 PM
billfish678 billfish678 is offline
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Quote:
Originally Posted by L. G. Butts, Ph.D. View Post
On another note, I read from your reply that I might have been coming off as an asshole (what with the bolded I and you); if this is the case, sorry, wasn't my intention...

No problem !

Those names you reference ring a bell...its amazing what those old neurons "remember" isnt it ?

As for the bolded parts, I was just trying to be more clear...no intended assholishness here on this end (heh).

We all remember or learn something new once in awhile dont we ?

Though ya know, if you wanna come off as less of an asshole....you need to drop the Butts part of your user name

Seriously though, nobody in this thread has come off as an asshole as far I am concerned...just a little bit of not quite talking the same language....which I am sure I am as much to blame as anyone else here....

Thanks for taking the time to give us your expert input !

Its been a very bad day here for me, so if you cant make heads or tales of me today....forgive me.

take care

Blll
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  #42  
Old 10-21-2009, 07:33 PM
Cardinal Cardinal is offline
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So, if AR coatings necessarily create constructive interference for some frequencies of light, is that noticeable by the eye? My point is that the glasses sales people will say things like, "It helps driving at night." It may emphasize some frequency, but there obviously can't be more photons getting through than were arriving at the surface to begin with. Could it possibly be that these cheap coatings do anything worthwhile in assessment of dark surroundings?
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  #43  
Old 10-21-2009, 07:39 PM
Alex_Dubinsky Alex_Dubinsky is offline
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Originally Posted by Cardinal View Post
So, if AR coatings necessarily create constructive interference for some frequencies of light, is that noticeable by the eye? My point is that the glasses sales people will say things like, "It helps driving at night." It may emphasize some frequency, but there obviously can't be more photons getting through than were arriving at the surface to begin with. Could it possibly be that these cheap coatings do anything worthwhile in assessment of dark surroundings?
"Helps driving at night" means that bright headlights are made dimmer. In the case of the rearview mirror it amounts to, I believe, nothing more than darkened glass.
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