TIR, prism choice and its effect on image quality (1 Viewer)

[Maljunulo]

Another example would be optical ring resonators that use TIR to circulate light and achieve Q factors that are astronomically large, like 10^10.

Optical fiber transmission loss is what I used to get the 1 part in 10^4 estimate I made above. For visible wavelengths, attenuation is quoted at worst ~ 10 dB/km. The ratio of surface area for a 100 um fiber that is 1 km long, to the surface area of a prism is on the order of 1,000. The loss is fibers is almost all due to Rayleigh scattering in the volume, so any contribution from imperfect TIR is going to be in the noise. And this is not factoring in that the planarization processes applied to the few sq cms of a prism faces are going to be far more exacting that what will be done during the manufacture of a such immense lengths of cabling.

I've found nothing in the literature yet that gets at quantification of any measurable deviation for perfect TIR, but I'm still looking. Of all the mechanisms in an optical path that can incur loss, TIR is the last one to worry about.

Interesting, thank you.

 

[Lerxst]

For those interested in a derivation of roof prism image degradation and its solution, I recently wrote an article on this, located here:

The Physics of Roof Prisms and Phase Coatings - Michael Hurben, PhD

I've written a technical article on the topic of why the roof prisms used in binoculars require specialized coatings in order to improve their performance. I was prompted to pull this together because I couldn't find a single source that traced through this interesting problem in a non-trivial...

legallyblindbirding.net

As a follow-up to this article, I've written a less technical, but still detailed, accounting of the problem as well. It is broken up into three posts:

The Physics of Roof Prisms and Phase Coatings, Simplified: Part I - Michael Hurben, PhD

Introduction Some time ago, I began a fruitless search for a detailed treatment of the performance losses endemic to roof prisms, and the so-called "phase coatings" that rectify the problem. Optics manufacturers tout the advantages of their proprietary coatings, but provide few specifics...

legallyblindbirding.net

The Physics of Roof Prisms and Phase Coatings, Simplified: Part II - Michael Hurben, PhD

Phase shifts and interference The previous post introduced the topic of roof prism performance loss resulting from phase shifts that accompany total internal reflection (TIR). These shifts are endemic to TIR and to any device that employs it, including not only roof but Porro prisms. But in the...

legallyblindbirding.net

The Physics of Roof Prisms and Phase Coatings, Simplified: Part III - Michael Hurben, PhD

Diffraction patterns and resolution In the previous post we traced the passage of light through a roof prism, which required decomposing vectors into components, accounting for phase shifts upon reflection, and then looking at how the waves will be recombined. Nothing was quantified from any of...

legallyblindbirding.net

In order to avoid the mathematics I've had to rely on diagrams and animations. Feedback is welcome.

 

[Holger Merlitz]

As a follow-up to this article, I've written a less technical, but still detailed, accounting of the problem as well. It is broken up into three posts:

The Physics of Roof Prisms and Phase Coatings, Simplified: Part I - Michael Hurben, PhD

Introduction Some time ago, I began a fruitless search for a detailed treatment of the performance losses endemic to roof prisms, and the so-called "phase coatings" that rectify the problem. Optics manufacturers tout the advantages of their proprietary coatings, but provide few specifics...

legallyblindbirding.net

The Physics of Roof Prisms and Phase Coatings, Simplified: Part II - Michael Hurben, PhD

Phase shifts and interference The previous post introduced the topic of roof prism performance loss resulting from phase shifts that accompany total internal reflection (TIR). These shifts are endemic to TIR and to any device that employs it, including not only roof but Porro prisms. But in the...

legallyblindbirding.net

The Physics of Roof Prisms and Phase Coatings, Simplified: Part III - Michael Hurben, PhD

Diffraction patterns and resolution In the previous post we traced the passage of light through a roof prism, which required decomposing vectors into components, accounting for phase shifts upon reflection, and then looking at how the waves will be recombined. Nothing was quantified from any of...

legallyblindbirding.net

In order to avoid the mathematics I've had to rely on diagrams and animations. Feedback is welcome.

Thank you so much for your efforts - it is really a very complicated process even for me who has studied physics. If I look into your Figure 4a of part III, then I see that the partial waves travel different distances through the multi-layer setup. Is this the mechanism which eventually compensates for the phase shift - the introduction of a dynamical phase shift (by varying the travel distance) to somehow compensate for the differences between s- and p- waves?

There seems to be a close relationship to the Seil-effect, i.e. the loss of contrast in prisms which use some of their (coated) faces simultaneously for TIR and transmission, such as the SP-prism. I imagine how TIR occurs while some parts of the waves penetrate into the coating layer and thus introduce another unwanted phase shift.

Cheers,
Holger

 

[Lerxst]

Thank you so much for your efforts - it is really a very complicated process even for me who has studied physics. If I look into your Figure 4a of part III, then I see that the partial waves travel different distances through the multi-layer setup. Is this the mechanism which eventually compensates for the phase shift - the introduction of a dynamical phase shift (by varying the travel distance) to somehow compensate for the differences between s- and p- waves?

Hi Holger:

Good question; thank you for reading my write-up. My understanding of the answer involves several subtleties at play. First, TIR only occurs at the final interface between the n1 layer and the air, so that is the only location where the p- and s-orientations can "directly" acquire a phase offset. Because things have been tuned to get this TIR at a larger angle than it would be in an uncoated prism, we've already secured some reduction in phase offset, but clearly we have not driven it to zero. It should be noted that more than one reflection occurs there, however: light coming up off that final boundary will first encounter the n1/n2 interface, where some partial reflection must occur, so some light will undergo TIR again (and some of that a third time, and so on), so the net phase shift between the p- and s-components for the grand sum of TIR light will be different from what it would be if there was only one boundary. Second, all of this light which acquired a TIR induced phase offset obviously must be added to all the light that never saw TIR, so any final reduction in the net phase offset must result from this. None of that light will have gotten any net p- vs s- phase offset, and whatever total final phase it has will be a function of the layer thicknesses, angles, and wavelength. But upon addition with the TIR light, the resulting phases can add up such that s- and p- end up very similar (I am envisioning a case of the TIR light having p- leading s- by say a quarter wave, but the non-TIR light phase for both components at an eight wave back, in between them; sum them up and it will help "pull" the s- and p- phases together.) Hence the strong dependence of phase shift on total layer thickness in figure 4(b) of post III, or figure 13 of the PDF article.

The only other way I can see in which some differential phase offset might be accrued would be to have some of the partial reflections occur below Brewster's angle and some above. Below Brewster, the p-component would get a 180 degree phase shift while the s- would not. This would have some effect on the final phase offset, but I do not know if any coating designs try to make use of it. For the specific example here, all of the incidence angles for the partial transmission exceed Brewster's angle, so I don't see that affecting anything.

When I first sat down with this problem, I tried to work it out in the most simple manner possible, starting with the transmission and reflection coefficients at each interface individually and then adding them up, with the idea of seeing exactly where all the action was happening, so to speak. But such an approach failed to reproduce the results in the Mauer paper, and got ugly fast, so I gave up on that. I then set up the problem in terms of transfer matrices, and the correct answer literally popped right out. The matrix approach handles all the higher order stuff automatically. So what might seem to be only second order effects from multiple reflections would seem to be key in getting the net phase offset dialed down.

 

[tenex]

Feedback is welcome.

Your comment on the proprietary nature of various manufacturers' P-coating solutions (and scarcity of information on them) prompts me to wonder why we don't see more obviously superior resolution from the top makers today -- or to put it the other way, why much cheaper roof-prism products now seem to perform about as well, compared to older instruments lacking P-coating. Is resolution somehow more straightforward to correct for than other subtler (chromatic?) effects, which then contribute more to a brand's secret recipe, or has a point of diminishing return simply been reached?

 

[Lerxst]

Your comment on the proprietary nature of various manufacturers' P-coating solutions (and scarcity of information on them) prompts me to wonder why we don't see more obviously superior resolution from the top makers today -- or to put it the other way, why much cheaper roof-prism products now seem to perform about as well, compared to older instruments lacking P-coating. Is resolution somehow more straightforward to correct for than other subtler (chromatic?) effects, which then contribute more to a brand's secret recipe, or has a point of diminishing return simply been reached?

I suspect the latter, but it's just my hunch based on how other technologies behave. I spent my career in the business of making recording heads for hard drives. There at least there would be occasional big lurches forward, initiated by one company and then quickly matched by the others, leading to a long asymptotic trend in which nothing seemed to change much for a while. Only when the next breakthrough technology appeared would the process repeat, and that would come every few years or more. People catch up fast because they are not really that far away from one another.

Another similarity that I am sure my industry shares with the optics companies would be very aggressive, and most importantly, relative easy and quantitative, Competitive Analysis. As soon as we could get our hands on another companies product, we were doing destructive analysis and measuring the thickness and composition of every layer in every nook and cranny of the device. Reverse engineering a series of coatings is not hard, and I'm sure everyone knows what everyone is out there doing.

And unlike a hard drive, where we (still) have a ways to go before we simply run out of space to pack bits, resolution and transmission and so on all have a "hard stop" that isn't far from where real devices are. And human discernment limitations only make it easier to get to the point of diminishing returns on the technology.

As far as your question on resolution vs chromatic effects, I honestly don't know which is more challenging with whatever coatings are being used today, but based on the Mauer paper I would say getting a flat frequency response looks like a real pain. Lacking a full Analysis Lab, someone with yet far greater resources than myself could at least measure resolution and spectra and so on for a range of devices over the years and see where they seem to be changing the most. Either that or a meta-analysis on the reviews of discriminating users that can detect the differences; also way beyond my resources. I'm just here for the novel physics problems .

 

[Maljunulo]

Human vision varies between the ability to (figuratively) “see like an eagle or a mountain sheep“ to the proverbial “blind as a bat”.

We seem reluctant to discuss this here, and I, for one, question this factor whenever someone declares two instruments of different prices to be “the same”.

Discussions (arguments) break out here from time to time, with some declaring that “alphas” are really BS, and are actually “no better” than less costly glass.

Some appear to have vision which can detect the difference, and others do not.

 

[Paultricounty]

Human vision varies between the ability to (figuratively) “see like an eagle or a mountain sheep“ to the proverbial “blind as a bat”.

We seem reluctant to discuss this here, and I, for one, question this factor whenever someone declares two instruments of different prices to be “the same”.

Discussions (arguments) break out here from time to time, with some declaring that “alphas” are really BS, and are actually “no better” than less costly glass.

I actually dismiss those people, as people who don’t have the vision to detect the difference, or who are BS.

Some appear to have vision which can detect the difference, and others do not.

 

[Lerxst]

Human vision varies between the ability to (figuratively) “see like an eagle or a mountain sheep“ to the proverbial “blind as a bat”.

We seem reluctant to discuss this here, and I, for one, question this factor whenever someone declares two instruments of different prices to be “the same”.

Discussions (arguments) break out here from time to time, with some declaring that “alphas” are really BS, and are actually “no better” than less costly glass.

Some appear to have vision which can detect the difference, and others do not.

Yes on all counts. I avoid discussions regarding end user performance because my vision is so bad that I'll never see much difference. And my wallet is happier too.

I'd love to see some double blind tests between users and optics that span various performance ranges and that have as much of their performance calibrated as possible, just to verify what can and cannot be discerned.

 

[Holger Merlitz]

Hi Holger:

Good question; thank you for reading my write-up. My understanding of the answer involves several subtleties at play. First, TIR only occurs at the final interface between the n1 layer and the air, so that is the only location where the p- and s-orientations can "directly" acquire a phase offset. Because things have been tuned to get this TIR at a larger angle than it would be in an uncoated prism, we've already secured some reduction in phase offset, but clearly we have not driven it to zero. It should be noted that more than one reflection occurs there, however: light coming up off that final boundary will first encounter the n1/n2 interface, where some partial reflection must occur, so some light will undergo TIR again (and some of that a third time, and so on), so the net phase shift between the p- and s-components for the grand sum of TIR light will be different from what it would be if there was only one boundary. Second, all of this light which acquired a TIR induced phase offset obviously must be added to all the light that never saw TIR, so any final reduction in the net phase offset must result from this. None of that light will have gotten any net p- vs s- phase offset, and whatever total final phase it has will be a function of the layer thicknesses, angles, and wavelength. But upon addition with the TIR light, the resulting phases can add up such that s- and p- end up very similar (I am envisioning a case of the TIR light having p- leading s- by say a quarter wave, but the non-TIR light phase for both components at an eight wave back, in between them; sum them up and it will help "pull" the s- and p- phases together.) Hence the strong dependence of phase shift on total layer thickness in figure 4(b) of post III, or figure 13 of the PDF article.

The only other way I can see in which some differential phase offset might be accrued would be to have some of the partial reflections occur below Brewster's angle and some above. Below Brewster, the p-component would get a 180 degree phase shift while the s- would not. This would have some effect on the final phase offset, but I do not know if any coating designs try to make use of it. For the specific example here, all of the incidence angles for the partial transmission exceed Brewster's angle, so I don't see that affecting anything.

When I first sat down with this problem, I tried to work it out in the most simple manner possible, starting with the transmission and reflection coefficients at each interface individually and then adding them up, with the idea of seeing exactly where all the action was happening, so to speak. But such an approach failed to reproduce the results in the Mauer paper, and got ugly fast, so I gave up on that. I then set up the problem in terms of transfer matrices, and the correct answer literally popped right out. The matrix approach handles all the higher order stuff automatically. So what might seem to be only second order effects from multiple reflections would seem to be key in getting the net phase offset dialed down.

It is truly complex and it seems that just running the math remains the most efficient way to progress. Once again I would like to point you to the Seil-Effect: As far as I know, Konrad Seil has been the only one who reported these findings in any publication. He conducted some experiments and found an image degradation with SP-prisms which were multi-coated. As far as I know, nobody has ever published a theoretical explanation for his findings. With your considerable insights into these phenomena, you may be able to construct a solid theoretical explanation for Seil's experiments and thus be rewarded with a decent publication.

Cheers,
Holger

 

[Lerxst]

It is truly complex and it seems that just running the math remains the most efficient way to progress. Once again I would like to point you to the Seil-Effect: As far as I know, Konrad Seil has been the only one who reported these findings in any publication. He conducted some experiments and found an image degradation with SP-prisms which were multi-coated. As far as I know, nobody has ever published a theoretical explanation for his findings. With your considerable insights into these phenomena, you may be able to construct a solid theoretical explanation for Seil's experiments and thus be rewarded with a decent publication.

Cheers,
Holger

Hi Holger:

Thanks for that. I will read it.

 

[Patudo]

I'd love to see some double blind tests between users and optics that span various performance ranges and that have as much of their performance calibrated as possible, just to verify what can and cannot be discerned.

You can say that again!!!

 

[hopster]

You can say that again!!!

I used to perform these tests on audio equipment within an internationally well-known loudspeaker design company which at that time had a significant research budget. The methodology is called ABX testing. One can accurately measure the ability to discern differences with double blind testing. It needs to be done very carefully but the analysis is unambiguous and not tainted by 'preferences' or other subjective issues. Before we could talk about preferences, we needed to calibrate listeners, programme material etc just for the 'ability to perceive a difference'.

I am happy to attach the web page from my old consultancy (no longer online) if anyone is interested.

 

[Maljunulo]

Please do.

 

[hopster]

Please do.

Here you go. The web page saved as a PDF. Hopefully the formatting has survived.

M

 

[Binastro]

It is well known in the astro world that certain observers have or had exceptional vision.

Both as regards resolution and the detection of faint objects or both.
Either with telescopes or without.

In addition, some people who have better unaided vision do not have better telescope vision.
This I cannot explain.

Part of the problem is that the dark adaptation may be different and certainly the Seeing and Transparency are different.
Also location and altitude.

Age is a problem, but the very good observers seem to keep their skills till 40 years old.

There is also observational skill, which is very critical.

But results are probably known for these observers to 10%.

Unfortunately binoculars are so poor optically, similar results are probably not repeatable.

Another problem is that well known astronomers and some optics experts just have closed minds and don't believe observers with exceptional eye sight.
This is just human failing.

For instance, George Alcock was seeing and drawing details on Mars with a 4 inch f/12 Ross triplet refractor that others struggled to see with 16 inch Newtonians.
He just gave up planetary observations and switched to discovering comets and novae.
He just did not want the hassle of dealing with well known idiots.

I could list other cases, but prefer not to.

Regards,
B.

 

[Maljunulo]

Very interesting, but beyond my ability to understand.

 

[Maljunulo]

For instance, George Alcock was seeing and drawing details on Mars with a 4 inch f/12 Ross triplet refractor that others struggled to see with 16 inch Newtonians.
He just gave up planetary observations and switched to discovering comets and novae.
He just did not want the hassle of dealing with well known idiots.

A sad summary of human nature and behavior.

More than one knowledgeable person has been driven from public fora by the nitwits.

For some, the sun still goes around the earth, because they can see it every day.

 

[Scott98]

Some appear to have vision which can detect the difference, and others do not.

Yes, 100% yes. I've done astronomy "outreach" many times which means setting your telescope up and showing things to visitors or other club members. The range in what people perceive in the eyepiece is astonishing.

One night I showed a family member a view of Mars and suggested they look at the polar ice cap and a few dark markings on the surface. He could see nothing but a featureless red disk, no matter what magnification I tried. Other times, even larger, huge objects in the FOV are not seen by some people. Massive range visual acuity and what people are seeing. And of course, to be polite, you never mention this....what can you say? I'm just thankful I can still see pretty well.

 

[Binastro]

The range of ability with all our senses varies from person to person.

One person who was registered blind just managed to see the planet Venus in a telescope and was overjoyed.

I accept observations on their merit.

However, as comet section director I had two idiot beginners who discovered a comet near M31.
I got the top people up in the middle of the night to photograph the area.
There was no comet.

When I told the two beginners they didn't even apologise, and I lost some of my respect.
They probably saw one of M31s companions.

When I was much younger I joined Dateline.

I met all four young ladies.
One encounter was interesting.
I arrived at her home.
She lived in the basement.
I said, sorry can you put the light on, as it was very dark.

She apologised and said she was blind.
I treated her with respect, and she told me how to lead her.
We went to a cafe and had coffee.
We chatted and I took her home.

My mates said I had been on a blind date, which was cruel.

Another was a teacher.
She had no friends at all and lived in an attic room.
I took a sample large carpet rug from my car and gave it to her to cover the bare floor boards.

We all have different life styles and experiences.

Luckily I was born in the U.K. and enjoyed a fairly good peaceful life.

My eyes lately are not so good and I have given up solar observing.
But my eyes are still good enough for most purposes.

Best regards,
B.