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Scattering of light due to uncoated prisms.

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#1 Princess Leah

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Posted 19 April 2024 - 03:57 PM

I recently read a review where the author stated the scattering of light within the binoculars was due to the prisms being uncoated?

Does this happen?

 

I thought light scattering in binoculars was a product of the lens or eyepiece elements being poorly figured/polished.

 

I attribute internal reflections/glare being a symptom of uncoated prisms.



#2 barbarosa

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Posted 19 April 2024 - 04:32 PM

Long ago in physics classes the teaching was that internally prisms are 100% reflective, A quick online check shows that this is still the rule. However, coatings on the external incident surface improve transmittance.  internally light is refracted by amounts dependent on wavelength and the refractive index of the glass.  I have seen this refraction described as scatter, but I do not think that is correct.

 

Coatings on the exterior incident and exit surfaces are for increasing transmittance.

 

Roof prisms are a special case in that they often have a phase coating to change the path of some wavelengths.

 

This explanation is from Celestron seems a bit misleading at least as applied to poro prisms. It might be true of roof prisms.

 

"Dielectric Coating – a high-reflectivity prism coating that provides high reflectivity across the entire range of visible light and acts as a dielectric mirror - thus providing both sharper colors and crisper images.

High Reflectivity Metallic Coating – metallic material such as aluminum or silver is applied to the reverse side of a prism surface that is not totally reflective to raise the reflectivity of the prism mirror surface. Silver coated prisms are more reflective than aluminum coated prisms and can have reflectivity in the high 90% range.
Phase Coating – optical coatings that are applied to one surface of the shorter light path half of the prism. The coating slightly slows down the short light path half of the incoming light that passes through that surface, causing it to once again be in phase with the light that traveled the longer path when they halves are rejoined.  With phase-corrected prisms, no colors are reinforced or cancelled, thus giving a more accurate color reproduction. The effect is particularly visible when looking at a back-lit or silhouetted subject, where more color and detail can clearly be seen in the shadowed areas of the bird.
Silver Alloy – a high-reflectivity prism coating that acts as a mirror - thus providing both sharper colors and crisper images when compared to aluminum prism coatings. A silver alloy typically has a reflectivity of 95-98%."



#3 sevenofnine

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Posted 19 April 2024 - 07:51 PM

This explanation copied from Oberwerks.com may help. Their binoculars are all Broadband FMC on all air to glass surfaces including prisms. 

 

What’s “multi-coating”?
Coatings prevent reflection and scattering of light- which minimizes light loss and offers better image contrast. An uncoated glass surface can lose up to 5% of light transmission due to reflection and scattering. A single layer of anti-reflection coating can reduce loss to about 1.5%. Multiple layers of different anti-reflection coatings can further reduce loss to as low as 0.25%. Multi-coating therefore, provides a higher level of light transmission and image contrast.

What’s the difference between multi-coated and fully multi-coated optics?
Many binoculars have multi-coated objectives and oculars, but it’s also important that all internal air-to-glass surfaces are coated (fully coated) or multi-coated (fully multi-coated) as well. A binocular has between 10 to 20 air-to-glass surfaces on each side- so a loss of 5% per uncoated surface could result in a binocular that transmits less than half of the light that enters its objectives! Only the highest-quality binoculars are multi-coated on all glass-to-air surfaces (fully multi-coated). This costs considerably more to manufacture, but allows the highest level of light transmission (up to 95%). Note: All Oberwerk binoculars are fully multi-coated.

What does “broadband multi-coating” mean?
Broadband is the highest-quality multi-coating available.  If you measure the reflectivity of standard multi-coating across the entire range of visible wavelengths (380-780nm), you’ll see that reflectivity increases (more light is lost) at each end of the visible spectrum. Broadband multi-coating has less of an increase in reflectivity at the ends of the range, or in other words a “broader band” of efficiency, across the entire range of the spectrum. Note: All Oberwerk binoculars are fully broadband multi-coated...waytogo.gif waytogo.gif


Edited by sevenofnine, 20 April 2024 - 12:58 PM.

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#4 Princess Leah

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Posted 20 April 2024 - 04:49 AM

Thanks for your help. To be clear I was recently reading a post on CN where it was stating that coatings don't reduce scattering of light in either objective lens or eyepiece elements. They do however increase transmission.

 

So I was wondering why a prism would be more likely to 'scatter' light? 



#5 Ernest_SPB

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Posted 20 April 2024 - 05:20 AM

Complex optical systems with many elements (like most binoculars) raises a lot 2-order reflections.

One optical surface reflects back some amount of falling light, the reflection meets another optical surface that again reflects back to direct pass certain quite small amount of light.

In complex optical system number of such pairs of optical surfaces raised quickly: n*(n-1)/2 where n - number of optical surfaces in the system. For ordinary binocular n = 15-20 and number of such 2-order reflections more than 100.

This light produces flares and common light veil over final image. Level of the "scatter" strongly depends from effectivity of AR coating used in the bino. 


Edited by Ernest_SPB, 20 April 2024 - 05:21 AM.

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#6 Princess Leah

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Posted 20 April 2024 - 06:00 AM

That's really interesting Ernest and explain a lot! Thanks for sharing.



#7 Rich V.

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Posted 20 April 2024 - 10:39 AM

An uncoated surface may reflect up to 20 times the light that the best multi-coated surfaces will. On extremely bright objects like the Moon in particular, that light can cause noticeable scatter, veiling glare or ghosting depending on the curvature of the reflecting surfaces.  Even some of the "best" binos can show reflections of the Moon superimposed into the image when it's just out of the FOV.  Many BTs will also show "shooting stars" whizzing across the FOV at odd angles when panning because of off axis star's light being reflected off of prism surfaces that aren't baffled sufficiently to block them.

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#8 Princess Leah

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Posted 21 April 2024 - 03:54 PM

An uncoated surface may reflect up to 20 times the light that the best multi-coated surfaces will. On extremely bright objects like the Moon in particular, that light can cause noticeable scatter, veiling glare or ghosting depending on the curvature of the reflecting surfaces.  Even some of the "best" binos can show reflections of the Moon superimposed into the image when it's just out of the FOV.  Many BTs will also show "shooting stars" whizzing across the FOV at odd angles when panning because of off axis star's light being reflected off of prism surfaces that aren't baffled sufficiently to block them.

A prism isn't a curved surface.... Is it?



#9 Rich V.

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Posted 21 April 2024 - 04:12 PM

A prism isn't a curved surface.... Is it?

 

I thought it was obvious that prism faces must be flat, but there are other surfaces, in the interior of the body as well as optical, that also contribute to scatter, glare and ghosting if they are not properly baffled or coated as well.  Minimizing all reflections is the ideal.  Prisms are just one contributor.



#10 Princess Leah

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Posted 21 April 2024 - 04:32 PM

I thought it was obvious my question was to do with prisms.



#11 Rich V.

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Posted 21 April 2024 - 04:50 PM

Sorry I went so far off topic...  wink.gif


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#12 SMark

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Posted 21 April 2024 - 07:03 PM

For illustrative purposes...

 

Two famous Fujinon binoculars. One with coated prisms...

 

 

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#13 SMark

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Posted 21 April 2024 - 07:04 PM

And one without coated prisms...

 

 

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#14 GlennLeDrew

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Posted 21 April 2024 - 11:47 PM

I'll just toss this into the mix. It is crucial to use the correct terms. Scatter and reflection are two very different phenomena. I sense a use of "scatter" where "reflection" is intended. Prism faces are like lenses, whose polished surfaces--coated or not--produce unwanted reflections that exhibit as ghost images of varying degree of enlargement/diminution, sharpness/defocus and intensity. This variety derives from the radius of curvature (flat is an infinite radius) of the involved surfaces and their disposition in the optical train. The more surfaces, the more compound reflections.

 

The prime job of a coating is reduced reflection intensity. That higher transmission is naturally attendant is more of a bonus. Because contrast is EVERYTHING in optics. Which is worse? A reduction in image brightness by 50% or a reduction in contrast by 10%? The latter.


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#15 Princess Leah

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Posted 22 April 2024 - 02:56 AM

Sorry I went so far off topic...  wink.gif

No worries I learned a lot.   Good thing I'm not asking FLO for a curved prism ;)



#16 Princess Leah

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Posted 22 April 2024 - 03:12 AM

I'll just toss this into the mix. It is crucial to use the correct terms. Scatter and reflection are two very different phenomena. I sense a use of "scatter" where "reflection" is intended. Prism faces are like lenses, whose polished surfaces--coated or not--produce unwanted reflections that exhibit as ghost images of varying degree of enlargement/diminution, sharpness/defocus and intensity. This variety derives from the radius of curvature (flat is an infinite radius) of the involved surfaces and their disposition in the optical train. The more surfaces, the more compound reflections.

 

The prime job of a coating is reduced reflection intensity. That higher transmission is naturally attendant is more of a bonus. Because contrast is EVERYTHING in optics. Which is worse? A reduction in image brightness by 50% or a reduction in contrast by 10%? The latter.

Yes, this is why I questioned the term 'scatter' in the review. Ernest suggests a situation above where a ping-pong multitude of reflections conveys a sense of 'scatter.'

 

I have tried the Binoculars in question. I definitely would have described the effect as 'scatter' myself, although I guess it was probably a multitude of internal reflections as Ernest described.

 

I have usually attributed scatter in the past to a poorly polished lens or rough coatings.

 

Interesting what you say Glenn about contrast Vs brightness. Surely worthy  of an entirely new thread? I have noticed how the importance of contrast is more evident and even more important with astro viewing. With terrestrial viewing binoculars that exhibit good colour saturation due to broadband coatings can often give a false sense of good contrast.

 

I have also seen binoculars of quite modest aperture, out performing larger aperture ones. This was in my mind mainly down to the exquisite contrast the smaller binoculars produced.


Edited by Princess Leah, 22 April 2024 - 03:15 AM.


#17 Princess Leah

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Posted 22 April 2024 - 03:22 AM

Curiously I read on CN that the binoculars in question are fully multi coated, including the prisms.

Perhaps some of the reflected surfaces used in the binoculars are poorly figured/polished?



#18 SMark

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Posted 22 April 2024 - 11:50 AM

Curiously I read on CN that the binoculars in question are fully multi coated, including the prisms.

Perhaps some of the reflected surfaces used in the binoculars are poorly figured/polished?

Both of the binoculars in my pictures above were also supposed to be fully multicoated. I believe what my eyes tell me, and those prisms in the second pic are uncoated.  


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#19 pat in los angeles basin

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Posted 22 April 2024 - 01:22 PM

SMark, do you think it more likely a production error or deception (which I can't imagine fujinon committing). What was the Tasco you had  with the mixed bag of low/high index prisms?   Pat



#20 SMark

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Posted 22 April 2024 - 02:01 PM

SMark, do you think it more likely a production error or deception (which I can't imagine fujinon committing). What was the Tasco you had  with the mixed bag of low/high index prisms?   Pat

Yeah, the Tasco 110. I could probably more understand the Tasco workers in Japan mixing up the prisms since the workers were likely making the 118, which uses Bk7 prisms, along with the 110 which has BaK4 prisms, at the same time. Though I think the error most likely came from the glass supplier just getting the glass formulas mixed up.

 

With the Fujinon binos, I also doubt that it was deception, but I would suggest it was more of lack of quality control. They clearly had a good size lot of these uncoated 16x70 prisms loaded into finished binoculars and waiting to be shipped out to dealers. I got two of these shipped to me and I've heard or read of others getting them too. 


Edited by SMark, 22 April 2024 - 02:03 PM.



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