Binocular lens coatings. (1 Viewer)

[NDhunter]

There are many ways that optics makers use lens coatings to offer the view that we are to receive.

I have several binoculars and I have found there are objective reflections that vary and include shades of blue, green, magenta and even red.

This is one of mine, and this photo is unaltered.

This is from a binocular that I do not use often, but I had it out yesterday, and wanted to bring this subject up for discussion. What do the reflections from the objective end mean?

Jerry

 

[David in NC]

I *think* I read that the apparent color of the objectives don't hold correlation. I can't recall though...

 

[OPTIC_NUT]

When you look straight on you get a slight inkling of what they do, but off-axis
it's a different story. The modern multi-multi-layer coatings with
"97" or "99.7%" transmission per surface are a strong bandpass filter. One
consequence of the layers and multi-wavelength total thickness is much higher
reflection away from the intended angles. The multi-color sheen of some can be
quite a show at the eyepiece.

 

[ronh]

Jerry,

I have puzzled about the correlation between the color of reflections from coatings and the effect on the image. This is my conclusion, so take with a grain of salt, but I daresay it is at least plausible.

There are many surfaces in a binocular, and the predominant reflected color from the outer objective is often a minor factor in determining the overall color balance of a good binocular. All the colored reflections you see when you look deep into the binocular from the objective end contribute about equally. So, a binocular with a remarkable colored reflection from the outer objective surface can be quite good in the image, if all the colored reflections balance out reasonably well.

But cheap binoculars with Ruby coatings are different. Usually, the outer objective provides by a large margin the brightest and most strongly colored reflection of all the surfaces, and so will dominate the overall color balance. Since much of the red light reflects away and is missing from the image, the view will lean toward a green bias.

Such binoculars are often ridiculed in forums, but I think much of that is just because they are cheap and so darned flashy looking. Green bias per se won't do you any harm.
I have such a binocular myself, and through it grass looks like green fire! That one thing about it provides me about a yearly minute's worth of entertainment. But if you looked at grass a whole lot, you'd probably think it was the best, brightest, and most transparent binocular on the planet.

Ron

 

[peatmoss]

Basically, anytime you see a color tinted reflection on a coated glass surface, that means that everything except that color got transmitted through the glass. Sort of. A couple of factors also have to be considered. First is the actual "color" of the reflection. A ruby tinted reflection sort of looks red, but actually it likely includes percentages of blue and green in it too. Second is the intensity of the reflection. The ruby tinted reflections in your picture are very bright, so these are called "high" intensity. This means that, whatever component colors make up the ruby tint, a fairly high percentage of that light has been reflected from the coated surface, rather than transmitted.

If you were designing a binocular and wanted a fairly neutral image, then the various tints and intensities of the coated surfaces within the bins would have to roughly balance out each other. For example, if you put a reddish coating on the objectives (meaning that the light being transmitted through the objectives is now deficient in red), then you'd also want to have other coatings in the optical system that would present as blue/purple, and maybe yellow/green. And, to maintain as bright an image as possible, you'd probably want the intensities of all the coatings to be a low as possible. The problem with the typical ruby coated bins is that the ruby coatings have a very high intensity, so if you tried to balance out the color, the end result would be a pretty dim image. Of course, I think that, at least as far as ruby coated bins are concerned, the intensity of the coatings is intentional, and so they don't really bother trying to balance the image's color shift.

 

[elkcub]

I stashed away several articles about lens coatings. This short one addresses apparent color differences and filtering. How Lens Coating Works

Simple questions don't necessarily have simple answers.

Ed

 

[typo]

Simple questions don't necessarily have simple answers.

Ed

That's a lot more comprehensible than the info I've managed to track down. Nice find!

David

 

[OPTIC_NUT]

Jerry,

But cheap binoculars with Ruby coatings are different. Usually, the outer objective provides by a large margin the brightest and most strongly colored reflection of all the surfaces, and so will dominate the overall color balance. Since much of the red light reflects away and is missing from the image, the view will lean toward a green bias.


Ron

I got some super-cheap ones with metallic-red-looking front lens and
tore them apart. "$50 value for only $20" on clearance for $5.
5$ for a laugh? Why not. The were 7x50s...how do they ship that
for $5? The carry case was worth way more than the binocs.

The view:
---fairly sharp (hmmmm...)
---incredibly dim...twilight at noon
---very deep green: red and blue things looked almost black

After a minute, your eyes factor out the tint and it's like
black+white TV of high resolution. You sort of see some color.

About the cheap red objectives:
---they were one single element: no achromat
---the red coating wouldn't clear off with hard pressure and metal scrub
---it did clean off with #400 emery paper...it was a fired dip coating
over green bottle glass

The rest:
---a plastic field lens
----entirely non-achromatic eyepieces, of greenish bottle glass

So my take on it was:
---the super-red "coating" looks stylish, llike the IR-reject coatings
on a few fancy binoculars
---the main purpose is to make the system monochromatic so
achromats and nice glass can be eliminated
---the 7x50 design allows a lot of light for spectrum-stripping


I'll bet you didn't imagine it could be that bad.
It takes some evil genius to design that.

 

[WJC]

Ooh, Ooh: from my section on coatings.

Cheers,

Bill

A WORD ON “RUBY” COATINGS

Ruby coatings are bright orange AR coatings often touted as a feature to increase contrast and, looking very different from traditional anti-reflective coatings, they are easily accepted as such. Yet, advertisers don’t tell the whole of the story.

They can enhance our view of a brown deer against a green foliage background—that’s true. Unfortunately, these “anti-reflective” coatings are so . . . reflective, they can appreciably reduce light throughput, as certain frequencies of the incoming light is reflected back whence it came, thus reducing the effective aperture.*

The result? A loss in the image brightness the coatings were supposed to help you obtain. Which is more important to you: brightness across the visible spectrum or higher contrast in certain situations?

If you’re thinking of buying a binocular with Ruby coatings, you might want to consider: if they’re as good as promoted, why don’t the legends of the industry—Zeiss, Leica, Swarovski, Kowa, and Nikon—feature them in their line-ups?

 

[kestrel1]

I think that color of coating that you see from side is IMHO part of spectrum where the binos has smaller transmission, because they reflect that wavelenghts. Zeiss has orange-red coating and results in blue tint of image due to lack of red part of spectrum, Leica trinovid that I have has blue coating and it incline to red tint, and ELSV has green coating and according to allbinos it has small dip in green part of spectrum. But I am not sure, it is only my opinion, because the color of objects is generally given by its surface properties, which means that object absorbs other wavelenghts and its color is the wavelenght which is reflected by surface. And also the color of coating depends on its thickness.

 

[David in NC]

http://www.bestbinocularsreviews.com/blog/ruby-coated-binoculars-coatings-09/

 

[OPTIC_NUT]

Just remember...the color you see from the side on multi-coating is not
the actual response of the filter on-axis. Most nowadays practically disappear
looking into the objectives.