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What does the colour of my Regractors front lens mean

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#1 simjp81

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Posted 05 November 2019 - 09:12 PM

Hi All

 

Can anyone educate me on what a green tinted lens means. I have seen other lens as a purple or blue tint.

 

Thanks in advance



#2 TOMDEY

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Posted 05 November 2019 - 10:05 PM

The reflection spectrum of an all-dielectric coating is the compliment of its transmission spectrum. If that coating is designed to enhance the visual transmission (to as close to 100% as possible), then all visible wavelengths will reflect very little... typically less than 1%, even below 1/2% at all wavelengths, for good multilayer MARC coatings. [MARC = Multilayer Anti-Reflection Coating.]

 

Those small residual spectral reflectivities combine to render the photopic color that we see, when looking at the lens, rather than through it. The coating on the external surface is usually the most obvious. Depending on the exact transmission spectrum (and therefore also its reflected compliment that we see as coloring the surface), what color we see rendered can be just about anything... red, yellow green, cyan, blue, magenta and any combo of those... gold, copper, mauve puce...

 

The coloration hardly affects the intended function of the coating --- enhanced transmission. Therefore, some manufacturers intentionally tweak the reflection spectrum color for purely aesthetic and/or marketing ~beauty~ appearance. What I'm saying... the color of the coating means absolutely nothing... functionally.

 

What does matter is the overall brightness of the reflected light, relative to e.g. a plain piece of uncoated glass. All coating engineers carry around a small piece of glass that is spray-painted black on the back side. This is their pocket reference 4% uncoated reflective surface. The engineer (tech, scientist or coater operator) holds that little 4% reference next to the product that has just come out of the coater... and looks for very feeble product reflectivity, relative to the 4% reference --- even as seen in plain ambient shop lights, is quite telling. The spectrophotometer is of course the final pass/fail before the product goes out the door.

 

[I worked as a coating engineer/scientist for twelve years.]

 

CONCLUSION: The color generally means nothing relative to MARC function, but may be designed for aesthetics and/or marketing.    Tom


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#3 Nippon

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Posted 06 November 2019 - 08:17 AM

I don't think it makes much difference what tint the lens reflects. More important is how dim reflections off the lens are. Looking at a coated lens is akin to looking at a fine gem for refractor fans and it would not surprise me if they have figured out that making coatings pretty can be a selling point.

 

Silly me I should have read TOMDEY's post first so I agree with what he said.


Edited by Nippon, 06 November 2019 - 08:20 AM.

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#4 simjp81

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Posted 06 November 2019 - 07:20 PM

Thanks to both of you for taking the time.



#5 SteveG

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Posted 06 November 2019 - 08:45 PM

You will also notice different color tints on your eyepieces. Same thing.



#6 Peter Nance

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Posted 08 November 2019 - 03:14 AM

The color you see is the highest reflectance point of the coating in the spectrum. So if it's green reflection = ~ 550nm etc for the other colors you see visually. The fun part is when the lens "disappears" and you only see the edge. The lens is usually a very light tint of color barely perceptible. It always made me crazy looking at gold coatings on binocular lenses. As Tom stated, the spectrophotometer is the final determination.

 

Reminds me of a funny story, ~ 30 or so years ago one of the guys in the shop wanted to know what type of glass a certain block was. The foreman put it to his nose and stated "It's SF-6" and walked away.

 

{I am just a trainee, been doing optical coating/design for 36 years} Hopefully I learn something new every day :)


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#7 beanerds

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Posted 10 November 2019 - 03:02 AM

Here are my 2 x 4 inch Synta ( Saxon ) retractors objectives side by side .

 

The one on the left in my 102mm f10 achro and is the typical ' China Green ' coatings .

 

The one on the right is my ED100 f9 Apo and it's more of a blue/brownish coating .

 

I have had these two out side by side many times and on deep southern skies from a dark site I find the views are identical using TV Radian's , Plossl's and Panoptics with my Celestron 2x Ultima barlow to get magnifications as close as possible . 

 

The 102mm achromat performs really well on fainter deep sky , basically identical to the ED100 , it's only when the objects get brighter > mag2 and above 150-200x  does a difference start to be seen , the ED100 has less scatter but I think thats in the lens itself not the coatings .

 

Beanerds.

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#8 Rich V.

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Posted 10 November 2019 - 12:02 PM

A photographic representation regarding the brightness of the reflections Tom speaks of in post #2:

 

https://www.csun.edu...d/coating2.html

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#9 Tamiji Homma

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Posted 10 November 2019 - 01:17 PM

You will also notice different color tints on your eyepieces. Same thing.

 

Here is an eyepiece example, both Nagler 16T5, purchased several months apart.

I can see no difference except reflection color :)

 

large.jpg

 

Tammy



#10 Kunama

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Posted 10 November 2019 - 01:59 PM

Here is an eyepiece example, both Nagler 16T5, purchased several months apart.

I can see no difference except reflection color smile.gif

 

large.jpg

 

Tammy

Tammy, you have maritime versions there, red for port.... green for starboard......navigation Naglers 



#11 25585

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Posted 10 November 2019 - 02:05 PM

Hmmm Ruby Port 

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