True Index

[John R]

Hi Guys and gals, I am need of a bit of help :D I need to find out what the true index is of the diffrent "plastic" materials that are avaiable..
Why, you ask...
Well i ham in the process of making a chart to show how the change of index affects the curves needed to get a given power.
I have done one for Glass here ,but would like to do one for plastic as well.
Many thanks in advance :D

[JRS]

Each manufacturer is slightly different - in most case the differences would only make changes in the thousandths of a mm. Here is a list of one's I remember - sure others will add more.

1.498
1.502
1.530
1.537
1.554
1.555
1.556
1.558
1.586
1.595
1.600
1.660
1.672
1.710

[shanbaum]

Also, 1.549 and 1.592.

[John R]

Erm yes rather a lot, but not a lot of diffrence. so i hed to remove a few...
How does it look now.
glass & plastic
Thanks folks..:D

[David Wilson]

John,
Many, but not all, of the indices quoted are for the helium d line (wavelength of 587.56nm), favoured by Americans, British and Australian. You will also find different indices quoted by European manufacturers such as Essilor, Zeiss and Rodenstock. These figures will be different for the same materials because they are for the mercury e line (wavelength 546.04nm). So, for example, JRS gave 1.498 and 1.502; both are CR-39. The material is no different, just the wavelength of the light they use as their reference. The Europeans put their stopwatch on light of 546.04nm (green) while we put ours on 587.56nm (yellow). Since our wavelength is longer our light travels faster through the medium faster and hence the index is lower.
Regards
David Wilson

[Wes Trayner]

I was just working on the new files for Transitions Next Generation and they come in 1.497. Add this to JR's list.

[Jeff Trail]

David,

Actually both of those are not CR39.. the 1.5 is CR307 :-) They changed the material slightly because of the chemicals used for the transitions.

Jeff "half a dozen or one or another you still only have half your egg cartoon full" Trail :-)

[David Wilson]

Jeff,
I'm aware of CR-307 but CR-39, when quoted by European manufacturers, is 1.502 not 1.498. Trust me, they do use a different reference. In fact the standards for lenses and focimeters give the option of either. If you are calibrating a focimeter you must use test lenses produced for either the d line or the e line. The lenses will have different curves and will measure differently on a focimeter calibrated for the other wavelength.
Leaving aside CR-307 which is a different monomer, the CR-39 with the index 1.502 is exactly the same material as the CR-39 with the index 1.498.
Regards
David

[JRS]

Being in (from) the US, I was referring to Transition index in my initial post.
However, David's point is quite right. I just spent 5 days in France and Spain, and seeing 'our' indices expressed in the 'e line is a bit bewildering at first. I seem to recall a great discussion over who's method should be standard, a few years back.

[Jeff Trail]

David,

I think that a lot more people should take note of this thread and see how "location" can make a great deal of confusion. I know I deal with some optician over seas and at times we seem to be going in circles when talking about materials.
I start going off in my tangent and talking about curves and such and than their print outs don't match and at times it was a real pain in the behind.
The basic formula's go over but it's very interesting about how things are done so different international compared to where your own little "optical world" is spinning and how you are making decision based on the physics of your own world while someone else is doing the same thing but since the root number we both started from is different we end up plugging different numbers into the equations.
Hey I even knew about the index difference and how we differ, but since JR is a over on this side of the ocean I knew his 1.5 and your answer were two different things.
The tricky part is not everyone seems all that interested in anything other than their own little part of the "optical world", to bad I just love passing info around that is not exactly the same based formula's as I use..
To mind comes the discussion about the Kryptok's, it had been years since I even heard that phrase, other than from a few old timers. I know that not one..yep not ONE of the guys who work for me have even heard of a kryptok lens. Which I think someone wanted to know about when it came around it was developed in the late 1800's and started being produced in the US in the early 1900's..but back to it, I find that the hardest part when we start passing info back in forth is that our answers might not make sense to someone who does not know the root umbers from the person doing the posting.
I enjoy your postings and have gleamed a few bits here and there that got added to my files of what my wife refers to as the "optical abyss" (I call it my home office) ;)
Oh well makes for interesting conversation, maybe to a die hard optical person.

Have a great day down under....


Jeff "you can never ask a dumb question but you can be silent and stupid" Trail

[John R]

Seems i started quite a thread here, just why everbody cant get a standard for measuring indexs is a bit of a joke to me, anyway thanks for all the answers and i have learnt a lot more than i had bargined for now.
Any way the charts are finished, i have even added a sag chart as well, decided to do it in 3 index's 1.498 ,1.523 and just for our americian cousins 1.530.
Just go to my site and you will find it, i would apreceate any feed back on them.

[David Wilson]

Jeff,
In a sense we are in a similar world since we Aussies use the same reference line as you guys, and the European figures look just as strange to us. I find it sad that people can't remember the kryptoc. Not because it was such a good lens but because it reminds me how old I am. 26 years ago I worked in a lab where we actually fused them. Incidentally, Jeff, I too have enjoyed reading your posts.
John, the reason we can't decide on one reference is much the same as the reason we can't all drive on the left hand side of the road (I'm sure that you guys in America and Europe wouldn't mind changing to the British and Australian way) Changing would cause the "loser" a considerable amount: re-writing textbooks, brochures, recalibrating focimeters (vertometers), changing all of the tooling etc etc. The last time this was debated in the ISO the Europeans argued for one reference (their e line), saying that at 546.07nm it was closer to the eye's peak sensitivity (555nm). The Americans argued that we have been using two references for so long that it was not a major problem as long as we made it clear which we were using. They also pointed out that while the e line might be closer to 555nm, the d line at 586.57nm was closer to the peak wavelength of the light emitted by the lamps etc used in eye testing. ISO standards currently allow for both but there is an ominous clause where they predict one wavelength in the future.
Regards
David

[Jeff Trail]

David,

When you really step back and look at it it would not really change much at all other than printing on the boxes of lens. All our programs have tons of patches for compensation and you can plug any number of combinations into the programs.
Mathmatically you can plug in compensating numbers to mke it all come out the same and that is quite easy to do.
The major problems I see that the lens companies want to really hedge on is the "numbers game" .. it seems most people fall into that trap, like computer programs..oh I have 2.0 version and they just came out with 2.5 I have to go download it.. it all it amounted to is a small patch here or there to upgrade..the same would apply to the lens.. the material would be the same as before, BUT say I have a 1.71 index (US Standard) and than they came a long and wanted us to use another wave length to determine the index and all of a sudden that 1.71 goes to a 1.67, same material same benifits same thickness and curves (after you compensated) but because the number is less than an optician says I'm not paying the higher price for a lower index!! LOL
Same thing applies to Poly and Tri-vex, Tri-vex has a higher tensile strength than poly, a higher abbe value (30 poly, 42 to 45 in Trivex) depending on slight mix between the two US companies distributing it.. BUT the tri-vex is running around twice the price as poly so poly wins out on economics and not quality.
They all might argue about it on a scientific level but dollars to doughnuts it's all economics driving the debate
Oh well off to the races got to watch NASCAR and than they are having a car show here at the Daytona 500 track over 9,000 antique cars and hot rods!!

Jeff "stuffed to full to type anymore" Trail

[Darryl Meister]

Here are some more detailed threads describing the differences between the helium d and mercury e lines for measuring refractive index:

Ophthalmic Optics Thread...
http://www.optiboard.com/forums/show...&threadid=2601

Ophthalmic Optics Thread...
http://www.optiboard.com/forums/show...&threadid=1810

Best regards,
Darryl