Lens Identification

[NavyChief]

Do you have a way of figuring out the material of the lens a patient is wearing?

I can determine the differance between a poly and plastic, but how about whether it's a poly or hi-index?? I don't know if a hi-index sounds like poly or not. You?

Thanks

[HarryChiling]

If you were to drop it onto a counter the difference would be clear. The Poly makes a clanky noise. Also if you have 2 polarized filters and place them 90 degrees apart from each other then place the lens in between them both you will see a high amount of stress in the poly. Yet another way is to take the bevel to a wheel and poly will melt and flake off where hi index grinds to dust.

[Happylady]

I can tell poly from hi index because poly sounds like a poker chip when dropped, but I have trouble telling one hi index from another. For example telling 1.67 from 1.60. Any ideas?

[Excel-Lentes]

That is kind of tough to figure out at first. The quickest test is the sound it makes when lightly dropping it onto a counter top (hard to do when mounted in the frame). Sometimes you might be able to determine based on thickness but as more lenses are introduced it gets harder and harder to differentiate. Many stock lenses in poly are 1.2 - 1.5 CT on -Rx's but could be 1.0 on a surfaced lens. Place of origin might help; lots of chains really push the poly where an independent would prefer a finer high index product.


IDEA::

ask your lab for some samples of various lens materials edged to identical shapes and label them with a marker. Then compare their different qualites; sound, feel on the handstone, thickness, etc... You could even use them to show your customers the difference in thickness when choosing High Index.:)

[ilanh]

THis is a very legitimate question and one that I have posted here before. Of course it's easy to tell Poly apart from CR39. However, how about mid-index from 1.60. Or 1.60 from 1.67 or higher? I have had several instances when I ordered a 1.67 and had the distinct impression of receiving a 1.56. I would love to know how to tell them apart (I don't have edging equipment so I can't tell by flaking etc).

[Clive Noble]

Ilan, after you've glazed many pairs of 1.56 and 1.66 there is a difference in smell, but as you say you have no edging facilities.

You've only got one option...... you have to trust your supplier!!

Having said that, I just received a pair of 1.71, same Rx as the Pts old 1.6
Same CTs same size lens A and same PDs.... Rx about -6.00
There is hardly any difference in the edge thickness and the Pt is not impressed especially at the extra cost. I do trust my suppliers, maybe we are overselling these lower numbers as 1.71 where 1.6 will be sufficient

[Chris Ryser]

There is hardly any difference in the edge thickness and the Pt is not impressed especially at the extra cost. I do trust my suppliers, maybe we are overselling these lower numbers as 1.71 where 1.6 will be sufficient

Why then do you even bother selling a 1.71................???? Price and Profit?

[Rafael]

Hi,

here is a way to calculate the material Index....


http://www.optiboard.com/forums/show...773#post127773

[NavyChief]

Thanks for all your comments. Think I'll try comparing some lenses coming back from the lab, and see if I can tell the differance.

[lensgrinder]

There is a formula that estimates refractive index. It is:

nt=1+Ft(nlm-1)/Flm

Where

nt=refractive index
Ft=true power of lens from focimeter
nlm=refractive index of the lens measurer
Flm=sum of surface powers taken from lens measures

Example:
A lens reads +2.50 in the focimeter. The front curve reads 6.25 and a back curve reads 3.50 with a lens clock that is scaled to a 1.53 index. What is the index of refraction of the lens?

nt=1+(2.50(1.53-1)/2.75)
nt=1+(1.325/2.75)
nt=1+0.48
nt=1.48

Keep in mind these are just estimates.

[Clive Noble]

Why then do you even bother selling a 1.71................???? Price and Profit?

No Chris, this case in question is where parents want the thinnest at any cost for their 'spoilt little girl' They were told that there wouldn't be a big difference in thickness.

On the same subject, with prescriptions up to about -4.00 we've found that 1.56 is as thin as (sometimes thinner than) 1.6
CTs are the same......... why? Go figure!

[QDO1]

Why then do you even bother selling a 1.71................???? Price and Profit?

thats an interesting point to be asked of the industry. I can see why the lens is there - perhaps for the -8.00 rx's or the larger framed -5-6 rx's. I can see the need for a lower power, but only as a match in an aneisemetropic RX. It does stump me why someome would sell a lens with a poor ABBE value when a very similar lens is cheaper, and has better optics, and only a minute change in thickness. There is a element in the industry that sell a lens for the $$$ value compared to the optical. I would say that too b fiar, there are a lot of practitioners that consider only the optical values of a lens first. I personally fall in the second camp, and I imagine many of the posters here also do

[spartus]

It does stump me why someome would sell a lens with a poor ABBE value when a very similar lens is cheaper, and has better optics, and only a minute change in thickness.

I had a patient a month or two back, RX was about a -8.00 OU, and she wanted the "thinnest lens available". I made the mistake of mentioning 1.74, and then spent about 10 minutes talking her out of it, trying to explain that it's more money for almost no difference in thickness. She was skeptical, but ultimately I got her to go with 1.67. People are strange.