high index lenses

[cantona]

hi
im so stuck.
why are plastic lenses limited to a refractive index of 1.74?
any links would be greatly appreciated.
thank you

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[jymezg]

Say whaaa?

 

[ibalz]

hi
im so stuck.
why are plastic lenses limited to a refractive index of 1.74?
any links would be greatly appreciated.
thank you

To put it simply:

The U.S. has certain minimal thickness standards for lens safety set by the American National Standards Institute (ANSI Z-87.1 I believe). The higher the index of refraction, the thinner the lens. This is especially true of minus lenses' center thickness. Generally as the lens gets thinner, it tends to lose impact resistance, and the higher the incidence of breakage & possible injury to the wearer.

http://www.visionrx.com/library/enc/enc_ansi.asp

http://www.labsafety.com/refinfo/printpage.htm?page=/refinfo/ezfacts/ezf251.htm

 

[lnh]

But couldn't you also maintain the minimum thickness at higher indexes? I have seen a -1.00 rx in a 1.67 high-index, and of course it is the same thickness as if it were in poly or CR-39.

I figured the limit is based on the nature of the material. There are 1.9's, but I believe they are all glass. I suppose if you want to pay for diamond lenses you could manage 2.X refractive index. But I have enough trouble talking people into buying high-index plastic, so I don't think that would go over too well.

 

[ibalz]

But couldn't you also maintain the minimum thickness at higher indexes? I have seen a -1.00 rx in a 1.67 high-index, and of course it is the same thickness as if it were in poly or CR-39.

I figured the limit is based on the nature of the material. There are 1.9's, but I believe they are all glass. I suppose if you want to pay for diamond lenses you could manage 2.X refractive index. But I have enough trouble talking people into buying high-index plastic, so I don't think that would go over too well.

Yes, you could. But think about it. What's the point of putting the px into 1.67's or higher (unless you're gouging the patient ) if they have the same thickness as a lower index lens like poly, trivex, or CR-39? I mean the whole point of getting hi-index is to get thinner lenses than they could in CR-39.

For example - the higher the Rx, the more pronounced the difference in thickness (for 1.67's the thickness difference is ~40% at about -6.00 and above, depending on how it's ordered from the lab). Of course those -1.00's will be virtually identical in thickness.

I believe 1.80 plastic is either available or just on the horizon - but in Japan. The thickness will not meet the most recent ANSI standards in the U.S. Any index above that would be glass, and also not be available in the states because they couldn't survive a drop-ball test (another impact resistance test).

Believe me, the next time I go to Japan, I'll be ordering those 1.80's for myself (I hate my OU +3.50 -2.00 x 180's thickness even in 1.70's).

Unfortunately, colorless diamonds large enough to be lens blanks just don't exist, though I wish they did.

Hypothetical situation:

The problem with diamonds is that they cleave predictably in only 4 meridians. Outside of those meridians, they would fracture unpredictably in a spiral or conical fashion. This would not allow for grinding any appreciable cylinder power into the diamond lens. The final problem is this: what would you use to cut down a hypothetical diamond? A diamond wheel! You'd have to replace the edger's wheel a few times in order to edge 1 lens!

That was a fun walk through fantasy land.

Hope this explained some.

 

[lnh]

Oh yes! Let's just say I have seen one too many customers with plenty of money, vanity, and little sense, and a commissioned-based "optician" who is all too willing to oblige (but let's save that for another rant). How else will we see that kind of rx in a high-index everyday? I don't even think you can use chromatic aberration to justify that one.

Yeah, I found lens thickness gets marketably noticeable for 1.67 at -5 and up, and for the 1.74, somewhere closer to the double digits and up. Any less and they don't notice/care enough to fork over the money (except the afore mentioned people).

Leave it to Japan to come up with that! What did they put in it? Can you even call it plastic anymore? You are rather stringent with your lens thickness Surely that can't be more than a mm difference? Although I'd really like to see pictures or something if you get them.

Thanks for the explanation and entertaining hypothetical diamond lens. Care to try that with contacts?

 

[ibalz]

Oh yes! Let's just say I have seen one too many customers with plenty of money, vanity, and little sense, and a commissioned-based "optician" who is all too willing to oblige (but let's save that for another rant). How else will we see that kind of rx in a high-index everyday? I don't even think you can use chromatic aberration to justify that one.

Yeah, I found lens thickness gets marketably noticeable for 1.67 at -5 and up, and for the 1.74, somewhere closer to the double digits and up. Any less and they don't notice/care enough to fork over the money (except the afore mentioned people).

Leave it to Japan to come up with that! What did they put in it? Can you even call it plastic anymore? You are rather stringent with your lens thickness Surely that can't be more than a mm difference? Although I'd really like to see pictures or something if you get them.

Thanks for the explanation and entertaining hypothetical diamond lens. Care to try that with contacts?

Not a chance.

 

[ric87]

the advantages in thickness of high-index lenses are primarly seen in the edge thickness for myopes.
in hyperopes be careful to chromatic aberrations in case of high rx..

not always the high index is the best choice, you reduce thickness and weightr, but consider also the disadvantages they have.

i.e. i don't prescribe hi index( >1.74) because they have too many disadvantages..

sorry for english mistakes..i'm from italy..

 

[cantona]

thank you everyone for your help
i got some information but it was so difficult to find out specifically about plastic high index lenses i had to make up some information.