Different Indexes?

[Amit]

VERY quick quesion.

In the UK, it is commonplace to be advised NOT to dispense different index lenses in each eye, due to possible loss in Binocular VA's, as a result of different lens performances.

Theoretically, depending on RX and degree of anisometropia as well as other factors, the final VA's from the eye test may not match the final product.

Any theories? Is the poteintal loss in VA's negligable? Does the cometic balances in terms of lens thickness in anisometropia outweigh thses risks? Assuming various circumstances on RX.

Thanx.

[Darryl Meister]

Unless the Abbe values of the two materials differ considerably, the difference in visual acuity produced between the right and left central zones will be minimal (and even then it would depend upon the color). In the periphery, differences in Abbe value can lead to more pronounced color fringing in one lens material over the other, but the effects upon binocular vision may be difficult to characterize.

However, there are other, more practical reasons for not mixing indices. The use of two different lens materials can result in a number of potential problems. Off the top of my head, these could include:

1. Durability. One material may be less abrasion-resistant than the other, which will lead to uneven wear. Other mechanical differences may also become important, such as differences in tensile strength for rimless mountings.

2. Color. Some lens materials have a slight hue to them (e.g., yellowness or bluishness) even in their untinted state. While this color is generally negligible, it becomes more obvious when materials with two different hues are compared. The brightness of surface reflections will also differ between the two.

3. Tinting. The material/coating combinations will almost certainly differ in their tinting properties. It may be difficult to match exactly the tint color and density between two different lens materials.

4. AR coatings. If the two lenses use different hard coatings, it is possible for the reflex color of the coatings to differ between the two lenses for custom-applied coatings. Also, one material provide a better substrate for AR than the other, which may also lead to uneven wear. For finished lenses with stock AR coatings, the differences would be even more significant since different AR coatings will most likely be used.

Best regards,
Darryl

[chip anderson]

I have done this quite often, mixing CR-35 with whatever high index (except poly, which may or may not do well, I just haven't done it) seemed cosmeticlly best for the application. While tints, coating failures, scratch resistance, etc are all conciderations, I have never in the last ten years had this come back to bite me.


Chip

[drk]

While theoretically, what you say is true, practically, the vision effects should make almost no difference.

Having said that, it seems impure to put two different materials in a single frame, like Darryl says. I've never done it. I assume it's to make one thicker and one thinner? Just use equal center thicknesses if it's anisoplus.

I'd bet it's anisominus, though, and your idea could work to add edge thickness to the lower power lens. I'd try it. A CR-39 SV or BF is fairly cheap to remake. Watch for impact resistance liability, however!

[Darryl Meister]

If you're interested in balancing thickness, it would probably be just as easy to use the same index/material for both lenses and increase the center thickness of the weaker lens slightly. This would accomplish a similar effect without any of the physical issues that you may (or may not) run into using two different lens materials. Magnification differences may be somewhat of a concern, but this seems to be a rare problem for wearers.

Best regards,
Darryl

[chip anderson]

Like I said, I do this all the time, if I have -2.00 on one eye and -4.00 on the other, I use CR-039 and 158 index. If I have -2.00 and -6.00 I use CR39 and 167. Works fine looks very good, patients are not asshame to wear thier glasses in public.

Although I do tend to mix mostly 158 or so with 169 not much with CR39 and higher indexs.


Chip:cheers:

[Amit]

very useful. thanx guys for your valuable resources :)