ask the labs

Lenses & Logic

by Karlen McLean, ABOC, NCLC

CRAZY AR

Q Is Trivex a viable option to high index? If so, which powers are appropriate to consider Trivex over high index?

A Our lab recommends Trivex anytime a good impact-resistant material is wanted that has none of the issues of other materials.

We use Trivex for children as it's the lightest material available with high impact, with a low specific gravity of 1.11. It's also not affected by chemicals, nor does it split. This makes it a great material for rimless.

Poly has an Abbe value of 28 to 31; Trivex is 43 to 45. Poly's refractive index, 1.598, is higher than Trivex's refractive index, 1.530. We recommend Trivex in Rxs that are equal to plastic.

This is due to its relatively lower index. This range would be around +3.00 to -4.00D.

— Jonathan Jacobs, vice president, Superior Optical labs, Inc., Ocean Springs, Miss.

THE HARD WAY

Q Are there certain types of PAL designs that are easier for flat-top wearers to adjust to?

A Yes, there are several types of PALs that would be easier for flat-top wearers to adjust to than others. PALs with a hard design would better mimic the functionality of a flat-top.

By making this statement, however, we are not recommending this course of action. Almost all the new PALs resolve many of the problems associated with hard design PALs, for example, field of view restriction and limited orientation at the periphery.

While these hard design lenses have good distance and near vision characteristics, they do suffer from an otherwise limited field of view, such as transitioning between near and distance zones and the movement of side-to-side images.

So consider hard design PALs, but also consider the latest PAL designs to achieve success with flat-top wearer conversions.

— Ed Dietz III, president, Dietz Laboratories, Inc., Fort Worth, Texas

If you have a question you'd like to have answered in Ask the Labs, send it to Karlen McLean. E-mail: mcleank@lwwvisioncare.com. Fax: 215-643- 1705. An archive of past Ask the Labs columns can be found on the Eyecare Business Website at www.eyecarebusiness.com.