Hands On - Ask The Labs
by Karlen Cole McLean, ABOC, NCLC

TATTLETALE LENSES

Q How can I determine--without patient records--whether  a spectacle lens is standard plastic, polycarbonate, or Trivex?

A The best way to determine a lens material is by using a Polariscope. New models are not expensive and can be used to demonstrate the difference between lens materials to patients. Most poly will show areas of color, while standard plastic and Trivex are clear.

If the lens is tinted, you can view it from the side. If the tint is only on the front and back of the lens, it's a poly; Trivex and standard plastic tint all the way through.

You can check lens thickness using a caliper. Or, you can also use a hand stone to carefully buff a tiny bit of the lens, then examine the dust. Poly has an acrid smell, while Trivex is tough and produces little dust.

Finally, there's the tried and true "drop test." Hold the lenses a few inches above a clean countertop and drop them lightly. Poly will make a "tinging" sound.

--Gerry Shaw and Pete Mayrand, Western Carolina Optical, Asheville, N.C.

BEST PAL

Q What's the best progressive addition lens for a specific seg height?

A Right now the big thing is small frames. More often than not, we recommend PALs that have been designed for today's popular smaller frames and feature lower minimum recommended fitting heights.

In making a determination of what PAL to use with a frame, we have to consider different lens material needs. Although choices are always expanding, not all materials are always available in specific progressive addition lens designs.

For example, if the eyewear is a rimless drill mount, we'd recommend poly or Trivex for strength, durability, and safety, and a particular Rx may or may not fit into the right PAL design in those materials.

Recently, though, lens manufacturers have been diversifying their design-to-material availability. These days, we can usually accommodate requests for specific seg heights in the right PAL for the patient.

--Michele Smith, Select Optical, Columbus, Ohio

Short Story

Q Should I prescribe short-corridor PALs instead of standard PALs, even if frame requirements don't necessitate short-corridor PALs?

A Although lens manufacturers have used new technology to create today's short-corridor PALs, these lenses should not be considered superior to standard PALs. Short-corridors represent a PAL category. To use them in anything other than small frames would compromise vision, since manufacturers have left less room to "progress" through the different fields of vision. Fashion should never take precedence over the best visual solution. Short-corridors don't offer the range of vision found in a regular PAL, so we recommend using them only if needed.

--Jeff Szymanski, Toledo Optical Laboratory, Toledo, Ohio

If you have a question you'd like to have answered in Ask the Labs, send it to Karlen Cole McLean. Fax: 215-643-1705. E-mail: mcleank@boucher1.com. An archive of past Ask the Labs columns can be found on the Eyecare Business Website at www.eyecarebusiness.com.