Ask the Labs
By Karlen McLean, ABOC, NCLC, and Martine Bruneni

LOOKING AT GLASS

Q Is glass still considered to be the best lens material for visual clarity and acuity?

--Barry Edewaard, OD,
Blountstown, Fla.

A Glass lenses are still the best when it comes to visual acuity and clarity. This is due in part to two characteristics: Hardness and rigidity.

The hardness of glass allows lenses to be ground and polished to a finer surface, that even under extreme magnification is free from variations in curvature that are often present in plastic or poly lenses. These variations can create distortions as light passes through the lens surface.

The rigid nature of glass allows lenses to be produced that will not deform from eyewire tension or temperature extremes. Plastic and poly lenses can alter from eyewire tension or day-to-day changes in temperatures, altering the prescribed power. The thinner the lens, the more noticeable the deformation.

Temperatures necessary to deform glass lenses would cause plastic or poly to burst into flames.

Glass lenses are still the standard for quality telescopes, binoculars, microscopes, and cameras.

--Dennis Tindall,
Vision Systems, Inc.

INDEX GUIDELINES

Q What is the cut-off index for the standard lens versus the high-index lens?

--Judy Canty, Holland-Canty Opticians, Fairfax, Va.

A I've seen no definitive source for the cut-off of a standard lens to high-index. However, for plastic: Standard index is anything under 1.501. This would include CR 39 and Transitions 1.501. Mid-index is from 1.502 to less than 1.599. High-index is 1.600 and above.

Although the 1.586 index of polycarbonate would classify it as a mid-index, it is referred to as polycarbonate.

--Daniel Torgersen, Walman Optical, Minneapolis, Minn.

EDGING POLARIZED

Q Can you recommend the best way to edge and groove a polarized lens?

--Grace Sutkus, student, Hillsborough Community College, Tampa, Fla.

A The improvements in polarized lens technology have made polarized lenses easier to process than previous generations, but polarized lenses have unique properties of which to be aware.

To ensure the best results, wet edging is recommended. Reduce the chuck pressure on the edger, use the slowest wheel speed, and use the finest wheel surface possible. When grooving, keep the groove toward the center of the back of the lens. Grooving too close to the front of the lens could disturb the polarized wafer.

--Eric Lindquist, Specialty Lens Corporation, Salt Lake City

Managing Magnification

Q What is a good starting-out hand-held magnifier power and size for a first-time user?

--Russell Tolar, Optometric EyeCare, Chapel Hill, N.C.

A This is a good question! Each patient must be individually evaluated. First, it must be determined which power is best for them according to the task or tasks they want to accomplish. Then, different styles of magnifiers for functionality need to be considered. For example: If someone wants to read the information on a prescription bottle, they need a handheld magnifier. If they want to read sheet music, you'd go with spectacle magnifiers. If they want to read a book, a CCTV is a likely choice. Or, there may be more than one solution for one patient; one person could utilize all the above options.

--Timothy Gels, Eschenbach, Ridgefield, Conn.

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