ASK THE LABS
By Joseph L Bruneni

Is Older Better?

Q We have a patient who claims his 15-year-old PGX lenses get as dark as regular sunglasses behind the windshield of his car. Is he mistaken? Also, how dark do the new Corning glass Thin and Dark lenses and the new Rodenstock lenses get behind the windshield of a car?

Are there any other products available in glass that are photochromatic that achieve a maximally darkened state under these same conditions?

-Gayle Misener, Dr. Robert G. Misener's office

A We contacted Corning, the primary international source for photochromic glass for the last 40 years, to see what they had to say.

The question led to a major investigation by Corning's technical director to help the doctor and his patient.

The patient drove a bus and wore a plus correction. This is what happens with plus PhotoGray Extra lenses that are this old. Plus PGX lenses can develop a "history" over a long period of time and do not clear as much indoors as the years go by. Their darkening action, however, is not affected. Consequently, this patient's lenses that originally darkened from 90 percent transmission to 25 percent were now clearing only to 75 percent indoors. Yet, when exposed to UV, they now darken to 20 percent. As a result, even behind the windshield of the bus, they get fairly dark.

This phenomenon only affects PhotoGray Extra lenses in plus or bifocal corrections. Thin & Dark lenses do not develop a "history," so their clearing and darkening action remain constant.

-Dave Kerko, Corning, Inc.

Eliminating Laser Markings

Q One of our customers has a patient who is complaining that the laser markings on the progressive lenses she is wearing are creating distortion for her in the lens. She wants to know if there are any progressives on the market without laser markings. Do you know of any?

A Though some early PALs used UV markings, we very much doubt that progressives without laser markings are still made by anyone.

Those important location marks are the only way in which fitters can reconstruct and determine the physical components of a progressive lens. There have been problems in the past when scratch coatings were applied to the front side of a progressive lens because scratch coating tended to cover the laser marks and that may be the answer for this patient.

Labs generally only apply scratch coating to the backside of a lens. Laser marks are always on the front surface. I suspect that if a scratch coating is applied to the front surface of the patient's PALs, the markings would vanish.

Safe As Poly?

Q Is a CR 39 lens in a prescription with a 3mm center thickness safer or as safe as a polycarbonate lens? This is for a patient that we are considering switching to a CR 39 lens.

-Dr. Pinchuk, Bowling Brook, Ill.

A The simple answer to your question is "No." Consider this: Currently, industrial safety lenses in glass, poly, or CR 39 are required to be 3mm thick at their thinnest point.

The new Z87.1 ANSI standard expected to be issued by the end of the year will permit polycarbonate (and the new Trivex lens material) safety lenses to be 2mm thick. The new standard will still require industrial safety lenses made of glass and CR 39 to be 3mm thick.

Incidentally, if the reason for considering CR 39 is because of patient complaints about distortion or color, lenses made with Trivex may be a good choice. Trivex has the strength of polycarbonate but with a considerably higher Abbe value. They are available from Younger Optics and Hoya Optical.