ASK THE LABS
By Joseph L Bruneni

Monocular PD for high-index

Q I know it is recommended that monocular PDs be taken when dispensing polycarbonate. Is this also true with other high-index materials?

A Apart from the obvious visual reasons for providing accurate PDs, any lens material with a lower Abbe value than glass or CR 39 benefits from accurate monocular PDs. All high-index materials have lower Abbe values. In addition, most people ordering high-index lenses tend to have strong corrections. Since most patients do not have their nose located in the exact center of their face, monocular PDs are always advisable for high index lenses (including poly).

Our suggestion is to take monocular PDs on everyone. As you take the measurements, tell the patient why accurate measurements are so important and how your instrument contributes to the accuracy of their final eyewear.

Drilling Poly

Q I have a lab in Mexico and have been working with polycarbonate for three years. I have had problems with drill lenses. I finished them successfully, but later they break at the holes. Do you have any suggestions?

�Carlos Becerra, Mexico

A The following information comes from a Vision-Ease Lens Technical Paper: Place lens with front toward drill bit and slowly operate drill through lens; back drill bit out of hole often to remove cutting debris that will increase heat damage. Excess pressures and speed create damaging heat, which may create eventual fractures.

Polycarbonate cuts best with a sharp burr operated at low speeds of 20 to 60 revolutions per second and minimal to moderate pressure. Twist style drills, cutting less aggressively, often leave subsurface damage. Cleaning should be done prior to mounting, and utilize warm soapy water with a clean water rinse. Wiping should be minimized to avoid abrasion from cutting debris.

For mounting, assemble lenses with appropriate hardware, minimizing excess tension. Frames should be fully aligned and formed to lens prior to final assembly. Excess stress on assembled eyewear may eventually create fractures. Use of plastic bushings is recommended. Mounted lenses should be inspected for excess stress and correct positioning and alignment.

Q Could you give us insight on backside AR coating on polarized lenses, whether plastic or poly? The patient is extremely light sensitive, lives waterside, and spends a lot of time fishing. He wants tintable polarized�preferably in the green (gray ok) family of tints. We will fit him with a FT bifocal, and there will be a little distance prescription as well.

Would AR coating on both front and back increase light transmittance to allow more light in and not help his light sensitivity? (Our preferred AR coating is tough and very durable, but they cannot do rear side only. So it must be applied to both the front and back.)

�Jane and Nancy, opticians for Drs. Jolly, Lewis, and Chlebina, Bradenton, Fla.

A If your preferred coating is not available in a backside coating, you shouldn�t have much of a problem in using a different AR coating for the backside. The rear surfaces on eyeglasses don�t get nearly as much abuse as the front surface. When ordering AR for both sides of the lenses, you are reducing the darkening effect of the polarized lenses by some 8 percent, which may not be what the patient wants, particularly for fishing. Go with a good quality backside AR and the patient should be pleased with the results.

Got a question you'd like answered in "Ask the Labs?" Send it now!