ask
the labs
By Joseph L. Bruneni and Martine Bruneni
PIERCING QUESTION
Q I have been working with polycarbonate lenses in Mexico and am having problems with drilled lenses. Some of the lenses we drill are cracking around the hole after they are used. Could you suggest a technique to solve this problem?
A Place the lens with the front facing the drill bit and slowly operate the drill through the lens.
Trying to push a drill through polycarbonate will actually push material out through the backside. To avoid this, it is best to drill down 1/2mm at a time, back the drill out, let it cool several seconds, and then continue -- 1/2mm at a time. Back the drill bit out of the hole often to remove any cutting debris that, if left there, will increase heat damage.
By slowing down the process, you avoid melting the material and end up with a clean drill hole.
Poly cuts best with a sharp burr, operated at low speeds of 20 to 60 revolutions per second and with minimal to moderate pressure.
Twist style drills cut less aggressively and often leave subsurface damage.
--Polycarbonate Lens Council
A MOST-ASKED QUESTION
Q When verifying bifocals in our
lensometer, why is the add power always 1/4 diopter stronger than requested? Our lab tells us not to worry, but we wonder if it really is an issue. In particular, the add is consistently off in plus powers.
A The add power of all multifocal lenses should be measured with the segment side of the lens against the lens stop of your focimeter (also called a lensometer).
For most, this is typically the convex, front surface. The reason for this is that manufacturers mark the add powers of bifocal segments based upon the difference between the front vertex powers of the distance portion of each lens and its multifocal segment.
The front vertex power is the power measured when the front surface is against the lens stop.
When you measure the distance Rx, on the other hand, you are measuring the back vertex power since the back surface is against the lens stop.
The same lens will differ between front and back vertex power measurements, and add powers measured using the back vertex powers will typically have more plus power than add powers measured properly.
Although the wearer actually uses the back vertex power of the multifocal segment, lens manufacturers cannot control the back vertex add power since it varies as a function of lens thickness.
For consistency, the front vertex power is used--since the difference between front vertex powers is not affected by lens thickness or surfacing.
For thin lenses (such as low- and minus-powered lenses), there is little difference between front and back vertex power measurements.
Consequently, flipping a minus lens around to measure it from behind generally won't affect the add power reading as much.
Plus lenses, however, should always be measured with the front surface against the lens stop, since the lenses generally have an appreciable center thickness.
In addition, some automatic lensometers might compensate for this effect, so you should refer to your automated lensometer manual if you are in doubt.
Law or Statute?
Q Is the Duty to Warn law a federal law, or a state law, or statute?
A There is no state or federal law mandating Duty to Warn. The courts have ruled that an eyecare professional has a responsibility involving consumers' right of "informed consent." Any person ordering eyeglasses must be informed that polycarbonate and Trivex materials are more impact-resistant than other lens options.
If you have a question you'd like to have answered in Ask the Labs, send it to Joseph L. Bruneni c/o Eyecare Business. Fax: 215-643-1705. Phone: 215-643-8116. E-mail: editor@eyecarebiz.com. Or mail questions to: Ask the Labs at Eyecare Business, 1300 Virginia Drive, Suite 400, Fort Washington, PA, 19034. An archive of past Ask the Labs columns can be found on the Eyecare Business Website at www.eyecarebiz.com.