Ask The Labs
By Joseph L. Bruneni

AND MORE ON OFF-AXIS POLY

This question was first featured in the December 2001 issue of Eyecare Business. Here we offer you a different take on the same problem.

Q Your recent column explained why edging polycarbonate lenses sometimes causes them to end up off axis when using small chucks. However, my edger does not have an axis adjustment. Is there anything else I can do to avoid these off-axis lenses?

--Diane Samuels, Kivlin Eye Clinic, Menomonie, Wisc.

A I've had the problem referred to, not only with poly, but with 1.0 CT high index lenses. This often happens with shapes that have a big difference between the A and B measurements. My solution is to edge the lens at least 2 millimeters oversize; check for axis; reblock if needed; and run on size. Not one lens has been lost since I've used this system. It takes a little more time, but it takes a lot of time and money to replace a 2.00D cylinder lens just five degrees off axis.

--Fernando J. Rueda, Houston, Texas

AR AND LIABILITY CONCERNS

Q I have read that coating plastic lenses with anti-reflective and anti-scratch can reduce their impact resistance. Can you tell me what plastic lenses may be involved (particularly CR 39 and polycarbonate) and to what extent this can happen? Your help here will be greatly appreciated.

--R.B. Pomeranz, OD, Rio Rancho, N.M.

A Your question touches on an important topic for eyecare professionals. The impact resistance of all lens materials is reduced by the application of either scratch coating or anti-reflective coatings (most AR-coated lenses are also scratch coated). The impact reduction is relatively slight from scratch coatings and is not an issue. The major concern is with AR coatings, particularly with higher index materials. In the case of polycarbonate, the reduced impact resistance with AR is far beyond the minimum requirements of the FDA. With the rapid increase in popularity of AR coatings, lens manufacturers monitor this issue very closely. In general, if your lab follows the thickness charts published by each lens producer, there should be no problem. In the case of stock lenses, lens producers and AR coating facilities batch test AR-coated stock lenses to make sure each batch passes the drop ball test. Quality labs will refuse to process lenses to a specified center thickness if it is less than the manufacturer's specified thickness for that power. The lab's refusal protects both you and the lab from liability concerns and should always be heeded.

LENSES
For Sports Goggles

Q  What are the ANSI standards as they relate to sports goggles? We are a lab and we shelf bevel them because they mount better and would break away from the face as opposed to a conventional v-bevel. Can lenses for Rec Specs be 1.5 mm thick or are they required to be 3mm?

A ANSI standards are for fashion eyewear (Z80 series) and industrial safety eyewear (Z87.1). The scope of ANSI Z87.1 specifically excludes sports from the standard. The Z80 committee also voted to exclude sports from the scope of Z80. Eyewear for sports must conform to the requirements of ASTM F803. The only lenses that are sufficiently shatter resistant for sports are polycarbonate and Trivex. CR 39, high index, and glass lenses all shatter with the impacts contained in many sports. At this time, lenses that are replaceable (all Rx and plano lenses in a spectacle that will hold Rx lenses) must have a 3mm minimum center thickness, as thinner lenses bow backward through the frame on impact.

--Paul Vinger, MD, Tufts University, Boston

If you have a question you'd like to have answered in Ask the Labs, send it to Joseph L. Bruneni. Fax: 310-533-8165. Phone: 310-533-4975. E-mail: joe@bruneni.com.  Or mail questions to: Vision Consultants, 2908 Oregon Court, #I-2, Torrance, CA 90503. An archive of past Ask the Labs columns can be found on the Eyecare Business Website at www.eyecarebiz.com.