ask the labs
By Joseph L. Bruneni

RIMLESS PD ISSUES

Q I must respond to an equipment question answered in the September issue, regarding frame PDs and rimless eyewear. I take issue with the comment that frame PD is "determined by many things, including the shape of the lens, the thickness of the lens, the depth of the groove, the tension of the cord, and flexibility of the frame." A frame's PD is simply the "A" measurement (boxed A) of the dummy lens plus the true DBL (not the DBL size marked on the frame). All the other factors don't apply to frame PD.

The two reasons for PD discrepancies are incorrectly measured DBL and incorrectly blocked lenses. A couple of half millimeters here and there add up to errors. Also, check rimless PDs by remarking carefully and comparing to a cut-out chart to mark (looking at the back side of the lens), as this reduces parallax and front curvature errors that can be created with a ruler.

I think your lab column is full of useful information, and read it regularly.

--Richard Powell, ABOC, RDO,
Tice Valley Optical,
Walnut Creek, Calif.

A You are correct about determining the frame PD for a rimless mounting. All you need is the "A" measurement of the dummy lens and an accurate measurement of the DBL.

Knowing the PD of the mounting is 68 and the patient's PD is 62 indicates the centers should be moved inward 3mm for each eye (68mm - 62mm = 6mm, divided in half [3mm]).

SPOILAGE FACTORS

Q Can you let me know what the industry benchmark is for lab breakage? I am trying to gauge what is an acceptable level for my own lab.

Do you have figures for edging and surfacing areas? I am particularly interested in what is an acceptable spoilage level in my edging lab.

--Jim Mazzerolle, Pacific Eye Doctors,
Chilliwack, British Columbia

A When looking at labs that have made reasonable investments in technology over the last five years, we find that in the surfacing room, 1.5 percent to 2.5 percent, and in the bench room, 3.5 percent to four percent, would be considered acceptable.

Overall, if you take a look at breakage, and it is over six or seven percent, it may be time to take an overall look to qualify it and then contact your equipment vendor for possible solutions.

--Steele Young,
LOH Optical Machinery

Trivex Pointers

Q The ANSI Z87.l-2003 portion mentions Trivex with polycarbonate. Are there hard numbers about impact resistance of Trivex compared to poly? What is its Abbe and can it be anti-scratch and AR coated?

--Robert Pomeranz, OD, New Mexico

A PPG, producer of Trivex, and two of the manufacturers making these lenses, Hoya and Younger, have not posted specific numbers on the relative strength of poly vs. Trivex, but claim Trivex is comparable to poly. The best indication of this is that the new Z87 standard accepts Trivex as poly's equivalent. Each lens manufacturer has made slight modifications to its Trivex resin so the specs vary between brands. Trivex lenses have an Abbe value ranging from 43 to 46 depending on the manufacturer.

Lenses come with a durable, front-side hardcoat, and they are more scratch-resistant than many plastic lens materials regularly dispensed without a back coating. However, for maximum scratch resistance, a backside coating may be applied. Anti-reflective coatings work well on Trivex lenses. Trivex lenses can be tinted to popular fashion and sunwear colors, and the unique molecular structure of Trivex provides a superior jewel-like appearance to edges that are rolled and polished.

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.eyecarebusiness.com.