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Troubleshooting Tips

Lens Misconceptions, Part I

Karlen McLean, ABOC, NCLC

There's a lot of confusion about how lens materials and measurements affect final eyewear appearance and performance.

Part I of this two-part series addresses misconceptions about 1.67 and 1.70 lenses and lens thickness.

IN THE THICK OF IT

Q What do incremental millimeters really mean when it comes to lens thickness?

A There is a misconception about the relationship between center thickness (CT) and edge thickness. If a lens is ordered with a 1.5mm CT, ANSI standards indicate the lab should provide a lens of no more than 1.8mm because there's a 0.3mm thickness tolerance.

Many ECPs believe that a 0.3mm increase in CT becomes exponential at the lens' edge. In reality, an extra 0.3mm in the center of a lens will result in an increase of 0.3mm at the edge of a lens, which is the thickness of around two Post-It Notes.

Many labs set their CT target at 0.1mm or 0.2mm above the minimal allowable lens thickness to avoid producing lenses that are too thin to pass FDA impact requirements. According to ANSI standards, if your lab provides a lens that's within 0.3mm of your ordered CT, the lens should be acceptable. Anything thicker and the lab should accept a return.

—Pete Hanlin, LDO, training and development manager, Essilor of America

NUMBERS GAME

Q What are common misconceptions about recommending and dispensing 1.70 lens material?

A That a 1.74 will be thinner because it has a lower refractive index. Substrate matching and unique cushion coating chemistry allow 1.70 lenses to be ground to a 1.0mm center thickness (CT) on moderate to higher minus powers and still pass FDA drop ball testing for dress eyewear.

The 1.74 materials have a minimum CT of 1.5mm on like powers. If all other parameters are equal, the 1.70 with a 1.0mm CT will be equal to or thinner than a 1.74 lens with a 1.5mm CT. Also, the 1.70 material has an Abbe value of 36 compared with the 1.74 with an Abbe value of 31.

—Brad Main, FNAO, director of training and technical resources, Hoya Vision Care, North America

If you have a question you'd like to have answered in Ask the Labs, send it to Karlen McLean. E-mail: KarlenMclean@WoltersKluwer.com. Or post a question at EB's page: facebook.com/eyecarebusiness. An archive of past columns can be found on our website at eyecarebusiness.com.