ask the labs

Thickness Issues

Susan Tarrant

Q Lens thickness is an issue for some patients with high-powered Rx’s. It can be even more of an issue when paired with the larger frame sizes. What can we do in the dispensary to make sure patients are happy with their finished eyewear?

A Some of the lens manufacturers have charts available to show edge/center thicknesses and material comparisons. The most important thing is to be honest with patients as to what they can expect. A good way to demonstrate thickness issues is to have frames with high-minus lenses and high-plus lenses mounted in them so that the patient can actually see what you are talking about.

— Mike Estes, Pech Optical Corp., Sioux City, IA

A I have always maintained that the definition of a good optician is one who knows what a pair of glasses is going to look like before it is ordered. It is essential to be able to determine how much power there will be across the horizontal axis line, then multiply the power times the number of millimeters of decentration. This will determine the amount of prism it takes to move the optical center that many millimeters of decentration.

On high-powered Rx’s it is a good idea to keep the decentration down to no more than about 4mm per eye. Remember, if you have a -10.00 Rx with 4mm of decentration, it will take four diopters of prism to move the optical center that far. A rule of thumb is that for every diopter of prism, it will add 1mm of thickness to the edge. That will be added to an already thick lens, because of the power of the lens. So, you can see that choosing a frame that requires no decentration, or as little as possible, will make a much thinner lens than one that requires 7mm or 8mm of decentration.

Another solution that many of our customers utilize is to call the lab with all of the Rx parameters. We enter a test Rx to determine the exact thickness while you wait. It is a very simple and accurate method.

— Mike Sutherlin, Sutherlin Optical, Kansas City, MO

A There are cut-out charts available for most progressive lenses, and asking your lab to show staff how to use them will help greatly. A similar approach for lined multifocals will also help. Knowing the blank diameter, decentration, and effective diameter (ED) allows for a simple formula for single vision lenses.

— Drake McLean, Dietz-McLean Optical, San Antonio, TX