ask the labs

Plus Lens Protocol

by Karlen McLean, ABOC, NCLC

BEST RESULTS

Q What are the top criteria I should consider to get the best results with plus-power lenses?

A Consider: 1. Frame PD vs. patient PD; 2. Frame shape; 3. Accurate frame ED; 4. Lens materials; and 5. Aspheric lens design.

Try to match the patient's PD with the frame PD to keep center thickness at a minimum. Angular styles can have difficult ED measurements that may result in lens blanks that are too small and/or too thick. To avoid this, send the frame or attach a tracing with your lab order.

Recommend lens materials according to their features. For example, 1.74 can make some Rx's up to 35 percent thinner, and poly or Trivex up to 40 percent lighter, than traditional 1.50 plastic.

Finally, we recommend an aspheric design on all Rx's of +1.50D and above, as well as −2.00D and above. This yields the flattest lens and improves vision away from the lens' optical center.

— J. Larry Enright, Jr.,
Perferx Optical, Inc.,
Pittsfield, Mass.

FINISHING TOUCHES

Q How should a lab finish plus-powered lenses to look best?

A Start with a lens design that will result in a cosmetically pleasing lens. In a single vision or multifocal this should be an aspheric in a higher index such as poly, 1.66/1.67, 1.70/1.71, or 1.74. An aspheric design flattens out from center to edge while maintaining maximum visual acuity.

Choose a frame that will hold a higher plus lens so the lab can match the base curve to the frame curve.

The width of the frame should be kept to a size that minimizes decentration, which allows the lab to better control the thickness and creates a more manageable fit of the nosepads.

We try to keep the front of the lens flush with the frame. Edging equipment lets us set the bevel specifically for each frame and Rx.

— Craig Giles and Greg Lieck,
Soderberg Optical, St. Paul, Minn.

thinness required

Q What can dispensers do to assure that they achieve the thinnest plus lens outcome?

A Frame selection is the key to providing patients with the thinnest lenses possible. Let's take a +4.00D to a +6.00D single vision Rx. What to avoid: 1. Nylor frames, since a 2.0mm edge is required; 2. A three-piece rimless mounting, which requires 2.0mm lens thickness at the drill points; and 3. High-wrap frames in metal or zyl, as an 8.00 base spherical lens is required. Best choices include: 1. A relatively flat metal or zyl frame with around a 4.00 to 5.00 base curve; 2. Selecting a frame size so the patient's pupil is close to the center of the frame with a maximum of 3mm decentration per eye; and 3. Ideally, a 1.67 material in an aspheric design, selected with a 1.2mm to 1.4mm edge. Also, premium AR is a must.

— Paul Zito, president, Encore Optics, LLC, South Windsor, Conn.

If you have a question you'd like to have answered in Ask the Labs, send it to Karlen McLean. E-mail: karlen.mcleank@wolterskluwer.com. An archive of past Ask the Labs columns can be found on the Eyecare Business Website at www.eyecarebusiness.com.