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Ask the Labs

Segments and Singles

Eyecare Business March 1, 2010
ask the labs

Segments and Singles

Karlen McLean, ABOC, NCLC

Many opticians would say that lens work can be tricky. Here, experts weigh in on lens fitting, identifying, and fabricating.

INVESTIGATING DISPENSER

Q I need to replace one lens in a pair of glasses. I'm unable to use the patient's previous records. How do I ensure the new lens totally matches the existing lens?

A 1. Determine the lens style. Single vision, segmented multi-focal, PAL, etc. Use the OLA Progressive Lens Identifier handbook to determine the brand, and sometimes material, of the lens if it's a progressive addition lens.

2. Determine lens design. Spherical or aspheric. Single vision and bi/trifocals may present a challenge here. You may have to make an assumption about design based on the power of the lens and relative thickness and optics.

3. Determine the material. Listen to the sound of the lens to be replaced when you drop it from around six inches onto a countertop.

A tinny sound likely indicates polycarbonate. Standard plastic will “thud.” If it's a fairly high-powered lens and doesn't sound like poly, it may be a high-index lens. Higher indices also have a tinge of gray when placed on pure white paper. Your lab can help; measure and specify the base curve of the other lens and note it on your lab order.

4. Measure the patient's PD, and fitting height if necessary. Determine the vertical placement of the optical center, relative to the segment line, on a bi/trifocal and specify to match with your lab order.

— Brad Main, training and technical resources, HOYA Vision Care North America

Double Take

In response to the November 2009 Ask the Labs story on double-D bifocals, I'd like to offer fitting techniques that work for me.

■ The B measurement of the frame really determines if a double D will work, and anything less than 34mm makes one or both of the bifocal segments unusable. Often the lower bifocal height must be adjusted to accommodate the upper bifocal, so there is approximately 10mm of usable area.
■ Take the B measurement. Divide it in half. Subtract three (rule of thumb for D seg placement). Add 14 (separation between segs). What's left is the upper seg.

If it's less than 10, adjust the lower seg down.

■ I demonstrate double Ds to patients by using pieces of Scotch tape and placing them on the top and bottom of the lens where the segs would go. Then I have them walk around. This helps stress that double Ds are not for everyday wear, and that even walking can affect their viewing.

Also, be aware that not all combinations of Rx's are available in the upper and lower add areas, so adjustments may have to be made. Especially for safety considerations, seg placement should be a priority.

— Jim Knapp, optician, customer service, AIRGAS Protective Optics

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

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