Good form

What are the basic steps in the laboratory process for

direct surfacing?

We process the direct surfacing design on the back surface of the lens. The customized lens has to be cut and polished using specific technology because the Rx is surfaced using exact curves needed and not the next closest curve to match the lap tool.

Once the lens has been processed, it can be verified using a lensometer and compensated value Rx sheet. The compensated value takes into account all the data that the Rx process program was given. Only the compensated value should be verified by the lensometer.

� Keith M. Cross, vice president, Northeast Lens Corporation, Newton Upper Falls, Mass.

Poly practices

How can I avoid the top three polycarbonate lens processing problems: birefringence, spider cracks around drill holes/lens edges, and chromatic aberration?

Chromatic aberration is a result of the dispersive power of the material, not a direct result from surfacing or edging.

The other two problems can be reduced in the lab. Use a dry-cut blade edger and make sure the blades are sharp. This reduces cracks around the edges of lenses.

For drilling holes, use a sharp blade and correct drill speed, and contact the drill manufacturer for guidelines. Then measure lens stress, with a polarascope, which helps you check to see if there is excessive unwanted stress in finished and mounted lenses.

If stress exhibits, the lens size may need to be slightly reduced or the drill holes modified. Stress in a poly lens may reveal itself in days, weeks or even hours later in the form of small cracks.

A little bit of extra time spent processing and verifying poly lenses will save time later on and will reduce spoilage and increase patient satisfaction.

� Donn McCarthy, practice development specialist, Polycore Optical USA

Tinting Tips

Can you recommend the best way to tint polarized lenses darker?

Good tinting practices would be to leave the lens submerged no more than 10 minutes at a time.

Then, allow the lens to cool back to room temperature between tinting cycles. Always follow the manufacturer�s temperature settings for the material.

Some caution should be exercised to not exceed the standards for color and darkness on plano sunwear. Although there are no color and darkness standards for Rx lenses, common sense should be used with tinting density on Rx lenses for driving, for example.

� Chris Bowers, operations manager, Walman Optical, Minneapolis, Minn.