Q. Vive La Diff�rence
I know there are differences in the technology of some photochromic materials. Can you explain how they differ?

A. The first effective photochromic plastic lenses imbedded photochromic chemicals in the front surface of the lens, using a process called "imbibing." These popular lenses are still produced in this way. A quite different process was later developed called "in mass." In mass lenses have their photochromic chemicals scattered throughout the entire substrate of the lens. Manufacturers using the new process claim a faster and darker reaction to sunlight over a longer period of time. In the meantime, the imbibing technology has been improved so that the entire photochromic field continues to evolve and expand. Glass photochromics have always been "in mass."

Q. Tinting Polarized Polycarbonate
Can polarized polycarbonate lenses be tinted? Mirror coated? What colors are they available in prescription form?

A. Polarized polycarbonate lenses can be tinted, preferably no longer than 15 minutes exposure. On the technical side, we express caution in adding tint to polarized lenses. The polarized lens is designed to meet ANSI transmission requirements and adding tint may affect driving and traffic signal recognition. Slight color modifications are achieved in a few minutes. Most polarized lenses designed for prescription use are currently available in both neutral gray and brown. Specialty colors may also be available in some lens forms. Mirror coatings are an excellent addition to polarized lenses, and are readily available in both current manufacturing inventory, or as a custom type coating.

-Denny Decourcy, technical service manager, Vision-Ease Lens

Author's note: Mirror coatings in solid, gradient, double gradient, and blush mirrors can be applied to polarized lenses on a custom coating basis.

We are having problems with drill mounts with polycarbonate lenses. They seem to turn out fine, with no wiggle in the lens. Six months later the lenses start to split at the screw hole. This seems to happen on different brands, surfaced, and stock lenses. We use a cordless drill to drill right through. Sometimes, I'll use a real fine drill bit, and then will make the hole bigger after I've checked placement. We don't do that consistently, so we can't determine if that contributes to the problem or produces lenses that don't crack.

-Cindy Tallay, Eyeota Optics

A. This common problem is preventable by incorporating some new drilling techniques. The cause of these eventual fractures is excess pressure while drilling, combined with heat damage. Polycarbonate can burn if you drill it too quickly. Avoid cracks by operating the drill at slow speeds of 20 to 60 revolutions per second with light pressure. Slowly drill 0.5mm at a time. Then back out the bit and allow the lens to cool for five seconds. Poly cuts best with a shaped burr. Avoid using twist drills that cut aggressively, often causing damage. Excessive stress with aligned assembled eyewear may also eventually create fractures. Use caution when mounting.

-Martine L. Breheney, educational director, Polycarbonate Lens Council EB

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