ASK THE LABS
By Joseph L. Bruneni

AR and Photochromics
Q I have heard that anti-reflective coatings should not be used on photochromic lenses. Is this true, and does it apply to all photo-chromics, glass and plastic?

A The original concern with applying AR coating to photochromic lenses came from the fact that AR coatings blocked a certain amount of UV light, and UV is what activates the darkening process. This is not a concern if the AR is on the backside only.

Currently, Transitions has qualified the AR coatings from several hundred coating labs that do not affect Transitions lenses. Both Rodenstock (ColorMatic Extra) and Corning (SunSensors) claim that AR can be safely applied to their photochromic lenses, which use a different photochromic technology. AR on photochromic glass is not a problem.

Keep one issue in mind, however. As with polarized lenses, applying AR on both surfaces of photochromic lenses means they will not darken as much because of increased light transmission. The decrease in darkening outdoors is about eight percent. On the other hand, the lenses will transmit eight percent more light indoors in the lightened stage. Determine what the patient's primary concerns are with their new photo-chromics before deciding whether to recommend AR.

With a full AR coating, photochromic lenses will have less backside reflections when dark and will certainly be more efficient for night driving. Some of the photochromic manufacturers claim a longer active life for their photochromic lenses when they are AR coated.

Only the Referee is Neutral
Q A lecturer I heard recently suggested using "neutral" polarized lenses for computer users. Can you tell me what neutral polarized lenses are?

-Confused in Alabama

A This requires a little background about polarized lenses. It is physically impossible to create a clear polarized lens. Generally speaking, the darker the lens, the more efficient the polarizing effect will be. The Gray C polarized lens absorbs 85 percent of the light and has 99 percent effective polarization. The Gray A lens is lighter and absorbs 59 percent of the light with 95 percent polarization. Neutral Gray was created specifically for indoor use. It absorbs 44 percent of the light and, because of the nature of polarizing film, has 50 percent effective polarization. A similar lens is available in Neutral Brown. These are excellent lenses for computer users because they reduce reflected glare. Neutral lenses should not be dispensed for outdoor use.

-Judy Miyasaki, KBco

More on Off-Axis Poly
Q Your recent column explained why edging polycarbonate lenses sometimes causes them to end up off axis when using small chucks. However, my edger does not have an axis adjustment. Is there anything else I can do to avoid these off-axis lenses?

-Diane Samuels, Kivlin Eye Clinic, Menomonie, Wisc.

A I have tried several different ways to solve the problem. I think your best option to resolve this problem is to multi-cut the lens down to size in 10mm increments. This will prevent some of the torque on the lens drive shaft during the cutting process. A more expensive, but perhaps necessary, solution is to replace the roughing wheel. In many cases, if the rough wheel is not cutting efficiently, it can cause undue torque on the system. Obviously, using the large chuck whenever possible and a good blocking tape are a given. On certain edgers, however, the issue cannot be corrected without two separate axis adjustments, one for plastic and one for poly.

-Keith Kerslake, Optic Owl