As anti-reflective lenses continue to increase in popularity in the U.S., more of the lenses are edged, drilled or hand polished in in-office labs than ever before. Yet along with the increased use is the increased risk that these lenses— which are often premium-priced—may be accidentally ruined during the finishing process. AR lenses, if improperly handled, are prone to scratching, cracking, crazing or other pitfalls. Here’s some practical advice about how to make sure your AR lenses safely cross “the finish line.”
—Andrew KarpWhen edging and mounting an AR coated lens, the lab technician is actually finishing two products—the substrate and the coating. AR coatings are a series of micro-thin layers (often referred to as a “stack”) of lightrefracting chemical compounds. Typical layers are zirconium-dioxide, titanium-dioxide, aluminum-oxide and silica dioxide (also known as quartz).
Heat and pressure can damage both the coating and, to a lesser degree, the substrate. Dispensers have to look at every finishing step—from edging to mounting—to upgrade their lab because what won’t affect an uncoated lens may damage a coated one.
The most common type of finishing-related damage is crazing. Crazing, which can be caused by either heat or stress, indicates a breakdown of the AR stack. If the substrate is unaffected, the coating can be stripped off and the lens re-coated by a coating lab. Otherwise, that lens will become part of the finishing lab’s spoilage rate.
Essentially, upgrading a finishing lab has more to do with proper protocols than specialized equipment. Handle the lenses carefully and as little as possible. Hold them by the edges and don’t wipe them with a dirty cloth. Avoid getting fingerprints on the lenses.
Cleanliness is key. Grit and excess lens debris in the lab area, especially around the edger, may scratch the lens and clog cutting wheels and spouts, resulting in excess heat. If you’ve edged a glass lens or a large number of other substrates, it’s a good idea to clean the edger before edging an AR lens.
than to warp a lens, some experienced opticians recommend increasing the water flow in your edger to keep the process cool. Consider attaching an extra valve to your edger’s water line.
AR coating will also craze from pressure. The chemical compounds used in an AR stack are glass-like and comparatively brittle in contrast to substrates such as plastic or polycarbonate. The substrate may have some flex. Relatively speaking, AR has none.
Before blocking the lens for finishing, lab techs recommend using surface-saver tape, now available in smaller sizes specifically for finishing. In addition to being a prudent scratch prevention measure, it keeps the lens from slipping and tends to ease the pressure of the chuck. Avoid keeping the chuck pressure too high, especially for myopic prescriptions in high-index materials, since these lenses will flex if the pressure is too high and the AR coating can craze.
Technicians should check with their AR supplier, as well as their edger manufacturer, for specific chuck pressure protocols.
Blocking and de-blocking should be ARspecific. Don’t leave an AR lens blocked for any extended period of time. Unlike uncoated lenses, AR lenses will craze if left blocked overnight. Also, when de-blocking the lens, submerse it in warm water when removing the block and leap pads. If you peel them off, you can take the coating with them.
Beyond the chuck pressure issue, patternless edgers have other advantages over older machines. Patternless systems tend to cut cooler, generating less heat. They also reduce handwork. Depending on the accuracy of the tracer, lenses require only one “cut”— and the polishing cycles eliminate the need for hand polishing.
Once edging is complete, lab techs must be sure handstones are clean.
Techs have to hold the lens steady so only the edge makes contact with the stone. The edge should not be pressed too hard or too long against it. Lab techs call this method “touch and kiss.”
Hand polishing, one of the most risk-free steps for uncoated lenses, contains potential spoilage pitfalls for AR coated lenses. To avoid problems, apply surface-saver tape while hand-polishing an AR lenses. If you slip or touch the buffing wheel to the lens surface, you can get brush-burns, which can’t be polished out on an AR lens. It’s also critical to check that the polish is AR safe. Some polishes will break down coatings. Avoid using acetone as a lens cleaner, which will break down certain AR coatings. One rule of thumb: Avoid anything ammonia-based.
Mounting, the final finishing step, also contains numerous hazards for AR. With nylon rimless and three-piece mountings, the desired effect is to have the eyewear “disappear” on the wearer’s face. By dramatically reducing glare on the front surface of the lens, AR coating completes the look. But these frames require more post-edging handwork, thus increasing spoilage risks. Using an automatic groover is relatively safe but the water has to be changed more frequently when etching the edge of an AR coated lens.
Debris and particles that wouldn’t affect an uncoated lenses can scratch the lens and even produce craze-inducing heat. Make sure that water is clean and cool or it could corrupt the coating.
Drilling the holes for three-piece mountings can also stress the coating. One way to avoid this is to fashion a make-shift rubber gasket into which the coated lens can be set when drilling. Drill the hole slowly and steadily at a straight angle while holding the lens firmly to prevent it from slipping. Don’t put too much pressure when using the template for drilling the holes. Often there’s a small spider web effect around the drilled hole, indicating tiny cracks in the lens. High-plus lenses are particularly susceptible because of extremely thin edges. The phenomena is a result of imperfect drilling—the hole being made too quickly or at an angle—but the marks are often so minute that they can be covered up by the nose and ear pieces. If the lens is cracked, however, the coating will also have stress-induced crazing.
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When tinting lenses to be AR coated, dye the lenses 10 to 15 percent darker than the final desired shade. Then bleach them back to about 5 percent darker than the final color. This better protects the integrity of the color during the coating.
When edging AR lenses, use protective tape or spray. Do not leave leap pads on overnight. Keep chucking pads and edgers as free from debris as possible. Lower the chucking impact and reduce constant pressure, not to exceed 60 lbs. P.S.I. Do not use an ultrasonic cleaner on AR lenses.
Use mild-non-lotion-based soap and lukewarm (never hot) water to clean AR coated lenses. Better yet, use a lens cleaner specifically formulated for AR lenses. Wipe the lenses with a clean, soft, lint-free cloth; your best bet is to use a special AR cleaning cloth. It’s okay to wash these cloths, but don’t use fabric softener as it can interfere with the cloth’s ability to clean the lenses. Never wipe a dry AR coated lens unless using one of these cloths. Recommend to your patients they clean their AR lenses once a day.
Edge AR coated lenses to exact size in order to avoid excess pressure when mounting them in a frame. Also steer clear of pressing at the center of an AR coated lens when mounting. Excess pressure on an AR coated lens may cause coating delamination, cracking or crazing that can compromise AR longevity.
Use an air blower—not a salt or bead pan— to adjust eyewear that sports AR coated lenses. Avoid excessive heat on AR lenses in general, even in ultrasonic cleaners, as high heat can damage AR coatings.