Reflections On Polarized Lenses |
|
|
Reflections On Polarized Lenses
 - by Vicki Masliah
"Reflections On Polarized Lenses"
As lens performance becomes a priority for practitioners and patients, polarized lenses are gaining a foothold as the sunglass lens of choice. Members of the Sunglass Association of America named polarization as the primary characteristic they expected to impact the sales of sunglasses.
Available for over sixty years, polarized lenses were predominantly offered to water and snow sport enthusiasts. Although the overall percentage of polarized lenses in; Rx sunwear is relatively small, the market is growing fast. That's because polarization is developing a reputation of being the premium feature to incorporate into outdoor lenses. The properties of elimination of glare, enhancement of color, and accentuation of contrast and acuity have make polarized lenses appealing to anyone seeking the most comfortable and effective sun and driving glasses. In the lighter shades, they are also proving to be helpful indoors for computer users or generally light sensitive patients.
As with all premium products, understanding how polarization works will assist the practitioner in educating the consumer. Light waves travel in every direction. When the light hits a surface, a reflection results, the reflected waves vibrating in a particular direction, usually horizontally. This reflected light is called glare. Reflective glare significantly reduces visual acuity and depth perception. It will also color distort and cause temporary blindness and squinting. While conventional sunglasses only reduce the amount of light transmission, polarization is the only way to eliminate the glare. Polarized lenses absorb light waves traveling in any direction other than the vertical plane. The transmitted light travels in one direction, hence the glare is reduced or eliminated.
To achieve polarization, iodine crystals are imbedded in a special film. The film is stretched in one direction, aligning the crystals in the same direction. This uniform direction creates the polarization. The film is then dyed. The color is, as in any lens critical to the absorption. A polarized lens without color would allow all the light to pass through, defeating the purpose entirely. The amount of polarization, and so its performance, increases with the density of the color of the film. Tinting a lens with a light color film does not increase the polarity, it simply makes the lens darker. Lenses are being produced with a large variety of colors and densities. Comfort and absorption requirements can be addressed with several general sunglass hues, as well as colors designed for specific types of fishing, boating, day and night driving, and hunting. No matter what the activity, there is an appropriate polarized color, with inherent UV absorption as well, available.
After the film is tinted, it is shaped to be incorporated into a lens, made from one of several choices of materials. The manufacturing methods have so greatly improved over the years, that the fear of lenses "separating" should finally be put aside. When the resin lenses are made, the film is suspended in the mold, and held in place by a gasket. After the mold is filled, the lenses are cured for as many as forty-eight hours, resulting in a solid, one piece, construction. Glass lenses are still produced by gluing the film between two layers of glass, but the techniques are far enhanced over the original polarized glass. In either case, glass or resin, consider the complexity in attaining purity of image, More than just the front and back surfaces of the lens must be perfect. There are the back surface of the lens portion in front of the film , both the front and back surfaces of the film itself, and the front surface of the material behind the film. All these areas must remain immaculate and distortion-free throughout the processes. This is one of the most difficult ophthalmic lenses to produce. In all cases, the modern production methods are offering a precise, sturdy, and versatile product, most often warranted against separation and scratching, and guaranteeing patient comfort.
Among the materials there are styles available in single vision - spherical and aspheric, bifocals, trifocals, and progressives. Labs are also now able to grind some of the lenses thinner than previously required. The placement of the film closer to the front surface reduces the possibility of grinding through the back to the film layer. In some cases, a thin point slightly over one millimeter can be achieved..
Product availability for prescription has been greatly expanded. Semi-finished polarized lenses are offered in glass, photochromic glass, mirrored glass, iridescent glass, plastic, high-index plastic, photochromic plastic, and polycarbonate. An entirely new market has been opened by the important addition of "Rx-able" polycarbonate. Combining the benefits of the other resin polarized products - a selection of absorptions and the ability to tint and anti-reflective coat, polarized polycarbonate also guarantees the safety factor so greatly required in sport eyewear.
It has become almost second nature to suggest progressives to presbyopes - patients are aware of "bifocals without lines." The same goes for high-index materials for higher prescriptions - the wearers know their lenses can be thinner. It can surely be agreed that if a person needs prescription glasses, they also need prescription sun, sport, or driving glasses. If a tint and UV absorption are good, then polarized is better. As is the case with every premium product, patient awareness is of key importance in successful dispensing of polarized lenses.
When discussing sunwear, explain the facts of scattered light rays and reflective glare. Emphasize that only polarized lenses can effectively control the light rays and eliminate the discomfort caused by glare. Liken the polarizing action to a vertical venetian blind. Only the light traveling in the same direction and the vertical panels are permitted to pass through. It should be noted that polarization should be worn by almost everyone, not merely boaters and skiers. Reflective glare is everywhere.
No matter how much explanation is offered, an image is worth a thousand words. Keep samples of several colors of polarized lenses at hand to demonstrate how they work. Patients are always impressed when two lenses are placed in front of their eye at crossed axes, allowing no light to pass through on either the horizontal or vertical planes. When one lens is rotated so the pair is parallel, the polarized rays are transmitted. Another great illustration is to point out a reflecting surface to the patient (counter-top, shiny linoleum, car hood). Hold a polarized lens ninety degrees off axis in front of their eye. The reflected light will be visible. Turn the lens on axis to eliminate the reflection. Many manufacturers have produced terrific demonstration displays. The counter cards hide an image that can only be seen when polarized lenses are worn. Some of the displays include a plano lorgnette, or use a plano sunglass in your inventory. Patients are amazed, and the lenses frequently sell themselves.
There are a select few patients to whom, it should be warned, polarized lenses should not be offered. These are people who need to see the reflection - pilots who land on or near water, professional skiers who, when moving at great speeds, need to spot ice patches, or mountain climbers, also to whom ice can cause danger. Also include speed bicyclists that require seeing wet or oily spots on the road. To these patient suggest a mirror coating that will eliminate some, but not all of the reflection.
When ordering lenses from the surfacing lab, do not expect them to be able to grind ultra-thin centers. When considering the thinnest point, recall that the polarizing film is, at least, almost one millimeter behind the front surface. Enough material must remain behind the film to surface the Rx. Surfacing a lens too thin will grind into the film. The lenses are no longer required to be excessively thick, but they cannot be one millimeter at the thin point. Thickness can also be reduced with the use of 1.57 index, polycarbonate, and aspheric polarized products.
Most frames work successfully with polarized lenses. Avoid eyewires with more acute angles that can place stress on the lens. If the polarizing film stresses, a rainbow effect will appear at that point.
There really are no special edging tricks. The way the lenses are now manufactured has removed the "separation anxiety." It is critical, though to note the orientation of the axis of the polarization. This is usually indicated by a notch in the lens or the film. Be sure the coolant in the edger is clean and flowing properly so as not to jeopardize the edge of the film. Do not edge the lenses too large, as forcing them into the frame will again place stress on the polarization. If the lenses require grooving, consider the placement of the film. Placing the groove too close could damage it. For the same reason, it is not recommended to use polarized lenses in drilled rimless or faceted frames.
Polarized lenses are made in a large variety of colors and densities. Grays, browns, ambers, yellows, blues, roses - each pair dispensed can answer very specific performance needs. Many plastic styles are produced in lighter colors, allowing them to be tinted to even more specialized hues. Remember, as mentioned, tinting the lenses does not increase the polarization amounts. The lenses are inherently UV absorptive, so the only tint treatment that might be done is the change of hue. If the dye temperature is at or over two hundred and five degrees Fahrenheit, do not leave the lenses in the vat for more than fifteen minutes at a time. If the temperature is cooler, use the normal manner of tinting. Under no circumstances use acetone to clean the edged lenses. If it contacts the film, the acetone will assuredly cause the film to break down. *Most polarized plastic products are compatible with anti-reflective coatings. It is suggested that since dark lenses cause back reflections, A-R coatings are most effective when applied as a backside coating.
It is no longer appropriate to predetermine the lack of willingness on the patient's part to purchase a premium performance sunglass lens. If they were willing to enhance their indoor lenses, it should be expected that they would want to do the same for their outdoor wear. Current patients aren't the only ones who should be introduced to polarized lenses. Previous patients who purchased premium lenses should be contacted by mail or phone about the new availability of their lens style in a polarized version. If the practice is not computerized to reference patients by product, check with the lab. Their systems, like the one at Hirsch Optical, should be able to supply patient names and lens styles ordered. Don't let the contact lens patients out the door either. There is no reason to let them shop the drug, department, or sunglass stores. Make sure the plano patients understand that eyecare professionals can provide ophthalmic quality non-prescription lenses.
Knowledge of the great strides being made in the quality, versatility, and ease of fabrication of polarized lenses should instill in the practitioner a new found security in dispensing the products.
|
More Ideas: |
|
|
|
|
