Aug
2005

Lens Choices

Whether metallized or dielectric, reflective lenses combine high-style and high-performance
By Andrew Karp

Mirror lenses have changed considerably in recent years as technology has made it possible to create unlimited mirror colors. Today’s mirrors offer a broad range of possibilities for both prescription and non-prescription sunglass wearers, whether you’re talking about color, intensity or translucence. In addition, mirrors offer the added dimension of optical performance, making them more than a fashion accessory. There are two fundamentally different ways to create a mirror: metallized or dielectric. Of course, all mirrors are reflective, but absorption is what differentiates the two. Metallized coatings are the ones that absorb light. They were the first type used for mirrored sunglass lenses and most closely match the traditional definition of a mirror. Dielectric coating is a newer process developed initially for the space program. It creates a mirror that reflects light without absorption. Although both processes begin with high temperature evaporation of either metal or oxide, the end products are unique, resulting in very different looks and performance characteristics. Metallized coatings deposit a layer of metal directly on the front surface of the lens, creating the optical equivalent of a one-way mirror. To allow the wearer to see through it, the layer of metal must be kept very thin. In turn, the thinness of the coating makes it susceptible to scratching and tarnishing. This problem is addressed by adding an overcoat to protect the surface and improve durability.


  Full Color Spectrum
Color in a mirror is a byproduct of the manufacturing process. Manufacturers have a choice of techniques for producing colored metallized mirrors. Each metal has an inherent coloration that is transferred to the lens. For example, the use of aluminum or chromium will create an icy-silver hue. Tint can also be added to the overcoat. Some metals allow you to create colors by varying the thickness of the coating itself, typically going from gold to blue to red as the thickness increases. Metallized coatings offer fashion versatility because they can be applied as flash mirrors with only a blush of reflectance, as full mirrors that mask the eyes or as gradient mirrors, which are particularly favored for ski and water sports because they provide extra reflectivity above and below the line of sight where glare is strongest. Because these coatings not only reflect but also absorb light, they decrease the transmittance of a lens and make them a bit darker. Dielectric coatings are the magicians of mirrors. As colorful as they are, the coating itself is clear, made from layers of oxides like titanium dioxide or silicon dioxide rather than metal. The colors you see are reflections of wavelengths of the visible spectrum bouncing back at you. Because they reflect but do not absorb light, lenses with dielectric coatings keep the wearer’s field of vision bright.



  Dielectric Technology

Unlike metalized mirrors that reflect all wavelengths in the visible spectrum uniformly, dielectric coatings use peak reflectivity to selectively block or filter specific wavelengths. The peak reflectance of the top layer determines the color of the lens. It takes a minimum of five layers to create a dielectric mirror, although most designs specify 20 or more. Each layer has its own peak reflectivity with a part of the spectrum. Manufacturers take advantage of this characteristic to create different looks. One option produces an overall color that appears the same from all angles, another produces a lens that changes color when viewed from different directions. These lenses can seem multi-dimensional, with vibrant cherry red, sapphire, emerald or sunburst yellow hues that appear almost opaque. The color range is nearly infinite. Technology has significantly improved the performance of mirrors in both durability and abrasion resistance. The challenge is greater with metallized coatings because these metals have to be thin enough to allow the wearer to see through them, making them inherently susceptible to peeling and scratching. The soft surface of a plastic lens compounds the problem. Premium sunwear with mirrored plastic lenses go through several manufacturing steps, each designed to increase the durability of the coating. A hard coat on the base lens establishes a surface that is more scratch resistant. It also provides a stronger bond between the lens and the mirror, which makes the chance of peeling almost non-existent. Once the mirror has been applied, an oxide barrier coat can provide additional abrasion resistance to the surface.

  Optimizing Performance

Another alternative uses an ion gun to hit the metals or oxides during the application process, making them denser and improving their abrasion resistance. Add a hydrophobic coating that sheds water, and you also improve the lens’s ability to shed smudges and grime. Because a hydrophobic coat makes the front surface slippery, it also helps with scratch resistance. Interestingly, these technologies also benefit some types of glass lenses, as well. Metal mirrors on glass lenses perform better than on plastic because glass can be heated before the mirror is applied, improving adhesion and forming a much stronger bond. A plastic lens pre-heated in the same way would melt. Yet for some glass lenses, like laminated polarizers or photochromics, a high-heat treatment can be detrimental. High temperatures can warp the plastic polarizing layer in a laminate or diminish photochromic performance. New technologies like those previously mentioned will enable lens coaters to engineer products that are much more durable, regardless of base lens material. Although technology is an important aspect of mirror coatings, fashion is still the driving force. Your patients want products that look good and feel comfortable. If a mirror is part of the look they’re after, they may be surprised to learn the option is available as a prescription add-on. Popular premium sunglass brands have given specialized mirror coatings a solid share of the plano market, but your patients may not understand they have the same high-performance choices in prescription sunwear as well. Don’t miss the opportunity to offer them a hot trend in cool shades.

L&T Basics: tip for dispensing mirror lenses
Mirror lenses are good in situations that require a reduction in the overall brilliance of light, such as sunlight on snow. Mirrors have become a significant category, largely due to prominent use by sports optics manufacturers. The look defines the product, with fashion taking the lead over performance benefits. Yet the more a customer knows about the features and benefits they receive, the greater their satisfaction with the purchase. The patient generally knows that higher priced sunwear means better quality, but may not understand the specific performance points associated with mirror coatings. The following information can guide your patient through the decision process:
  • Mirror coatings can customize prescription sunwear to match the patient’s lifestyle. If used primarily for driving or outdoor reading, a top gradient mirror helps reflect the intensity of the sun above while leaving a slightly lighter area below for better visibility of the dashboard or book. A double-gradient metallized coating works well for water or snow sports. Consider a dielectric coating on a polarizing lens for fishing or sailing.
  • Dielectric coatings that reflect infrared keep the lens a bit cooler. Patients who tend to have dry eyes may find them more comfortable.
  • Dielectric coatings that selectively filter blue light tend to improve contrast and visual acuity. Optically, these lenses offer the wearer better visual performance.
  • All mirrors reflect light away from the eye. For light-sensitive patients, this is a benefit beyond the standard absorption of a tinted lens.
  • UV protection is not necessarily included in the coating. When in doubt, check with the coating supplier who can provide specifics or select a base lens that already provides UV protection.
  • Long term durability and abrasion resistance can be improved with the addition of a hard coat and hydrophobic coating. Patients who will be using their sunwear frequently should consider these add-ons.
  • Back reflectance will be more noticeable with a mirror, especially on darker lenses. An AR coating on the back surface is strongly recommended.
  • Take advantage of the educational literature that your coating supplier provides and make sure it’s conveniently available in the waiting room. An educated patient will be more receptive to your recommendations.

 

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