Below is a review of several common lens coatings and treatments. This section will clarify these terms and explain what they do, how they work, how they’re made, and the benefits and drawbacks to each.
Antireflective Lens Coatings
Antireflective lens coatings (also known as simply AR or anti-glare or non-glare coatings) consist of a series of thin, hard layers of metallic oxide that are applied to a finished lens after it is surfaced for a certain prescription. This process involves meticulously cleaning a lens and then putting it is a vacuum chamber where the substance is applied, allowed to cured and then repeated to form a series individual layers on the lens1. This very hard coating is naturally scratch resistant and additional scratch coatings are not necessary. The better coatings have more layers and are applied to both sides of the lens.
Reflections off lenses are common, regular plastic lenses reflect about 8% of the incoming light and Polycarbonate and High Index lenses are even worse at about 12%1. These lens coatings work by preventing this light from bouncing away. As light contacts any clear surface (like a lens or a window), most of the light passes right through however some percentage of the light is reflected back from that clear surface. These lens coatings are designed and applied in a way that the light that would have been reflected off the lens gets caught within the individual layers of the AR lens coating and either eliminate the reflection or force the light to continue through the lens2. This seemingly simple task helps the wearer of glasses with this lens coatings a number of different ways:
- Most simply, if more light passes through a lens with AR lens coating, then more light will enter your eye and more light equals better vision. Just like trying to wear sunglasses in the dark, if the light doesn’t get through the lens the quality of the vision will not be a good.
- By eliminating light that would have reflected off the lens, the wearer will notice less reflections and eye strain during visual tasks where glare is common like driving at night or with computer, cell phone, or tablet use.
- Higher quality AR lens coatings also have this coating on the backsides of lenses. This prevents light (commonly from overhead fluorescent lighting) from bouncing off from the backside and entering the eye which also causes eyestrain and glare.
- Finally, that 8-12% of light that bounces off the prescription lenses is also seen others. By reducing this reflection you will also increase the cosmetic appearance of your eyes and glasses.
There are some downsides and considerations with anti-reflective lens coatings. Most important to note here is that an anti-reflective coating is inherently hydrophilic (attracts water) which means that fingerprints, water spots, and the natural “oils” from your skin and lashes will more readily smudge the lens and stick to the lens coating making it more difficult to clean and keep clean. Additionally, the lack of reflections from the lens will make a smudge look more noticeable. More advanced coatings like the options used at our office apply a final hydrophobic coat to the AR lens coating to aid in cleaning and some options even add an oleophobic top coat that repels oils and make smudges easier to clean1. As noted prior, AR lens coatings are naturally scratch resistant, but that does not make them scratch proof! The more advanced coatings use many more layers of anti-reflective film which helps with scratch prevention. Nonetheless, it is important for everyone with anti-reflective lens coatings to clean their lenses with only a microfiber cloth or a clean cotton rag (like a dish towel). Any product made out of wood pulp including tissues, paper towels, napkins, or toilet paper ARE GUARANTEED to scratch AR lens coatings over time.
OUR RECOMMENDATION:
Even with some downsides, at Precision Family Eye Care we view these lens coatings as essential to comfortable vision and visually pleasing glasses. We only use the best, most technologically advanced AR lens coatings available and are so confident in our product that we stand behind every pair of our anti-reflective coated lenses with our Bumper-to-Bumper 2 Year Warranty.
Scratch-Resistant Lens Coating
Despite all the advancements in the glasses technology, no lenses (even glass lenses) are immune to getting scratched over time. Scratch-Resistant Lens Coating is a very hard coating that is applied to both the front and back of a lens with a spray gun or through a dipping process. As it cures, this lens coating forms a permanent bond with the surface of the lens3. This lens coating is a near necessity for prone to scratch Plastic lenses (CR-39). Other more resilient materials including Polycarbonate, High Index, and Trivex® are naturally scratch resistant4 but will also benefit with this coating to improve their performance over time.
It is also important to note that scratches CANNOT be removed or buffed out of lenses once they occur. It is the curvature of the front and back of the lenses that determine what prescription they are and thus how well you see the world. Any attempts to buff or resurface out the scratches on lenses will change this curvature as well as the prescription and lead to distortions in the lens and blurred vision.
Ultraviolet (UV) Lens Coating
Ultraviolet or UV protection on glasses is very important. Just like wearing sunscreen on your skin, your eyes and the tissues around it need protection from the sun’s harmful raysl. Having your eyes continually exposed to UV light can increase your chances of developing skin cancer of the eye lids, pinguecula/pterygium (growths on the white of the eye), cataracts, and macular degeneration. Trivex®, polycarbonate, and high index lenses are naturally UV protective but having dual sided UV coating is often beneficial. Plastic lenses (CR-39), however does not block enough of this harmful light and UV protection for plastic lenses is a necessity4.
Photochromic or Photochromatic Lenses (Transitions Lenses)
Photochromic lenses is the correct technical term for lenses that react to ultraviolet light to darken into sunglasses outdoors and then return to clear lenses inside. However, because of the intense popularity of the Transitions® brand in the 1990’s, this lens type is often simply referred to as Transitions; similar to the generic use of band-aid, kleenex, or xerox. The first widely available, mass produced variable tint lens was actually developed by the Corning Glass Works company in the 1960’s5 which still produces the very popular PhotoGray and PhotoBrown photochromic lenses.
Photochromic lenses are 100% UV protected even in their clear state and are available in brown or grey colors. They can also be used in conjunction with any lens coatings, materials, or bifocal designs. The photochromic tint is not a lens coating that can be scratched or rubbed away. Instead, Photochromic Lenses are given a chemical bath where either silver halide or silver chloride molecules become imbedded within the lens itself to a depth of about 150 microns or about the thickness of two human hairs stacked. These molecules are naturally clear under normal, visible lighting conditions. However, when exposed to ultraviolet light from direct sunlight, the molecules absorb the UV light which causes them to change their shape and the molecules new “excited state” absorbs light in the visible spectrum causing the lens to darken. The more ultraviolet light (ie the brighter the sunlight), the more of these tiny molecules take up the UV light and the darker the lens gets! When you go back inside, the molecules no longer have the UV light needed to keep them in their “excited state” and the molecules snap back and the lens gets clear again6. The chemical reaction occurs quickly in both directions and allows a photochromic lens to go from clear to dark to clear again in only a matter of minutes. Who said science can’t be fun!!
The drawbacks of Photochromic lenses are minimal but there are two things to keep in mind before purchasing these lenses. First, as a group, Photochromic lenses will not darken as deeply as a typical pair of dark sunglasses and may not give enough relief to a very light sensitive patient. Secondly, as explained above, Photochromic lenses require intense UV light in order to darken and all vehicle window glass blocks UV light, therefore (aside from some newer products) Photochromic Lenses are not helpful from sun protection while driving.
OUR RECOMMENDATION:
At Precision Family Eye Care, we think Photochromic Lenses are just a terrific choice for both children and adults who need sunglasses and appreciate the freedom of spending time outside playing, gardening, golfing, or just reading a good book without the hassle of using clip on sunglasses or the expense of a separate pair of prescription sunglasses.
Solid Tint Lenses
Unlike Photochromic Lenses, solid tint lens coatings remain tinted at all times. Tints of virtually any color or darkness can be applied to eyeglass lenses. Lighter fashionable tints are often used primarily for cosmetic purposes while dark tinting can turn any pair of glasses into sunglasses. Additionally the color can be added as a solid tint where the darkness and color are consistent throughout, or as a gradient tint where the color darkness is greatest at the top of the lens and then gradually fades toward the bottom7. There are plenty of options here for customization.
Polarized Lenses
Polarization in glasses first became available in the 1936 when Edwin H. Land started incorporating his patented Polaroid filter into lenses8. Polarized lenses are essentially just that, a filter for light. Commonly used on sunglasses, these filter lens coatings are made by applying a chemical evenly to the lenses that naturally aligns in a parallel horizontal rows – picture horizontal slat blinds on a window. When light bounces from a reflective surface, like a lake or the hood of a car, it becomes polarized and the reflected glare and light is orientated in a particular, consistent pattern. It is this light that the horizontal rows of the polarized filter block and prevent it from reaching your eyes10. This vastly decreases problems from sunlight glare and decreases light sensitivity. If you have never tried a good pair of polarized sunglasses, you owe your eyes a treat!
References:
1. http://www.allaboutvision.com/lenses/anti-reflective.htm
2. http://en.wikipedia.org/wiki/Anti-reflective_coating
3. http://en.wikipedia.org/wiki/Anti-scratch_coating
4. http://www.allaboutvision.com/lenses/coatings.htm
5. http://en.wikipedia.org/wiki/Photochromic_lens
6. http://science.howstuffworks.com/innovation/science-questions/question412.htm
7. http://www.allaboutvision.com/lenses/photochromic.htm
8. http://en.wikipedia.org/wiki/Sunglasses
9. http://science.howstuffworks.com/innovation/everyday-innovations/sunglass6.htm
10. http://www.wisegeek.com/what-are-polarized-lenses.htm