Sponsored by IOT
By Tina Lahti, ABOC
It’s important that opticians know the best material for the job especially for groove and drill mount frames or high lens powers. We have a lot of material choices, and we must determine which combination of lens properties will produce the best lens performance for each job. For example, what are the most impact resistant and strongest lens materials? Strongest, in terms of tensile strength, means the ability of a lens to withstand a pulling force. Lenses with high tensile strength are good for drill mounts. High tensile strength lenses include Trivex® (also sold under the trade names Trilogy® and Phoenix), 1.60 (MR-8 and MR-8+), and if you want a high-end excellence, 1.70 lenses also have high tensile strength. Always ask your laboratory what materials they use for drill mounts rather than ordering solely based on refractive index like a 1.60 or 1.67. For example, how do 1.60 Acrylic lenses compare to 1.60 MR-8? Acrylic lenses have a low Abbe value of 32, compared to 1.60 MR-8 which has an Abbe value of 41. They also have poor impact resistance compared to 1.60 MR-8, which have very good impact resistance.
The most impact resistant lens materials are polycarbonate and Trivex. But understand the tradeoffs with these materials. Polycarbonate is not resistant to chemicals, and it has a lower Abbe value which can reduce peripheral optics in stronger prescriptions. Trivex has a higher Abbe but a lower refractive index, so it is slightly thicker in stronger prescriptions. Trivex is more resistant to chemicals than polycarbonate. One important thing to note when choosing between these two materials for impact resistance is that the application of a vacuum deposition coating like an AR or a mirror can greatly reduce the impact resistance of a Trivex lens.
Now let’s talk about lens tint. Most materials have good tintability except for 1.74 and Trivex, which rate “OK,” and glass and polycarbonate that are not tintable. Laboratories that tint polycarbonate lenses carry special blanks with tintable hard coatings, and sometimes they use a tintable primer coat which also absorbs a small amount of dye. To avoid disappointment when you receive the tinted lens from the lab, give them a sample of the color to be matched and the sunglass tint category desired. Otherwise, the lab will get as close to the standard color and sun lens category specified. If you add an anti-reflective coating over the tint, it will change the color of the lens. Also, attempting to tint polarized or photochromic lenses can damage the lens. Tinting is another area where you should always consult with your laboratory and ask them what they recommend. The goal of a tint is to reduce brightness by reducing visible light transmission outdoors. There are levels of tint.
Finally, let’s talk about lens coating. Hard coats are applied to the lens to increase durability, abrasion and scratch resistance, and to create a good base for AR coating adhesion. The best hard coatings are index matched to the lens index to reduce birefringence which looks like an oil slick on the lens. As opticians we know that AR coatings selectively cancel specific wavelengths to minimize reflections. While this has the cosmetic benefit of removing the glare from the lens that obscures the wearer’s eyes from view and making the lenses appear clearer, it also improves night vision. Without AR coating, internal lens reflections from cars and streetlights result in glare and halos or ghosting around lights from internal lens reflections. I would be remiss if I didn’t address the gorilla in the room—blue light filter coatings.
To touch briefly on the topic, there are lenses and coatings with claims about protecting the eye from blue light or increasing wearer comfort in certain conditions, especially night driving or the use of digital devices. If these products make wearers more comfortable, I am all for them. But I am always concerned when I hear and see product claims that are very different from what I know the product is doing. Know this, a blue filter coating is not blocking all blue light. It is only filtering or reducing transmission of certain blue wavelengths. Do not oversell these coatings as blue light protection. With sunlight exposure as the predominant source of harmful blue light, it is sunglasses and photochromic lenses that you should be recommending. Blue filter coating on clear lenses cannot provide this level of protection. In the next article, we will speak more in depth about blue light and blue light filtering lenses.