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Lens Features
Common Lens Materials: Regular Plastic, Glass, and Polycarbonate
Regular Plastic
- Plastic lenses are now by far the most common type of material used for eyeglass lenses.
- We generally recommend plastic over glass lenses, because they are: a) safer/more impact resistant, and b) lighter/more comfortable.
- The main disadvantage compared to glass lenses, is that are softer and more prone to scratching.
- This disadvantage can be overcome to some degree by proper care of your eyeglasses and the use of scratch resistant coatings.
Glass
- If you decide to use glass for your lenses, then they must be chemically or heat tempered to improve their impact resistance (they will still not be as safe/impact resistant as plastic lenses).
- Glass lenses are usually somewhat thinner than the same power regular plastic lenses.
- It should be noted that even glass lenses can be scratched and proper care of your eyeglasses is still a required.
Polycarbonate
- For situations where you wish the safest and truely impact resistant lenses, then polycarbonate is the most appropriate material.
- This material is also a little lighter than regular plastic, but is also the softest material and most prone to scratching.
- Scratch resistant coatings are needed to reduce surface scratches and help protect regular plastic and polycarbonate lenses lenses from normal day-to-day wear and tear.
- At the Mission Eye Clinic all of our regular plastic and polycarbonate lenses automatically come with double-sided scratch resistant coatings (some places do charge additional fees for this lens feature).
- For additional scratch resistance, premium scratch resistant coatings are now available (Please ask our optometrist or staff for more information).
- Even with regular and premium scratch resistant coatings, plastic and polycarbonate lenses are only scratch "resistant" and not scratch "proof". To help ensure the successful ongoing enjoyment of your new eyeglasses, proper care of your eyeglasses is required.
Anti-Reflection (Anti-Glare) Coatings
An anti-reflection coating is a multi-layered coating on your lenses that reduces the reflections from the front and back surfaces. It is undoubtedly one of the most beneficial lens options for the following reasons:
1) Improved cosmetic appearance. Because of the reduced reflections, your eyes are clearly visible almost as if there were no lenses in your glasses. This can be particularly evident in photographs, and when having face-to-face conversations with family, friends, or business colleagues.
2) Improved visual function: Under glare conditions (such as during night-time driving or often with computer use), reduced reflections result in decreased glare and increased lens transmission. This has been shown to result in improved contrast and improved visual function.
The following are some special considerations when choosing anti-reflection coatings:
1) Clean your anti-reflection lenses regularly. Because these lenses have increased light transmission, they also show fingerprints and smudges more easily.
2) Premium, name-brand anti-reflection coatings such as Nikon HCC or Crizal are recommended. These coatings come with "built-in" scratch resistance. They also include the manufacturer's warranty against abnormal defects. Inexpensive, no-name anti-reflection coatings are often not durable enough to endure normal day-to-day wear and tear.
3) Anti-reflection coatings are always recommended with high-index lenses. High-index lenses (which are made of materials that allow the lenses to be thinner) have more reflections from their surfaces compared to regular lenses.
- High-index lenses are made of materials (plastic or glass) which allow different front and back curvatures of the lens surfaces to result in lenses with the same optical power. This results in lenses which are much more cosmetically appealing, thinner, a little lighter, and more enjoyable to wear. The higher the index of the material, the thinner the lens.
- This option is especially welcomed by patients with relatively high myopic or hyperopic refractive errors who would otherwise have fairly thick and heavy lenses. For myopic patients (minus lenses), this results in lenses which are thinner at the edges. For hyperopic patients (plus lenses), this results in lenses which are thinner in the centre.
- Anti-reflection coatings are always recommended with high-index lenses. (High-index lenses have more reflections from their surfaces compared to regular lenses.)
Aspheric Lenses
- Aspheric lenses are designed so the lens surface curvatures flatten out towards the periphery of the lens. This results in lenses which are thinner and have improved peripheral optical properties.
- In addition, the
minification or magnification (due to high myopic
(minus lenses) or hyperopic (plus lenses) which cause the eyes to appear smaller or larger behind the lenses) is reduced.
- This option is available in conjunction with high-index lenses for even further improved cosmetic appearance and easier adaptation.
Tinting and UV Protection
- There are three types of invisible UV (ultraviolet) radiation (A: 315-400nm, B: 290-315nm, and C: 200-290nm) which come from the sun. UV-C is blocked by the earth's atmosphere and does not occur naturally on the earth's surface. UV-B is the primary cause of sunburn and has been associated with skin cancer. Research also suggests that UV-B (and maybe UV-A) may cause the earlier development of cataracts when exposed regularly for many years. UV radiation and short wavelength/blue light may have a role in the development of age-related macular degeneration.
- In order minimize any possible damage to your eyes and vision which can occur from a lifetime of exposure to UV radiation, a UV protective coating is recommended for your lenses. This can be done in the form of sunglasses: in conjunction with tinting to also reduce the amount of visible light that reaches the eye. Or this can also be applied to a regular clear plastic lens. Certain lens materials such as polycarbonate, high-index, or photochromic lenses automatically provide a degree of UV protection (an additional UV protective coating may be required to obtain complete UV400 protection).
- In addition to tinting and a UV protective coating, wearing a large-brimmed hat and suntan lotion is also recommended when heading outdoors.
Photochromic Lenses
- Photochromic lenses are a very popular lens option, especially with the advent of modern plastic materials (previously only glass photochromic lenses were available). These lenses automatically change their level of tinting in response to the amount of UV light to which they are exposed.
- They are particularly useful when someone is often both indoors and outdoors. When indoors they lighten their tint and when outdoors they darken. Depending on the type of photochromic, they can substitute for sunglasses in some situations. Photochromic lenses do provide UV protection.
- However, in certain situations "real" sunglasses (which are always darkly tinted) are better such is inside a car where most of the UV light required to activate a photochromic lens is blocked by the windshield. Although modern photochromics change from light to dark fairly quickly, there is some delay which may pose a problem under some conditions. Some photochromic lenses do not fully lighten when indoors and can cause some difficulties seeing.
- There are many different brands, materials, and lens types now available as photochromic lenses. For extra fun, lenses are even available which come in different colours that change when they are activated by UV light: yellow to orange, pink to purple, and blue to green.
Polarized Lenses
- Polarized lenses are designed to reduce reflections and glare due to sunlight which is reflected from surfaces such as water or snow. This lens feature always comes with tinting and UV protection, and is to be used as sunglasses.
- Sunlight itself is naturally polarized (360 degrees). When it is reflected from a surface such as water it becomes "horizontally polarized". Polarized lenses are designed to reduce these "horizontally polarized" reflections.
- Depending on the viewing angle, this type of lens can sometimes interfere with the visibility of LCDs (Liquid Crystal Displays) such as sometimes used on a car's dashboard.
- If you are often
on the water or near the water, this lens feature will provide significantly
reduced glare and improved vision.
Select one of the following topics for more information about eyeglasses from the Mission Eye Clinic:
> Understanding Your Eyeglass Prescription: Common Refractive Errors
> What to expect when you receive your new eyeglasses
> Proper Care of Your Eyeglasses
> We are especially experienced in eyeglass problems and difficult prescriptions