What You Must Know . . . Before You Buy New Eyeglasses!
Your prescription needs to be made into lenses that will fit into the frame you have chosen. The materials used and the design chosen can often be the difference between a pair of glasses you love and a pair that you hate.
Think of the prescription as a "blueprint" for a building. The architect designs the plans for the building and this is converted into the "blueprint" for the builder. If the builder uses substandard materials and poor workmanship the architects vision will be poorly realized. Your prescription must be made into a pair of glasses using the proper materials and the best workmanship. Then you will be thrilled with your new glasses.
More Than One Pair Might be Necessary
Having only a single pair of prescription eyeglasses is like having a Swiss Army Knife. They are a great little tool. I'm looking at mine right now and remember who gave it to me (he was from Switzerland) and how many years ago it was (longer than I care to admit!) It has a couple of knife blades of different sizes, a screwdriver, a saw, tweezers, a corkscrew and lots of other stuff. It does an amazing number of things, but nothing really great. If I really want a screwdriver, I use a real screw driver. If I need to cut a tree down, I guarantee that I won't use the saw-blade on my Swiss Army Knife. If I want a knife to slice tomatoes, I use a kitchen knife.
If you want a pair of glasses that work best on the computer, its best to have computer glasses. If you want the best glasses for driving on sunny days a lens called "Drivewear"is the best. If you want glasses for skiing, ski goggles are your thing. Using one pair of glasses for every activity means that there are going to be unavoidable compromises.
All lenses allow light to pass through them and they control the way the light is focused. Some materials do a better job than others at doing this. Also, some materials are stronger, thinner and lighter in weight for the equivalent focusing power. It is always a matter of "trade offs" to get the best material for your prescription.
Lens Material Characteristics
A little terminology will be useful in understanding the characteristics of the different lens materials.
Abbe Value- This refers an optical characteristic of a lens called dispersion or chromatic aberration. The higher this number the better. Materials with lower Abbe value numbers have greater chromatic aberration which the wearer perceives as color fringes around high contrast objects, especially in the periphery of the lens. Some will simply perceive it as blurriness or distortion. Values in the 30's can be a problem in higher prescriptions. An Abbe value number of 45 or greater is ideal.
Index of Refraction- This is simply a measurement of the lens material's ability to bend light. The higher the number the thinner the material needs to be to give the same amount of focusing power.
- Normal index (1.48 - 1.53)
- Mid-index (1.54 - 1.59)
- High-index (1.60-1.69)
- Very high-index (1.70 - 1.74)
Specific Gravity - Refers to the density (the weight) of a given amount of material. A material with a specific gravity of 2.00 will be twice as heavy one with a specific gravity of 1.00. Lens materials range from 1.11 (for Trivex) to 2.93 (for 1.7 High Index glass).
Reflected Light - When light strikes the lens some of it is reflected from the front surface. The remainder travels through the lens where it is useful for vision. The light reflected from the surface causes annoying glare for those looking at the wearer and makes it difficult to see her eyes. The light reflected from the back surface of the lens interferes with vision. Antireflective coatings (AR) eliminate these distractions and improve vision. AR also dramatically improves the appearance of the glasses.
Materials Used for Eyeglass Lenses
Plastic (CR39) lenses - The original plastic lens material used as an alternative to glass. It weighs only about 1/2 that of glass. The abbreviation stands for "Columbia Resin #39," because it was the 39th formula of a plastic developed by the Columbia Resins project in 1940.
The first commercial use of CR-39 was to help create fiberglass reinforced plastic fuel tanks for the B-17 bomber aircraft in World War II therby reducing weight and increasing the range of the bomber. After the War, the Armorlite Lens Company in California is credited with manufacturing the first CR-39 eyeglass lenses in 1947.
- Abbe Value 58 - CR-39 plastic is an excellent optical material with an Abbe number of 58 giving excellent optical characteristics and virtually no chromatic abberation.
- Low Index of Refraction of 1.498 - A disadvantage is that it has an index of refraction of only 1.498, This means that it will result in a thick eyeglass lens in moderate to high prescriptions, especially in larger frames.
- Specific Gravity 1.31- A relatively dense (heavy) material combined with a low index of refraction results in a thick heavy lens for any given prescription. However, this may not be an problem with a small prescription in a small frame.
- Reflected Light - 7.97% - 92% of the light striking the lens travels thought the lens.
CR-39 must be treated with additional coatings to make it scratch resistant, UV absorbing, or anti-reflective.
Polycarbonate lenses -
Polycarbonate is a very strong, thin, lightweight material. The main advantage of polycarbonate over other types of plastic is its unbeatable strength combined with its light weight. Bulletproof windows and enclosures inside banks or at drive-throughs are often made of polycarbonate.
It is an ideal lens material for children or for hazardous activities because of its safety. It inherently blocks UV rays which cause damage to the eyes. It is a very soft plastic so it always comes with a scratch coating. It does not tint well.
- Abbe Value (30) - It has a low Abbe Value which can created problems with chromatic aberration (or blurriness) in the periphery of a high prescription lens.
- Index of Refraction ( 1.59) - A very good index of refraction for a relatively inexpensive cost.
- Specific Gravity ( 1.2 ) - A medium weight plastic lens combined with a good index of refraction results in a relatively thin, lightweight prescription.
- Reflected Light ( 10.27%) - 90% of the light goes through the lens.
Trivex has many of the same properties of polycarbonate. It blocks UV and is shatter resistance. It has a high Abbe value and therefore has superior optical qualities. It is an excellent choice for rimless frames because of its strength. It can be tinted more easily than polycarbonate.
- Abbe Value (44)
- Index of Refraction (1.532)
- Specific Gravity (1.11) - (the lightest material available)
- Reflected Light - 8.7%
High index lenses (Index of Refraction 1.60 to 1.70)
- Advantages of High Index Lenses:
Farsighted correcting lenses will have a thinner center than a low index lens of the same prescription. Nearsighted correcting lenses will have a thinner edge thickness. Because less light can enter in through the thinner edge there will be fewer internal reflections. Because less material is used to make the lens it will be lighter in weight and more comfortable. There will also be a better cosmetic appearance.
- Disadvantage of High Index Lenses:
Compared to CR-39 the Abbe value will be lower and therefore will have more chromatic aberration. This is noticable as color fringes next to high contrast images especially in the periphery. Some wearers may be aware of blur.
It is critical to use anti-reflective coatings (AR) since reflections are even more bothersome with high index lenses.
- Index of Refraction (1.6 - 1.7)
- Abbe Value (32 -42) - The higher the index of refraction, the lower the Abbe value.
- Specific Gravity (1.3 -1.5) The higher the index of refraction the higher the specific gravity.
- Reflected Light (10.65% - 13.44%) Only about 88% of the light travels through the lens and the rest is reflected. Anti-reflective coatings (AR) are highly advisable.
Super high index lenses (1.7 or greater)
These have all the same benefits and issues as high index lenses, but more so.
Crown glass lenses
This is the original lens material. Glasses are called "glasses" because glass was the material used to make lenses. (CR-39 plastic lenses were introduced in 1947, but took a long time to catch on.) Now only about 7% of lenses are made of glass. It must be hardened to prevent it from shattering.
- Index of Refraction (1.525)
- Abbe Value (58.5)
- Specific Gravity (2.54)- This is twice as heavy as most plastic lenses.
- Reflected Light (8.58%)
There are five major categories of tints - solid, gradient, mirrored, photochromic and polarized.
- Solid - A tint that does not vary throughout the lens. There are many tints available, ranging from very light to very dark sunglasses.
- Mirrored - A sunglass lens that appears like a mirror on its front surface.
- Gradient - A tint that fades from the top to the bottom of the lens.
- Photochromic - These lenses (Transitions, SunSensor, Photogrey etc.) darken in the presence of sunlight. Over the years there have been many improvements in these lenses so that the recent versions react more quickly, darken more completely and lighten more fully after coming indoors.
Most do not work well in a car since the windshield absorbs much of the light necessary to change the lens. "Drivewear" is specifically designed to wear when driving because it gets as darkens fully and is polarized.
- Polarized - Many sunglass lenses are polarized and block out light that is reflected from a surface such as a lake. Polarized lenses block bright reflected glare and allow useful light to enter the eye giving a more comfortable viewing experience. As an example, fisherman can see fish swimming below the surface of the water better than otherwise.
- Scratch Resistant Coatings - These coatings make the plastic lens less likely to scratch but not scratch-proof. If the lens does scratch, lenses will usually be covered "under warranty" within a specified time period, usually a year.
- Anti Reflective Coatings (AR) - They make the eye behind the lens more visible and give a better cosmetic appearance. They reduce the reflections off the back of the lens which results in a better image for the wearer with more contrast. At night AR reduces glare from headlights, street lights and neon signs.
Because AR reduces reflections from the front surface of the lens it allows more light to travel through the lens resulting in better vision, especially at night. Lenses without AR only allow from 86% to 92% of the light to go through the lens.
In the past AR coats scratched easily and did not adhere well to the lens. They smudged and were hard to keep clean. Premium AR coats available now have made these problems a thing of the past.
- UV Coatings - Ultraviolet radiation (UV) can be damaging to the eyes (the lens and the retina). Some lens materials inherently absorb UV radiation (Polycarbonate or Trivex). Lenses designed for outdoors use (Transitions, Sunsensors, Drivewear) also protect from UV. Other lenses must be UV treated in order to receive protection from damaging UV radiation.
- Single Vision - A lens design that has only one focus distance. For a younger individual (under age 40) this lens will probably be adequate for any distance since they have the ability to accommodate (refocus) their eyes from one distance to another.
- Aspheric - These lenses are designed to make a thinner lens and to cause less distortion of the wearer's appearance. A lens that corrects for a significant amount of "farsightedness" will make the wearer's eyes appear "bug-eyed" unless it is made in an aspheric form.
- Bifocal - After about the age of 40 we lose our ability to focus at near through the same prescription that we can see at far. Then we need two prescriptions - one for far and one for near. A bifocal lens is made with the prescription for far at the top and for near at the bottom.
- Trifocal - A trifocal lens divides the lens into three prescriptions - far, intermediate, and near.
- Progressive- These lenses are very complex in their design and over the years have gone through many design improvements. They are commonly called "no line" bifocals and gradually (or progressively) vary in power from the top of the lens to the bottom.
A progressive lens not only eliminates the telltale line (and therefore gives a better cosmetic appearance) but gives a more natural visual experience that is easier to adapt to. Lens design always involves "trade-offs" so it is important to have a lens that is specifically chosen for your needs.
In a frame with little vertical height, the lens power will have to transition more quickly from the the top of the lens to the bottom, which means that each area of view will be a little smaller.
There are differences in the qualities of the optics of various progressive lenses. Some premium progressive lenses are available in "high definition" freeform optics that result in a prescription with the sharpest optics.
Occupational Progressive Lenses
This is a type of progressive lens that is specifically designed for the office worker and computer user. It allows for a wider field of view and a more natural visual experience for up to a distance of about 7 to 10 feet. It is not intended for full time use since the distance vision is compromised.
At Littlefield Optometry we will help you choose the best lenses for your needs and the frame that will be the most comfortable and flattering for your appearance.