Great Lens Materials are Available to Make Lenses Thinner, Lighter and Shatter Resistant
There's much more to buying a new pair of eyeglasses than just selecting the perfect frame. There are many different lens materials, lens coatings, tints, etc. and it can sometimes feel overwhelming. Understanding the different lens options available can make the process much less stressful. This information will explain the basic types of lens materials available and the most common lens options.
Basic Optical Lens Science
Eyeglass lenses are often categorized by their refractive index (also called index of refraction).
The refractive index of any substance is the speed of light in that substance compared to the speed of light in a vacuum expressed as a ratio. Light travels fastest in a vacuum and slower through different materials.
Refractive Index = speed of light in a vacuum / speed of light in the comparative material
The refractive index of a basic plastic lens (CR-39) is 1.498 meaning that light travels 1.498 times faster in a vacuum than it does through the plastic lens.
The higher the index of refraction the thinner a lens can be to get the same refraction effect. Optical lenses are now classified into the following categories:
- Normal Index: 1.48-1.54
- Mid Index: 1.54-1.64
- High Index: 1.64-1.74
- Ultra High Index: 1.74 and above.
Abbe value is a measure of the lens material’s dispersion of light. A lens with a low Abbe value causes a higher dispersion and leads to unwanted chromatic aberration. Chromatic aberration is a distortion of the image due to the inability of the lens to focus all colors onto the same focal point. This leads to the perception of undesirable color fringes when viewing objects for many people.
High index lenses offer a thinner lens but usually have lower abbe values. The Abbe value determines to a large part the optical integrity of the lens. The higher the abbe value the better the optical clarity and less distortion. It is a delicate balance to find a lens that not only satisfies in terms of aesthetics and weight, but also features acceptable optical clarity.
In the United States, most high index lenses are made from the various plastic materials. Some other countries still use a lot of glass material for high index lenses. Glass is available in very high indices such as 1.8 and 1.9 but because of its density it is still very heavy. Glass lenses also take longer for optical labs to fabricate. High index plastic lenses can be used in desired rimless and 3 piece mount frames.
Options of Lens Materials for Eyeglasses
Normal Index Lenses
Standard Plastic CR-39 Lenses
These are conventional plastic material lenses with a refractive index of 1.498 that have been widely used since their introduction in 1947. One of the biggest advantages of this lens is its affordability. It also provides good optical clarity and is easy to tint. Some disadvantages of this material are that it scratches easy, does not provide adequate UV protection, is not as impact resistant as other lenses and is also heavier and thicker than the higher index plastics. It should be noted however, that a scratch resistance coat, as well as a UV coating can be applied to the lens helping to reduce some of these deficiencies. CR-39 plastic lens material is usually only recommended for those with a weak prescription where lens thickness and weight are not a concern and for dress eyewear in which safety is not an issue.
CR-39 lenses have a high Abbe value of 59.3 making them the lenses with the least distortion from dispersion/chromatic aberration.
Polycarbonate lenses have a higher index of refraction (1.586) than CR-39 plastic (1.498) and are therefore thinner in an equal lens prescription. With a higher index (of refraction), light is bent more efficiently, therefore it requires less material to achieve the eyeglass prescription making the lenses much lighter. Another significant advantage of these lenses is that they are impact resistant. Polycarbonate lenses offer up to 10 times more impact resistance than regular plastic lenses, hence along with Trivex, are the lenses of choice for safety glasses, children’s eyewear, and for anyone who wants durable yet thin lenses. It's important to recognize that impact resistant does not mean shatterproof. They can still break, therefore official safety glasses should be worn for eye-hazardous jobs and sports. Additionally, Trivex and polycarbonate naturally block most UV light (385 nanometers and below) and do not need the application of a UV coating. They are also lab favorites for use in rimless frames, because polycarbonate lenses can be easily drilled and resist cracking around the drill holes. A disadvantage of polycarbonate is that it is naturally a soft material causing it to scratch much easier. However, unlike CR-39 basic plastic lenses, a scratch resistant coating is almost always standard as it is applied when the lenses are made. With higher refractive indexes, there are often more chromatic aberrations, meaning visual disturbances of light, that can be interpreted as blur by some. Chromatic aberrations are higher in polycarbonate lenses. The abbe value of polycarbonate is 30, the lowest of all lens materials making it the worst lens for optical clarity and integrity. Plenty of people cannot adapt to a polycarbonate lens. For those reasons Trivex is the better choice in a mid-index lens.
Trivex is a relatively new optical lens material. It has the ultraviolet blocking properties (380 nanometers and less) and shatter resistant properties of polycarbonate. However, trivex lenses have a much higher abbe value (43-45) vs. polycarbonate (abbe value 30) making it much better in optical clarity with fewer chromatic aberrations. Trivex also has a slightly lower density so it is lighter than polycarbonate. In fact, Trivex is the lightest weight lens material available. Another advantage that Trivex has over polycarbonate is that it can be easily tinted.
Trivex along with polycarbonate is great for rimless frames and those frames requiring drill mount screws through the lenses.
High Index and Ultra-High Index Plastic Lenses
An even higher index lens should be considered to achieve the thinnest lenses possible. This means the index of refraction would need to be greater than that of polycarbonate and Trivex (>1.60). High index lenses are classified by numbers that represent their refractive index and range from 1.64 to 1.74. High index lenses can be up to 50% thinner than regular glass or plastic lenses, and they’re usually much lighter, too. Although these lenses are generally recommended for people with high optical prescriptions, high index lenses can benefit anyone who would like a thinner lens profile. The higher the index, the thinner the lens will be relative to basic plastic. One of the biggest disadvantages of high index materials is the higher cost compared to other materials including polycarbonate and Trivex. High index plastics also have a lower abbe value (32-42) and therefore have some problems with chromatic abberations. Because of the way that light interacts with high index lenses (chromatic aberrations), it's highly recommended that an anti-reflective (AR) coating is also applied to the lens to help with reflected light.
Because of the density of the high index plastic material it isn’t usually the lightest material even though it is thinner. Trivex is usually the lightest weight lens material.
High index plastics do offer good ultraviolet inhibition (below 380-400 nanometers) properties as well as shatter resistance.
A lens with a higher index of refraction tends to reflect light more than standard CR-39 plastic or glass lenses. The extra reflections are usually quite bothersome for the wearer especially at night and while using a computer monitor. The glare from the lenses is also cosmetically unappealing. Because of this most high index lenses come with an anti-reflective coating as a standard option.
Aspheric Curved Lens Design
An option with high index lenses is an "aspheric" design, which can contribute to a thinner lens appearance and profile. Aspheric means that instead of having a round, or spherical curve on the front surface, they have a curve that gradually changes from the center of the lens to the periphery of the lens. The curvature in a minus lens will get gradually steeper toward the periphery and a plus lens curvature will gradually flatten. This is important because it provides consistent optical clarity in the lens periphery, not just the center. It also makes it possible for someone with a strong prescription to wear a larger selection of frames without worry of the lenses being too thick. This not only affects the appearance of the lens but also reduces the enlarged appearance of eyes ("bug-eyed" look) with high plus power prescriptions that are used for farsightedness. The opposite occurs in high minus power lenses for nearsightedness. In non-aspheric lenses, eyes can appear minified, although the effect is not as dramatic as it is for farsighted people. Aspheric lenses do help reduce this effect. Aspheric lenses are also available in lower index plastics but are more common in high index, because high index is so often chosen for very high prescriptions. The aspheric design itself produces a thinner lens.
Glass lenses provide excellent optics, the most scratch resistant lens and blocks UV light. However, glass lenses are heavy, thick, dangerous if broken and cannot be used in certain frame styles.
Eyewear Frame Size is a Major Factor in Lens Thickness and Weight
It is a simple law of physics. The smaller the lens the lighter and thinner it is. Choosing a small lens size frame is the biggest determinant of lens thickness and weight! Smaller frames also decrease the amount of lens distortions/aberrations.