Last Updated on April 21, 2023 by amy
High-index lenses are lenses that have higher “indexes” of refraction. They are specialized lenses that can be made thinner and lighter than average prescription lenses.
As the use of eyewear steadily increases all over the world, there is a greater demand for glasses. Children are beginning to wear glasses at younger ages than in the past, so prescription strengths are increasing.
High-index lenses are not the kind typically used in single-vision glasses. They are usually used for people with extreme nearsightedness, farsightedness, or astigmatism.
HOW HIGH-INDEX LENSES WORK
Understanding how the eye works helps better inform how glasses and high-index lenses work. Within the human eye is a small pocket that contains a lens. This lens allows light to pass through to photoreceptor cells which then convert this light into electrochemical signals that are sent to the brain. Once these signals reach the brain, it makes sense of the object within the field of vision.
Poor vision typically comes from a misalignment of this lens within the eye or refractive errors. These refractive errors account for blurry vision and why people need glasses; glasses correct the misalignment of the original biological lens. These lenses fix the refractive errors by bending light as it passes through the lens. With glasses, the light passes through precisely so that proper vision is restored, and the right electrochemical signals are sent to the brain.
Depending on how misaligned the lens is, the light bending capability or lens power may need to be lower or higher. Usually, the lenses that people employ to correct their eyesight are plastic or glass. But both become problematic as the need for a higher prescription—the lens power—becomes greater.
While this does not mean that plastic or glass lenses cannot correct vision, they tend to be thicker and heavier the higher the prescription becomes. Fortunately, this is where high-index lenses come in handy. The way high-index lenses are made allows them to bend light more efficiently and, thus, do not need as much material to bend the light as typical plastic or glass lenses. Owing to their efficiency, high-index lenses allow for thinner and lighter makes of the same prescription.
INDEX OF REFRACTION
When it comes to glasses, there is a “refractive index.” A refractive index is a number that is a relative measure of how efficiently a material bends light. This efficiency depends on how fast light travels through the material: if light travels slower, it results in greater bending.
High-index lenses have higher “indexes” of refraction. To get a bit technical, the refractive index of a material is the ratio of the speed of light in a vacuum divided by the speed of light in the material. For instance, the refractive index of high-index lenses is 1.74, meaning that light travels nearly 70 percent slower through high-index lenses than through a vacuum.
There are a variety of materials that glasses lenses can be made of; depending on the material, these lenses will have lower or higher indexes of refraction. Some materials listed may be better suited for some people based on their eyewear use and prescription.
Crown Glass/Glass Lenses
The introduction of glasses and early vision correction with lenses all used glass. Glass lenses offer exceptional optics at a low cost. However, they are naturally heavy because they are made of glass and are more prone to break. As effective as glass lenses are, they have a refractive index of 1.523 which is suitable for average prescriptions but not as powerful for correcting higher refractive errors.
CR-39 Plastic/Plastic Lenses
Plastic lenses were introduced by the Armorlite Lens Company in 1947. The lenses were composed of a plastic polymer called CR-39 which is still used today for many glasses. Plastic lenses can provide almost as good optical quality as glass. They are also low-cost and lighter than typical glass lenses. However, plastic lenses’ refractive index is 1.498, meaning the higher the prescription, the thicker the lenses become. Thus the downside to plastic lenses is their inefficiency in refracting more light and the consequential need for thicker lenses.
Polycarbonate lenses were initially developed for physiological applications, such as helmet visors for the Air Force and bulletproof glass for banks. But in terms of eyewear, polycarbonate lenses for safety glasses were introduced in the 1970s. Due to their impact-resistant quality, they have a refractive index of 1.586 and are used primarily for children’s eyewear, safety glasses, and sports eyewear.
Of course, high-index lenses are in a separate category precisely because their refractive index is far above the rest. Their refractive index rises above 1.59 and can range from 1.60 to 1.74. Thus, anything above 1.59 on the index is considered a high index.
ADVANTAGES OF HIGH-INDEX LENSES
Comfort is a significant advantage. As mentioned earlier, high-index lenses allow for thinner and lighter lenses: these are their prime advantages. The fact that they are thinner and lighter makes for a more comfortable experience when wearing glasses.
Style. High-index lenses are helpful for aesthetic means. Regular lenses for nearsightedness tend to be thicker at the edges. This means that conventional plastic or glass lenses protrude from the frames, and their thickness becomes highly visible from the side. High-index lenses resolve this issue as they are thinner and more compact—they are either less exposed or fit within the frames. Unlike lenses for nearsightedness, lenses for farsightedness tend to be thinner at the edges but thicker at the center. This may seem reasonable in terms of fitting the frames but tend to create a “bug-eye” look depending on how strong the prescription is. Since high-index lenses can be more compact, the center area can be thinner, creating a more attractive profile.
DISADVANTAGES OF HIGH-INDEX LENSES
Price. The main disadvantage of high-index lenses is the price point. Compared to conventional plastic or glass lenses, high-index lenses can be up to three times the regular price, but again, depending on the prescription. The cost is due to the manufacturing process, which is more complicated and requires more precision in crafting thinner and more compact lenses.
Brittle. High-index lenses tend to be more brittle than the average pair of glasses. They may scratch and even shatter more easily than their other counterparts, but there are usually anti-scratch coatings that can be added if necessary.
Reflective. In addition to being brittle, high-index lenses are more reflective than other materials, posing problems with being in the sun as they can cause glare. But like the anti-scratch coating, an anti-glare coating can also be applied to remedy this issue.
GETTING HIGH-INDEX LENSES
High-index lenses pose some advantages for modern eyewear. However, the issue that remains is if they are truly necessary. High-index lenses are primarily designed for people with nearsightedness, farsightedness, or astigmatism. Simply put, high-index lenses could be considered a cosmetic remedy for thick lenses.
As discussed earlier, some materials are not as efficient in refracting light. Plastic and regular glass, for example, would require an enormous thickness to correct vision that requires a refractive index of 1.60.
A basic understanding of the script in prescriptions can help determine whether high-index lenses are necessary. On a typical prescription, there will be two categories marked “OS”(oculus sinister) and “OD” (oculus dextrus) and perhaps even a third one labeled “OU” (oculus uterque). These are simple abbreviations of Latin derivations that mean the left eye, the right eye, and both eyes, respectively.
There will be numbers under these categories, which are measured in diopters. A negative number indicates nearsightedness, while a positive number indicates farsightedness. The further from zero either of these numbers is, the greater the lens power must be to correct vision. For instance, a number of -1.50 would suggest nearsightedness which does not require high-index lenses. But a prescription of -6.00 would most likely need high-index lenses as low-index plastic or glass lenses for such a prescription would be thick and heavy.
Suppose the prescription is somewhat high, and getting glasses based on that prescription would make using glasses difficult because of the weight and thickness of the lenses. In that case, high-index lenses will probably be beneficial. Otherwise, getting high-index lenses may not be worth it. In any case, the best method to determine the need for high-index lenses would be to take an eye exam and seek the professional guidance of an optometrist.