What are High Index Lenses?
by Julius Im

High index lenses are lenses that have higher “indexes” of refraction. Simply put, they are essentially 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 and as such, prescription strengths are increasing.

High index lenses are not the kind typically used in single vision glasses. They are usually used for people who have 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 which 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 that is within the field of vision.

Poor vision typically comes from a misalignment of this lens within the eye, or what are called refractive errors. These refractive errors are to account for blurry vision and also the reason 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 in such a precise way that true vision is restored and the right electrochemical signals are sent to the brain.

Depending on how severely misaligned the lens is, the amount of 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 do the job of correcting 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 would 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 which 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, then 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 at nearly 70% slower through high index lenses than it does 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 of the materials listed may be better suited for some people based on how they use their eyewear and their prescription.

Crown Glass/Glass Lenses
The introduction of glasses and early vision correction with lenses all made use of glass. Glass lenses offer exceptional optics at a low cost. However, they are naturally heavy because they are made of glass and more prone to break. As effective as glass lenses are, they have a refractive index of 1.523 which is good when it comes to 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 in use today for many glasses. Plastic lenses can provide almost as good of a quality in optics as glass does. They are also low cost and lighter than typical glass lenses. However, plastic lenses’ refractive index is 1.498 which means that the higher the prescription, the thicker the lenses become. Thus the downside to plastic lenses is its inefficiency when it comes to refracting more light and the consequential need for thicker lenses.

Polycarbonate Lenses
Polycarbonate lenses were originally developed for physical 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. They have a refractive index of 1.586 and due to its impact resistant quality are used primarily for children’s eyewear, safety glasses, and sports eyewear.

High Index Lenses
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 high index.

ADVANTAGES OF HIGH INDEX LENSES
Comfort is a big advantange. 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 useful for aesthetic means. Regular lenses for nearsightedness tend to be thicker at the edges. This means that with conventional plastic or glass lenses, the 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 good 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 made thinner and create a more attractive profile.

DISADVANTAGES OF HIGH INDEX LENSES
Price. The main disadvantage of high index lenses would be the price point. Compared to the 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 as it 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 which can be added if necessary.

Reflective. In addition to being brittle, high index lenses are also more reflective than other materials which can pose problems with being in the sun as it can cause a glare. But like the anti-scratch coating, an anti-glare coating can be applied as well to remedy this issue.

GETTING HIGH INDEX LENSES
Evidently, 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 seen as a cosmetic remedy to 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 in order to correct vision that requires a refractive index of 1.60.

A basic understanding of the script in prescriptions can help determine if high index lenses are necessary or not. 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 what are called 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 needs to 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.

If 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, then high index lenses will probably be very useful. Otherwise, the cost of 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.