Reflections, Density, and Index

The high index of a lens is connected to most of the qualities worth considering when shopping for a lens material.

Reflections, Density, and Index

Index of refraction, Abbe number (and optical clarity), amount of glare, thinness, weight, scratch resistance, and overall aesthetics are all tied to the lens’s index. If you are shopping for lens materials, index is a fair guide to deciding what you are getting. There are exceptions to the rules, but there are reliable trends to follow as index of refraction increases. If you are interested in knowing more about lens materials, it is useful to know more about these trends. Here are the things to consider when browsing lenses by index:

  • Thickness is the most well-known attribute that is affected by a lens’s index, as they are directly correlated and there are no exceptions to this rule. The higher the index, the thinner the lens can be to accommodate the same prescription.
  • Generally speaking, the higher a lens’s index, the denser it is. This does not mean that the lens will necessarily be heavier; in fact, the general trend is that, as the lens’s density and index go up, the index of refraction makes it thinner, and therefore lighter, more quickly that the added density can weigh it back down. In a nutshell, the higher the index, the lighter the prescription lens will be.
  • A lens’s density and index of refraction are tied into the amount of glare and reflections that it produces. The higher the index, the more a lens reflects light. This is because, just as higher indexes bend light more efficiency, they also block a small amount of light from passing through the lens. The light that does not pass through the lens is reflected instead, leading to glare. This is generally taken care of by applying anti-reflective (AR) coating to the lenses.
  • Higher indexes are also tied into Abbe number, which is related to optical clarity. Basically, the lower the Abbe number, the more distortion the lens produces. This is why crystal makes white light turn into a rainbow as it passes through. The higher the index, in general, the lower the Abbe number, and the worse it distorts light. This distortion is especially apparent in your peripheral vision.
  • Another general trend regarding index has to do with scratch resistance. Generally, the denser the lens is, the harder it is, and therefore the more scratch resistant it is. Higher index lenses generally are more scratch resistant.
  • This hardness related to scratch resistance does the opposite for impact resistance. While higher index lenses are harder to scratch, they are generally more brittle and crack easier, especially upon impact.
  • Finally, overall aesthetics play (arguably) the most vital role in the popularity of high index lenses. The higher the index, the thinner the profile of the lens, which makes them look sleeker and your prescription look less strong. The lens’s thinness also makes the distortion of your eyes less apparent; thicker lenses tend to make your eyes look larger or smaller than they are, depending on the type of prescription. By making the lens thinner, higher indexes eliminate much of this distortion.

As you can see, the index of a lens has a lot to do with its strong and weak points. The materials that stray considerably from these rules are polycarbonate and high index glass. Polycarbonate has a higher index than a standard plastic would, but it is less scratch resistant and more impact resistant. It also has a lower Abbe number than some 1.67 high index plastics. High index glass lenses follow the rules, but they are far and away more brittle than the highest index plastics, and they also cause much more distortion because of their very low Abbe numbers. For the most part, high index glass is best avoided. If you are curious about why this is, or if you would like to know more about specific lens materials, browse our other blog posts and you may find what you’re looking for. Thanks for reading, and happy shopping!

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