How do Plastic High Index Lenses Work?

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  • Written By: R. Kayne
  • Edited By: Niki Foster
  • Last Modified Date: 15 October 2019
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High index lenses made from specialty plastics allow thinner, lighter lenses than traditional glass or plastic. These lenses are geared towards those with very strong prescriptions, both nearsighted and farsighted. Glasses correct vision by bending the light that passes through them. A stronger prescription requires more bending than a weaker one, resulting in a thicker lens. A high index lens is made of denser material than conventional one, bending more light through less thickness, resulting in ultra-thin, lightweight glasses.

When viewing conventional glasses, one can easily see lenses designed to correct nearsightedness. The lenses are thicker at the edges than in the middle. The stronger the prescription, the thicker the edges become.

Most stylish frames are too thin to hide the thick edges of nearsighted lenses. Thick lenses also distort the eyes, making them look unnaturally small. High index lenses eliminate both problems.

Those with extreme farsighted vision have lenses that are thicker in the middle, with thin edges. The thick middle has the opposite effect of a nearsighted lens, making the eyes look unnaturally large. Again, these denser lenses offer a solution.

All lenses are categorized by how much light they can bend using an index of infraction. This index compares lightspeed through air to lightspeed through the lens material. Glass bends light at an index ratio of 1.52, while conventional plastic has an index of 1.50. Lenses with a greater index are considered high index.


There are several plastic materials now available with index ratios between 1.53 and 1.71. The higher the index, the thinner, lighter, and more expensive the lenses. High index glass lenses are also available, with a ratio of 1.90. However, glass lenses are not as light or durable as plastic lenses. In fact, high index glass is actually heavier than standard glass, canceling much of the benefit of the thinner lens.

High index lenses can correct astigmatism and are available as bifocals or trifocals. They look nice in thin, wire, and “frameless” frames. The index that will best suit you is determined by your prescription, but don’t assume a vendor will make the best choice for you. Some suppliers do not sell all types, and you could end up with a lower index lens than is ideal. Ask your optometrist which index is best.

This type of lenses are available from many optical centers. They can make wearing glasses more enjoyable, improve self-image, and restore confidence. They are durable and shatterproof, perfect for children and active adults. After you’ve worn a pair of high index lenses, chances are you’ll never go back to standard glasses again.


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Discuss this Article

Post 9

The Refractive Index isn't everything. The Abbe value is at least as important. The standard CR-39 lens with its high Abbe value is optically superior to all high index lenses, even though it's a bit thicker.

Post 8

I have a strong prescription, as well: -11 in L eye, -9.5 in right eye. To the person wondering about lens size, yes, unfortunately, the bigger the lens, the thicker the lens on the outer edge. Some websites have a lens thickness calculator where you can input all your data--lens width, bridge size, and strength, and it will calculate the approx thickness at the edge. I have always been encouraged to get the smallest possible lenses because of my scrip.

As far as buying high strength glasses, I don't live in the UK, but I have just taken a chance and ordered glasses online. I am also looking at other online stores. Unfortunately, not all of them offer lenses for over a -10, but at least there are places out there. For the savings, it was worth the gamble.

My normal glasses always cost 4-500 dollars, while these cost only $85. Good luck!

Post 7

does anyone know where i can buy high index lenses in the uk? i have a super high prescription. -11 in both eyes.

Post 6

I have no need to be anonymous.

I have tried a few Rx glasses. My eyes are reasonably good, I still am a legal driver without the glasses.

My best Rx glasses so far are a bifocal.

Problem: The "Line" at the top of the bifocal is not really a "line". It's a smear with finite vertical loss of vision.

What does anyone recommend to improve?

Thanks. Gene L.

Post 5

my bifocals prescription is right eye+8 left eye+6 add+4. how strong is it and would high index lenses be better?

Post 4

i am also 54 and left eye +6 right eye +8 add+4 with ordinary lenses the right ens was thicker than the left. plastic frames and high index lenses are a god send rather than metal frames.

Post 3

i am also 54 my prescription is right eye+8 and left eye+6 add+4. this is the first time i have worn high index lenses. what a difference it makes to plastic frames.

Post 2

I just had a prescription made with high index. I am nearsighted at about 6.5. They told me the larger the frame the larger the lens will be, and that is just how it is. I need to know if this is true.

The frames are not that large. One is for bi-focal and the other just regular, and they are thick.

Please reply with the true answer as to weather the size of the frame makes a difference or is it the material? Thank you.

Post 1

My prescription is +6.00,and I am 54 years old.

I tried a high index progressive for a year, but was in the process of sending them away to have new lenses made and when I switched back to my old trifocals I realized how much better my vision is with the trifocals.

The optical person says that they don't make trifocals that have the same index as the progressive and that you can't get the same anti-reflective coating too, so if this true I am wondering why not? I did decide to go with a poly carbonate trifocal.

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