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Index Of Refraction

If you have a high prescription (give or take +/- 3 diopters) you've probubly been told that you should go with a Hi-Index lens material in order to make the lenses thinner.  So what exactly is Hi-Index?

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All substances that allow light to enter and leave can be assigned an index of refraction (IOR).  The number essencially defines how that substance bends (refracts) light.

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Here are some optical lens materials with their IOR:

  • CR-39 - 1.498

  • Trivex - 1.54

  • Polycarbonate - 1.586

  • Hi-Index - 1.67

  • Hi-Index - 1.74

So let's say someone has a -4.00 prescription; depending on the size of the frame they select, using a low index material like CR-39 can lead to undesired thickness.  A higher index material will provide the same correction but will produce less thickness.

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So this is generally why opticians will suggest going with a Hi-Index material when a patient has a high prescription.

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Now, it may seem relative, but a Hi-Index material can produce a thinner lens but that doesn't make it lighter weight.  In fact, the density of Hi-Index lenses often leads to a heavier weight over materials such as Polycarbonate and Trivex.

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There have been plenty of situations where patients have preferred a lightweight lens over a thinner lens.

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