High Index Lenses…Avoiding the “Coke Bottle” look
When choosing new glasses, most people spend a lot of time carefully choosing a frame, but hardly spend anytime time at all considering their lenses; material, design, coating, etc. Right now, let’s focus on materials.
Back in the early days of vision correction, all lenses were made from crown glass, hence the term “glasses.” Now in more modern times, plastic is most commonly used. Starting in the late 40’s, plastic started being used for lenses, and became very popular because it was roughly half the weight of glass lenses, low cost, and still offered excellent optics all the while being safer then glass. The only downside was the thickness of these lenses in medium to higher prescription. So as time went on, and technology became better, “high Index” lenses were created which offered thinner lenses. The index refers to the “index of refraction” which is a measurement that shows how efficiently the material bends light. Therefore, the higher the index, the more efficient, the thinner the lens and the higher the cost. Any lens that has an index of refraction higher than 1.52, which is the index of glass, is a “high index” lens.
Another thing to consider when choosing the right index is the Abbe value. The Abbe value, named after the physicist that discovered it, measures how widely the lens disperses different wavelengths of light as it passes through the lens. A lens with a low Abbe value, will have high dispersion. This causes “chromatic aberrations.” Wears of lenses with a low Abbe value may notice rainbow like halos around objects and especially lights. Abbe values range from 30, to the best being glass at 59, and plastic a close second at 58.
“Trivex” is first of the high index lenses. It has an index of 1.53 and an Abbe value of 45. This material became available in 2001, and is 15% thinner than plastic, and is the lightest material available. It is also very impact resistant, so it is a good choice for safety frames, children and sports glasses. Before 2001, polycarbonate was the choice for these types of glasses, but Trivex is quickly becoming the preferred material because it offers a much higher Abbe value, and is much lighter.
Polycarbonate lenses are next. Polycarbonate has an index of 1.59, and the lowest Abbe value of 30. It was developed in the early 1970’s and is 20% thinner than plastic. It was originally developed for the Air Force’s helmet visors, “bullet proof” glass, and for safety glasses.
Next is 1.6. This material is slightly thinner than polycarbonate and has an Abbe value of 36. As an added bonus, it also blocks 100% UV! 1.67 is the next material, and it is about 20% thinner than polycarbonate, has an Abbe value of 32 and also blocks 100% UV.
1.74 is the highest index available. It is 10% thinner than 1.67, but much more expensive, so it is best used on only very high prescriptions. 1.74 has an Abbe value of 33.
This is all probably very hard to visualize, so here is a diagram showing what different prescriptions would look like across the different indices:
Other things to consider when choosing your lenses are the frame size and the lens design. All this may feel overwhelming when you’re deciding on your new eyewear, but rest assured, We at Village Creek Optometry will take all the factors into account when helping you decide on your new frames and lenses to give you the best results for your budget. After all, your new lenses, even more so than your frames will determine how happy you are with your new eyewear, and if you’re not happy, we’re not happy!