From Ben Franklin til the mid 20th century, glass was the only lens material available for eyeglasses. That changed with the introduction of CR-39 plastic lenses in the mid 50’s. They were far lighter and much more resistant to breakage. The optical quality was not as good as glass but still acceptable.
CR-39 lenses gained popularity in the 1970s and from then on, plastics displaced glass as the lens material of choice. Many advancements were made in plastic lens materials over the years, which has led to the incredible array of plastics available today.
The Index of a lens material, in it’s simplest form, refers to the amount of lens material required to achieve the power needed. As CR-39 has an index of 1.49-1.50, any material with an index higher than 1.50 is considered high-index. There is a rule-of-thumb that says- The higher the index, the more it costs. And, while a fairly accurate statement, there are many things that our opticians take into consideration before recommending a particular lens design or material.
With over a dozen different lens materials and indices available, it is important to bear in mind, when shopping for lenses, that 2 different opticals can be considering 2 different ‘high-index’ lenses and you will see a vast difference in the price. So its important to know exactly what your comparing.
High index lenses reflect about 50% more light than ordinary plastic or glass lenses. These reflections can interfere with your vision especially when driving at night. However, this difficulty doesn’t exist when you use them with a premium anti-reflective (AR) coating. An AR coated high index lens transmits up to 99.5% of light to your eyes. This allows unimpeded vision while driving at night, eliminating lens induced glare.
Also, If thickness is the overriding concern, ultra-thin 1.8 and 1.9 high index lenses are available in glass. They are fairly tough and impact resistant, though not at the level of plastics so are not a good choice for kids and active adults. These lenses are the thinnest currently available and fit in many stylish frames. However, they aren’t as light-weight as plastics.
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