Spectacle Lens Materials
Lens materials fall into two categories: glass and plastic (organic).
Plastic lenses are subdivided into two groups: thermosetting plastics
and thermoplastics (polycarbonates).
BENEFITS:
The use of one material over another may be determined by opthalmic and/or cosmetic
factors as well as the practical application of the lens (for example in safety
spectacles as opposed to reading glasses). The Refraction Index
of the lens material is another contributing factor.
The benefits obtained are therefore likely to be a combination of the following:
- lightness
- thinness
- transparency
- shock resistance
- UV protection
REFRACTION INDEX
The refraction index is a ratio between the speed of light in one
optical medium and the speed of light through air. The higher the index, the less
curved a lens needs to be, for a given correction, it is therefore possible to make
a thinner lens with a higher index material.
The refraction indexes of the most common spectacle lens materials are as follows:
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Glass
|
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1.5 - 1.9
|
|
|
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Plastic (Low and medium indexes)
|
|
1.5, 1.56
|
|
|
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Polycarbonate
|
|
1.59
|
|
|
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Plastic (High and very high indexes)
|
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1.6, 1.67, 1.74
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