The thinnest type of plastic lenses allowed by FDA to make glasses is known as 1.74. The name comes from the index of refraction of this type of plastic material. The index of refraction of a lens material determines thickness. As a comparison, Standard Plastic lenses, or CR-39 lenses, have an index of refraction of 1.49. Use 1.74 Thinnest Lenses if you have a strong to very strong prescription and your lenses will be lighter in weight and more cosmetically appealing.

Light responsive lenses darken when exposed to ultra violet rays (UV) from the sun. They generally get very close to completely clear inside and almost as dark as tinted or polarized sun lens gets when outside. It also tends not to take too long changing from light to dark or dark to light. One cautionary part of Light Responsive lenses must be told: In your car they will not change from light to dark because your windshield does not all UV rays to pass through.

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