1.74 High Index Lenses | Progressive Glasses

FAQ

Common indexes of ophthalmic plastics:

1.50 (CR-39 plastic), 1.56, 1.59 (Polycarbonate), 1.61, 1.67 & 1.74

What about weight?

With few exceptions, the higher index materials weigh more than lower indexes. 1.74 weighs the most with a specific gravity of 1.47 - However, since less lens material is needed to achieve the desired outcome, the overall weight of the lens is less compared to other materials with the same refractive power (Refractive power is called Diopters)

Do they have UV protection?

High index lenses, compared to CR-39 also block 100% UVA & UVB. With no exceptions, higher index plastics have higher rate of reflectance. As a comparison, CR-39 Plastic reflects 7.7% of all light. 1.74 reflects 13.6% of all light. Because of this, higher index lenses should have Anti-reflective (A/R) coatings. 1.74 is the only lens that automatically includes A/R, giving it ~ 0.5% reflectance.

Other considerations?

Higher index lenses, because they refract better, do not need to be as round. The front surface can be more flat than lower index lenses. This allows them to better fit in frames which are typically more flat on the front surface

Flatter front surfaces also reduce "Spherical Aberrations". This is "variation in focal length of a lens or mirror from center to edge, due to its spherical shape." Basically spherical aberration means that the farther from the center of the lens, the more imperfect refraction. Flatter lenses have less spherical aberration.

1.74 is also have great tensile strength, so lenses that need to be drilled, don't fracture as much as lower index lenses (except polycarbonate which is also great for drilled lenses).

Most of these properties apply to all higher index lenses, with 1.74 being the most advanced, highest index plastic lens, therefore these good properties are best in 1.74 material.

1.74 = thinnest, lightest lens with least amount of spherical aberration. 100% UV blocking and A/R included.

FAQ

What are the common indexes of ophthalmic plastics:

The most common forms of index materials and prescriptions are 1.50 mid-index lenses (CR-39 plastic), 1.56 mid-index lenses, 1.59 polycarbonate lenses, 1.61 high-index lenses, 1.67 high-index lenses, and 1.74 high-index lenses.

What's proper weighting for a lens?

As a general rule of thumb, the higher the index, the less they weigh. 1.74 weighs the most with a specific gravity of 1.47. Since less lens material is needed to manufacture this lens, the overall weight of the lens is significantly less compared to other materials with the same refractive power (diopters).

Are our glasses UV protectant?

Yes. 1.74 high index lenses are the only lenses on the market that naturally includes an anti-reflection percentage. The percentage is 0.5% reflectance whereas other materials like high index plastics have a higher rate of reflectance. Plastics like CR-39 reflects 7.7% of all light. High index lenses, compared to CR-39, also 100% block UVA & UVB. 1.74 reflects 13.6% of all light. Because of this, higher index lenses should have Anti-Reflective (A/R) coatings.