High Refractive Index Lenses
Craig Banda B.Optom(UJ), ND Optical Disp(CPUT)
What are high index lenses?
These lenses have a higher index of refraction and therefore bend light more efficiently than regular lenses. As a result of this, high index lens materials produce a thinner lens. The higher the index, the thinner the lens. They are ideal for people with high spectacle prescriptions, where cosmetics are of concern.
Lens index range:
A standard index is 1.50 and 1.52 for plastic and glass lenses respectively. Plastic goes up to 1.74 whereas glass 1.80. Glass lenses are usually thinner, but also heavier when compared to plastic. Plastic is also a safer option as it is less likely to shatter. It’s definitely the most prescribed material. Commonly used high indexes are:
1.60 (High Index Plastic)
1.67 (High Index Plastic)
1.74 (High Index Plastic)
How much thinner are high index lenses?
This depends on several factors, including:
The choice of index
Design of lens (aspheric)
A lens with a refractive index of 1.67 and higher can be less than half the thickness of a regular plastic lens. In saying that, a polycarbonate lens (1.59) may only be 20% thinner than standard index lenses.
A high index lens with an aspheric design will also further reduce the thickness of your lenses. Aspheric lenses are made with flatter curves and therefore have a slimmer side profile. They also reduce magnification or minification, not only of the image you are looking at but also of the eyes, as often experienced with stronger lenses.
*Choose the smallest frame suitable for your face; the smaller the frame, the thinner the lens.*
Due to the nature of high index lenses, they unfortunately cause reflections within the material, which can be disconcerting for the wearer. An anti-reflective coating is therefore essential to reduce this effect.
Anti-reflective coatings also result in high index lenses, virtually appearing invisible. Others see your eyes, not the distracting reflections in your lenses. Due to fewer reflections, your vision is also crisper, especially at night when faced with lights of oncoming traffic.
As with anything, high index lenses have advantages and disadvantages but ultimately can be the solution for you. Before purchasing new lenses, discuss your options with your optometrist.