Why we need different types of index lenses and how it is used in different frames
Chante Roets (B.Optom (UOFS) CAS(SA)
What is Lens Index?
The lens index of eyeglass lens material is an indicator of how efficiently the material refracts (bends) light. This efficiency is related to how fast light travels through the material.
The higher the refractive index of a material, the slower light moves through it, which results in greater bending (or focusing) of the light rays. So, the higher the refractive index of a lens material, the less lens material is required to bend light to the same degree as a lens with a lower refractive index.
In other words, for a given lens power, a lens made of a material with a high refractive index will be thinner than a lens made of a material with a lower refractive index.
Types of Lens Materials
1.5 Standard Index - Individuals with a slight prescription who wants to save money on lenses may opt for a 1.5 standard index lens. Full eyewear frames are most suitable for this lens index.
1.56 Mid-Index - The difference between 1.56 mid-index and 1.50 standard lenses is thinness. Full eyewear frames are most suitable for this lens index.
1.59 Polycarbonate - Polycarbonate lenses offer UV protection. The unique polycarbonate material makes lenses stronger and impact-resistant, which makes it a great lens for sport activities.
1.6 High Index - These ultra-thin lenses are suitable for relatively thin eyewear frames and high prescriptions. These lenses are also used in rimless frames as the lenses needs to be hardened and thinned.
1.67 High Index – These lenses surpass in thinness and lightness. Also used in rimless frames.
1.74 High Index - Prescriptions in need of the highest lens power may find favour with this type of lens. Also used in rimless frames.
Do I Need High Index Lenses?
It is always best to speak with an optometrist to determine whether you need high-index lenses for your prescription. Individuals with a relatively strong prescription may find high-index lenses ideal for vision correction. Other additional reasons that may influence the need for high-index lenses include:
Possibility with semi-rimless or rimless frames
Reduction of the “coke bottle” effect caused by thicker lenses