Thin High Index Lenses
High index lenses or thin lenses are a great upgrade for most eyeglass wearers. They provide a few key benefits over traditional lenses, which make them appealing. The primary one being cosmetic appeal, as no one likes the look of 'coke bottle' like glasses. They also tend to be lighter in weight because of the reduced thickness, but not always. High index lenses can provide improved visual acuity to people with higher eyeglass prescriptions, as the optical image does not get distorted as much especially in the periphery (think about regular windows vs glass block walls).
How do thin high index lenses work?
All ophthalmic lenses are designed to bend or focus light. The lens must be able to bend light infront of the retina for farsighted individuals and behind the retina in nearsighted individuals. Multiple factors determine how much the light gets bent, but the 2 key principles are as follows:
1. The shape of the lens. The rounder the curve the more the light is bent. Therefore higher eyeglass prescriptions require a greater curve. Unfortunately as the curvature increases, so to does the thickness of the material. In nearsighted people this causes the edges of the lens to appear thick, while in farsighted people the centre of the lens becomes thick.
2. The index of the material. When light enters a material it slows down and is shifted in direction. The higher the index of the material, the more the image is shifted. This allows for a reduced lens curvature to achieve the same image shift, and therefore a thinner lens can be produced. A great example of this is water. When we look down into a pond, the rocks or fish seem to be close to the surface. But when we actually reach in, we find out that its deeper than we thought. The water acts like a lens and bends the image closer.
Lens now come in numerous index's. Traditional CR-39 plastic lenses had an index of 1.50, while crown glass lenses had an index of 1.52. The newer products all range in index from 1.53 - 1.74. There is also one lens on the market made out of glass, with a index of 1.90, which can make even the thickest prescriptions seem paper thin.
Please keep in mind though that high index lenses also reflect light very well, which causes a glare across the lens surface. For this reason, we always encourage people to get an anti-reflective coating applied to their new high index lenses.
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