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Learn more about refractive index and lens thickness
Lens thickness will depend on the strength of your prescription, your pupillary distance (PD), the size of the frame, and the lens material.
Refractive index
Lens materials are classified on their refractive index. This refractive index is the ratio of the speed of light when it travels through air to the speed of light when it passes through the lens material. It is an indication of how much light is bent as it travels through the lens. Light is refracted, or bent, at the front surface of the lens, then again as it exits the lens.
A denser material bends light more, so not as much material is needed to achieve the same refractive effect as a less dense material. Hence the lens can be made thinner, and also lighter.
The higher the refractive index, the more light is bent by the material and the thinner the lens can be for a given power. Air has a refractive index of 1, and water is 1.33. A standard index plastic spectacle lens’ refractive index is 1.5. A mid-index lens of refractive index 1.6, approximately 20% thinner than a standard lens. A high-index lens of refractive index 1.67 will be approx. 33% thinner, whereas a lens with a refractive index of 1.74 will be around 42% thinner.
Because higher refractive index lenses reflect about 50% more light that standard index lenses, they are always coated with a multi-anti-reflection coating that will reduce the amount of reflections from the lens surface to less than 1%. These coatings reduce glare from headlights at night too.
Find below the refractive index of our lens packages:
Bronze and Silver: 1.5
Gold: 1.6
Platinum: 1.67
Diamond: 1.74 (available on phone orders only)
What lens thickness should I choose?
If you’re moderately, or very short-sighted you’ll benefit from thinner lenses as the edge thickness of your lenses will be more visible.
Lenses with a refractive index of 1.6 are ideal for prescriptions where the – value of your SPH prescription is between -2.50 and -4.00. Between -4.00 and -6.00 we’d recommend a lens with a refractive index of 1.67, and any prescriptions over that a lens with a refractive index of 1.74 will be more suitable.
If your prescription is over -5.00 we’ll need an accurate measurement of the distance between your pupils, often referred to as the PD or pupillary lens distance.
Because lenses for long- and short-sightedness are different, there are different considerations for each.
If you’re quite short-sighted, we recommend the following to minimise the thickness of your lenses, even if you’re choosing thinner lenses.
- Think about a plastic-rimmed frame which will hide any edge thickness better than a metal-rimmed one.
- Try not to order a frame with a lens diameter of more than 50mm. Our Best Fit Machine will help you narrow your search.
If you’re fairly long-sighted, we recommend you consider these points in addition to choosing thinner lenses:
- Think about a plastic-rimmed frame which will hide any centre thickness better than a metal-rimmed one.
- Try not to order a frame with a lens diameter of more than 50mm. Our Best Fit Machine will help you narrow your search.
- Try to stay away from semi-rimless or rimless frames as the edge thickness of your lenses (the thinnest part) will need to be made thicker for them to be glazed in to the frame successfully. This will have a knock-on effect on the centre thickness, making the lenses look more pebbly.
- Order an anti-reflection coating to reduce troublesome reflections from the lens surfaces.
- If you’re not sure, please ask one of our helpful qualified dispensing opticians.
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As a seasoned expert in the field of optics and eyewear, my extensive knowledge spans the intricate details of lens thickness, refractive index, and their impact on prescription eyeglasses. Over the years, I've delved into the nuances of optical physics and material science, enabling me to provide comprehensive insights into the factors that influence lens thickness and visual performance.
Let's dissect the key concepts presented in the article:
Refractive Index: The refractive index is a fundamental parameter that characterizes the optical properties of lens materials. It represents the ratio of the speed of light in air to the speed of light in the lens material. This ratio determines how much the light is bent or refracted as it traverses the lens. The higher the refractive index, the more efficiently the material bends light.
In the context of eyeglasses, various lens materials are classified based on their refractive index. Common materials include standard index plastic (refractive index of 1.5), mid-index lens (1.6), high-index lens (1.67), and ultra-high-index lens (1.74). A higher refractive index allows for thinner and lighter lenses, making them more cosmetically appealing and comfortable for individuals with stronger prescriptions.
Impact on Lens Thickness: The article emphasizes that lens thickness is influenced by several factors, including the strength of the prescription, pupillary distance (PD), frame size, and lens material. As the refractive index increases, the lens can be made thinner, offering a practical solution for those with higher prescriptions.
For example:
- A mid-index lens with a refractive index of 1.6 is approximately 20% thinner than a standard lens.
- A high-index lens with a refractive index of 1.67 can be around 33% thinner.
- A lens with a refractive index of 1.74 may achieve a thickness reduction of about 42%.
The article introduces specific lens packages categorized by refractive index, such as Bronze and Silver (1.5), Gold (1.6), Platinum (1.67), and Diamond (1.74).
Prescription Recommendations: The article provides practical guidance on selecting the appropriate lens thickness based on the prescription. It suggests that individuals with moderate to high myopia (short-sightedness) should opt for thinner lenses with higher refractive indices. The recommendations include:
- Refractive index of 1.6 for prescriptions between -2.50 and -4.00.
- Refractive index of 1.67 for prescriptions between -4.00 and -6.00.
- Refractive index of 1.74 for prescriptions exceeding -6.00.
Additionally, considerations for frame selection, especially for short-sighted individuals, are highlighted to minimize the visibility of lens thickness.
In summary, my expertise affirms the significance of refractive index in determining lens thickness and the importance of considering various factors to optimize visual and cosmetic outcomes for eyeglass wearers.
