When it comes to the thickness of your lenses there is quite a lot to consider. The thickness is usually down to the prescription level that you need and the material that is used.
Some of the materials that you may find on lenses are as follows:
At the very start of the vision correction journey, all glasses lenses that were made were made from glass.
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Although this type of lens offers exceptional optics, it can be broken easily and they are heavy. This can cause potential harm to your eyes and in some cases even loss of an eye. For these reasons, glass is no longer commonly used for glasses.
The Armorlite Lens Company in California was the first to introduce lightweight plastic lenses for glasses in 1947. These lenses were made from the plastic polymer Columbia Resin 39 (CR-39) because it was the 39th formula of the thermal-cured plastic that was developed by PPG Industries,1940s.
This lightweight material weighed about half of the glass. It was a low cost solution and had brilliant optical qualities. The CR-39 plastic is still a popular choice for material in today’s spectacle lenses.
The Gentex Corporation introduced the very first polycarbonate lenses in the early 1970s. These were designed for safety glasses. Later in the 1980s, polycarbonate lenses became more popular and they are still popular today.
They were originally designed to be used as helmet visors in the US Air Force, for many safety applications, and for the bulletproof glass you see in banks. Compared to CR-39 plastic, polycarbonate is significantly more impact-resistant and lighter which is why it is a much-preferred material for children’s eyewear sports eyewear, and safety glasses.
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The lightweight glasses lens material that has similar impact-resistant properties as polycarbonate is known as Trivex (PPG Industries), This has been used in eyewear designs since 2001. Trivex has a higher Abbe value which is seen as an advantage.
High-Index Plastic Lenses
Over the last 20 years, there has been a demand for lighter and thinner glasses. So, a number of manufacturers of lenses, have now introduced high-index plastic as an option. These are much thinner than CR-39 plastic because of the higher index refraction and are lighter.
The materials that you find in glasses are classified by looking at their refractive index. The refractive index is the ratio of the speed of light when it is traveling through the air compared to the speed of light when it has to travel through a lens material. It gives you an idea of how much light is bent as it goes through the lens. The light is bent or refracted on the front surface of the lens material, and also when it leaves the lens, on the back surface.
When using a denser material more light will be bent, therefore, not as much information is needed to get the same level of refraction as a less dense material. This means the lens can be made lighter and thinner.
A higher refractive index means that more light is being bent as it travels through the material. The thinner the lens and the more light that is bent can be for a given power.
Have a look below at some levels of refraction:
- Air = 1.1
- Water = 1.33
- Standard index plastic glasses lenses = 1.5
- The mid-index lens of refractive index = 1.6 (which is approximately 20% thinner than a standard lens)
- A high-index lens of refractive index =1.67 (approximately 33% thinner, a lens with a refractive index of 1.74 will be approximately 42% thinner)
A higher refractive index lens will be able to provide a 50% higher refraction than that of a standard lens. The higher refractive index lenses are always coated using a multi-anti-reflection coating that provides a reduction in the number of reflections from the lens surface ( less than 1%). These are able to reduce the glare from computer screens and headlights too.
What Lense Thickness Should I Choose?
If you are very short-sighted or moderately short-sighted it is likely that you will benefit from using thinner lenses. This is because the edge thickness of your lenses will be more noticeable. Professionals will usually use a lens thickness calculation to assess the prescription. Lenses that have a refractive index of 1.6 are recommended for prescriptions where the SPH value stands between -/+2.50 and -/+4.00. If you have an SPH value between -/+4.00 and -/+6.00 you would be better off with a lens that carries a refractive index of 1.67.
If the prescription level that you require is more than -/+5.00 you may be asked for accurate measurement of the distance between your pupils, this is known as the PD. It’s a good idea to read more about pupil distance if this applies to you. Because the lenses that are needed for long-sightedness and short-sightedness are different, there will be different considerations that need to be looked at for each one.
If you are slightly short-sighted, then it’s best if you minimize the thickness of your lenses, even if you are opting for thinner lenses. Try some of the following to achieve that:
- Consider plastic rimmed frames that will hide any edge thickness much better than a metal frame.
- Try to avoid ordering any frames that have 50mm plus lens diameter.
If you are slightly long-sighted, then you may want to consider the following additional points to choose thinner lenses:
- Consider choosing a plastic rimmed frame that will help to hide any central thickness much better than a metal frame.
- Try to order a frame that is greater than 50mm in diameter.
- Try to avoid semi-rimless or rimless frames. The edges of these lenses will need to be thicker in order for them to be glazed correctly. This could impact the thickness of the central point and make the lenses look a little pebbly.
If you want to reduce troublesome reflections such as headlights or computer screen glare then choose an anti-reflection coating to help.
If you are unsure, always speak to a professional who will be able to point you in the right direction.