An Understanding Of Ophthalmic Lenses

All lenses serve one simple function, to bend light in a desired manner. This allows us to focus light where the user needs it. Whether it is in a telescope, camera or glasses (even contact lenses), they all behave in the same manor. Different lens properties allow this to be customized in ways that change weight, thickness, distortions, reflectance and many other properties. Through this article, you will develop a basic understanding of how these lenses work, some specific advantages and disadvantages to each, and why one might be better than another for use in your glasses.

When light enters any substance, it slows down. If the substance – let’s say glass – is uniform in thickness, it will slow down until it exits the glass, and then speed back up. If it is passing straight through, it will exit straight through.

If however, the glass is thicker on one end than the other, the slower moving light will bend towards the thicker end then straighten out once it regains speed upon exiting the light.

Concave Vs Convex

Knowing this property of how a light is bent, we are able to design lenses to bend light, thereby focusing it, exactly how we need.

Indexes Of Refraction

The index of refraction is obtained by dividing the speed of light in a vacuum by the speed of light in the medium. So a vacuum has an index of refraction of “1”. The slower the light travels in a substance, the higher it’s index of refraction. Air for instance, slows down the speed of light ever so slowly and has an index of refraction of “1.000293”. Water is “1.33” and Crown Glass – the 1st material glasses were made of – has an index of “1.523”.

Here is a list of the most common indexes of ophthalmic lenses:
1.50 – CR-39 (Plastic)
1.523 – Crown Glass
1.56
1.589 (1.59) – Polycarbonate
1.61
1.67
1.74

The higher the index of refraction, the slower light travels, therefore the more it s bent. Higher indexes allow light to bend the same amount in a thinner material as compared to lower indexes. This is why, especially in larger prescriptions, higher indexes are often recommended.

Weight, Reflectance & ABBE Values

Weight

It makes sense that the thinner the material is, the lighter it will be, and in many cases that is correct. However, as a general rule, higher indexes of material, carry a higher density, or essentially, weight. This density is measured as “Specific Gravity” and again here is a list of common lenses along with their indexes of refraction and specific gravity:

1.50 – CR-39 (Plastic)specific gravity = 1.32
1.523 – Crown Glassspecific gravity = 2.54
1.56specific gravity = 1.28
1.589specific gravity = 1.21
1.61specific gravity = 1.3 – 1.34
1.67specific gravity = 1.37 
1.74specific gravity = 1.47

Let’s look at opposite ends of the spectrum – 1.50 plastic versus 1.74 plastic. The higher index of 1.74 will bend the light much faster than 1.50, however the weight of the material is 11.36% more. On a small prescription, the light does not need bent much. The thinner 1.74 material might achieve a 5% thinner lens, but the weight would still be more because of its’ higher specific gravity. On a much higher prescription however, the lens might be 50% thinner with a 1.74 lens. The material is 11% heavier, but you only need half as much. So for the higher prescriptions, 1.74 plastic is not only thinner, but also lighter.

Reflectance

All lenses have surface reflections. These are noticeable as glare from headlights & streetlights while driving at night, off of computer screens, etc. Again as a general rule, the higher the index of refraction, the more reflectance off of the lens surface. Most eye care professionals recommend Anti-Reflective (A/R) lenses and this is more necessary as the lens’ index of refraction increases. 1.74 lenses for instance, are not even made without A/R. At Payne Glasses, rest assured all of our lenses come with A/R included!

ABBE Value

When light hits a prism, it is not only bent, but the color becomes separated. This means white light can be split into all the colors of the rainbow. The amount to which this happens is called “Chromatic Aberrations” and is measured as ABBE Value. Once again as a general rule, the higher the index of refraction, the more chromatic aberrations there are.

A Little More About the Lenses:

1.523 – Crown Glass

The 1st lens material for ophthalmic lenses, in use for hundreds of years. This material has excellent optics and is nearly impossible to scratch. However it’s excessive weight and break-ability have made it mostly obsolete with newer materials.

1.50 – CR-39 Plastic

A revolution in eyeglass lenses. This was the 1st alternative to glass. It maintained great optics with a significantly lighter profile than glass, it scratches easy and does not block out Ultra-Violet radiation well therefore needing to be coated. It is still used, however only the smallest of prescriptions would not benefit from more advanced, higher index lenses.

1.56 – or is it 1.57?

Many places offer 1.57 index lenses, leading people to believe it is thinner than 1.56. In reality, it is just a rounding error. Places advertising 1.57 lenses are technically using 1.56 lenses. They are the same! Regardless, this is an excellent lens. At Payne Glasses, our software shows that under most low, medium and even some higher prescriptions, this is the lens of choice. It combines great optics with a thin, lightweight design. It really marries the advantage of 1.50 plastic and the higher index lenses to give a great choice under many circumstances.

1.589 – Polycarbonate

Polycarbonate has a slightly better index of refraction than 1.56 lenses, however its’ main drawback is the high amounts of chromatic aberrations. Its’ advantage on the other hand is it is practically shatterproof. This property makes it a must use for children, sports or safety glasses, and others needing the ultimate protection.

1.61

This is a lens which struggles to find its’ place. It is slightly thinner and lighter than 1.56 or polycarbonate, however not much. Add to this the increased cost and it becomes a lens material not often recommended.

1.67

This is the 1st lens that offers a true advantage over 1.56 lenses. Higher prescriptions will certainly appreciate the reduced weight and edge thickness of this lens and will most likely tolerate the slight increase in aberrations.

1.74

The thinnest and lightest lens available for the highest prescriptions. Costing more than other lenses can make it less appealing, however when the prescription needs it, this is the lens to choose.

Again, at Payne Glasses, our software does all the work. This insures you are recommended the best marriage of weight, optics and thinness for your unique prescription, eyes and frame.

What About Transition Lenses

Transition lenses are a brand name of lenses that we call “Light Responsive”, or photochromic. This means they react to light by darkening when in sunlight. More specifically they adapt to UV radiation. This keeps them from becoming dark indoors. Perhaps their greatest feature is the variability of darkening that occurs. Of course inside or low sunlight areas they are clear, and outside in the direct sunlight they darken. What about when you are in the shade or a partly cloudy day? They partly darken! They really do give you just the right amount of tint for the sunlight you are in. Because they are triggered by UV radiation, they do not get very dark while driving because the windshield blocks most of the UV..

Transitions have been around for quite some time. The 3 big areas that have seen improvement in Transition lenses are:

Darkening ability – newer Transitions darken significantly more than prior generations.
Speed at which they darken – newer Transitions darken significantly quicker.
Speed at which the lighten – again newer Transitions lighten much quicker.

There are many other brands of photochromic lenses. They generally darken as fast as the prior generation of Transition lenses. They are significantly less expensive, however the technology may lag behind the brand name Transition.

Hopefully you now have a better idea all the variables in choosing lenses for your new glasses. Overwhelming? Maybe. But rest assured, we are here to help you!