Surely you’ve heard of Abbe value, but what exactly is it? This article is not intended to show you a formula and assume you’ll make sense of it. This article is a comprehensive outline of Abbe value with considerations of the origins and the “why it matters.” With a greater understanding of Abbe value, you will be able to confidently factor in all variables when considering the best options for your patient. Key points, as well as a downloadable Abbe value chart, are also included in this article.
The origin of Abbe value
Because I appreciate the historical importance of things, here’s a bit about the who behind the Abbe value we all reference today. Ernst Abbe (1840 to 1905), was a German professor of physics, mechanics, and mathematics whose associates were Carl Zeiss and Otto Schott.1
“Abbe was a forefront researcher in optical glass, telescopes, optical aberrations, microscopes, and is known for his Abbe refractometer, Abbe prism, and Abbe value.”
In 1889 Abbe founded what is now one of the oldest science-funding foundations and named it after his friend and business partner, Carl Zeiss. The Carl-Zeiss-Stiftung Foundation provides funding for STEM research and teaching, financed from the dividends of the foundation companies Carl Zeiss AG and SCHOTT AG. The philosophy of Abbe’s original foundation statute of 1896 remains a model for modern labor laws today. In addition to all of that, the Abbe crater on the southern hemisphere of the moon was named after him.
What is Abbe value?
First of all, know that Abbe value only applies to transparent materials.2 Additionally, this formula applies to visible spectral lines; a different formula is used for non-visible light. The Abbe value (V) represents the amount of light dispersed into individual wavelengths while passing through a material as a numerical value.
The Abbe value, or Abbe number, is also sometimes called the “V-number,” however the latter of the three is more commonly used when referring to the non-visible spectrum of light. The higher the Abbe value, the less dispersion (chromatic aberration) in the material, and the better the optics.
Figure 1 demonstrates the formula to calculate the Abbe value of a material.
Figure 1: Created by Carissa Dunphy, ABOC, inspired by https://nvlpubs.nist.gov/nistpubs/jres/52/jresv52n6p303_A1b.pdf
Understanding the concept of Abbe value
All wavelengths of light (various colors) travel at the same speed of 186,000 miles per second through the air but travel through different materials at varying velocities.3 Longer waves have less interference and pass through material faster, whereas shorter waves have more interference and pass through slower. When referring to the formula above, each wavelength has its own value, known as Fraunhofer lines.4
“Essentially, the greater the distance between the values, the farther apart the rays, which results in a lower Abbe value.”
On the other hand, when the calculated values are closer and more equal, the light disperses less and has a higher Abbe value. When factoring in the index of refraction, the light hits the material and disperses more or less depending on its Abbe value. The selection of material, with consideration to many factors, is vital to your patient being satisfied with their vision.
Figure 2 shows the color ranges within the visible light spectrum and demonstrates spatial periods of wavelengths.
Figure 2: Created by Carissa Dunphy, ABOC, inspired by Curtis Barnes' Invitation to Biology, 5th edition
Download the Abbe Value Chart and Quick Reference Guide
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Abbe Value Chart and Quick Reference Guide
These guides will help you optimize the frame selection process by comparing the Abbe value of various lens materials to other key factors.
Using Abbe values to choose a lens material
There are many things to take into account when selecting a material, and we will touch on each of them below.
Abbe value and the index of refraction
The index of refraction is the amount that light bends (refracts) while passing through a medium. All light travels at equal speed through the air, but different mediums with different properties refract light differently. As the index of refraction increases, the amount that the light bends also increases.
Figure 3 displays the Abbe value of various lens materials in comparison to their respective index of refraction. The data is taken from Figure 5 below.
Figure 3: Created by Carissa Dunphy, ABOC
Within the variety of materials available to make spectacle lenses, there tends to be a correlation wherein the higher the index of refraction, the thinner the material becomes. In Figures 3 and 4, you can see how the Abbe value changes by material, and in general, the Abbe value decreases as the index of refraction increases. All materials possess their own properties, resulting in some that refract and disperse more light than others. Both the Abbe value and the index of refraction should be considered, and sometimes compared against each other, for every patient.
Figure 4 outlines the Abbe value and index of refraction of various materials. The data used for the table was confidential and collected from multiple optical laboratories.
Figure 4: Created by Carissa Dunphy, ABOC
Lens material index and specific gravity
While we are still considering materials, let’s also consider the weight. The term specific gravity refers to a material’s density ratio compared to that of water. If a material’s specific gravity is 2.54, it is 2.54 times heavier than water (which has a value of 1.0). The higher the specific gravity, the heavier the material.
On top of that, the higher the prescription, the thicker and, thus, heavier the lens. A higher index material is beneficial in a larger lens and/or higher prescription; however, thinner does not always mean lighter. We now have the Abbe value, the index, and specific gravity to keep in mind.
In a perfect world, the simple concept of thinness equating to lightness would be true, but unfortunately, it’s not. When comparing two lenses, where the material is the only difference, it is possible to have a lens that is both thicker and lighter when compared to a thinner and heavier lens because of the material’s properties. Take a look at Figure 5 and see how the values do not maintain a parallel trajectory when the index of refraction increases. To see another breakdown of lens materials and their respective index of refraction at a glance, download the cheat sheet.
Figure 5 demonstrates the index of refraction and specific gravity of commonly used lens materials. The data used for the table was confidential and collected from multiple optical laboratories.
Figure 5: Created by Carissa Dunphy, ABOC
The tenets of frame selection: frame choice, measuring, and optical labs
Power, lens thickness, and material choice can all increase dispersion. Additionally, frame selection is very important and can dictate the material when opting for groove mounts or drilled lenses; if the patient requests glass or wants a large or small frame, make sure your office has a wide variety of frames to accommodate any patient’s needs.
Well-measured glasses not only put the best vision for the wearer right where it’s supposed to be, but they also minimize extra material, especially around the periphery. Taking and providing any and all applicable measurements of the lens design can also ensure the lens ends up being as thin as possible.
Factors that can all play into minimizing chromatic aberrations for your patient:
- Select the appropriately sized frame to compliment the prescription.
- Be knowledgeable of lens designs and material availability so you can get the best within the options.
- Use a great lab that will select the best base curve for each prescription and frame.
- Choose a material based on frame size and prescription.
- Consider the Abbe value of the chosen lens material.
- Thicker edges mean more dispersion, so consider how much, or little, of a roll and/or polish would help minimize this.
- Discussing lens options and reviewing the prescription before frame selection can save time and avoid re-selecting frames.
A knowledgeable lab is vital to this process; your lab is there to help you succeed because when you do, they win too. A good lab will balance optics and cosmetics, which will, in turn, ensure your patient ends up with the best options. When in doubt, always call your lab to discuss options. They can look at availability and run lens thickness simulations based on lens design, material, and frame selection.
Reflections change with the index of refraction
Another factor for selecting the right material is considering the amount of reflections, which vary by the index of refraction. This makes recommending an anti-reflective (AR) treatment more important than ever. In the most basic of terms, the higher the index of refraction, the more reflections the material creates, and the more the wearer would benefit from AR to minimize reflections.
When comparing CR-39 to 1.74 ultra-high index, the percent of reflections nearly doubles, with all other indices in between falling in order respectively by index. It’s important to note that adding an anti-reflective coating or lens treatment can change the tensile strength of a material.
Availability of material or lenses
A lens material can dictate which lens designs are available to select from. For example, nearly every lens design on the market will be available in CR-39, slightly less in polycarbonate, and even fewer in trivex. It then gets more intricate when you get into high index and/or higher prescriptions.
If you want glass, then you have significantly fewer options and no newer technology designs available to select.
Whatever go-to lens family you sell the most of in your office, try to get a really good grasp of the lens design and material combinations or limitations. On top of “availability,” there is also what can actually be obtained—we all saw what happened during COVID-19 with trivex-lined multifocals; they are now non-existent. Sometimes the perfect combination is not an option to order, making this puzzle all the more complex.
Factoring in the cost to the patient
When taking all of the above into account, your cost from one patient to the next can vary tremendously.
Points to address with patients before ordering frames:
- All materials have different costs.
- Material and lens design can dictate one another as not every combination may be available to order.
- An anti-reflective coating will add cost.
- Lens material can dictate anti-reflective coating, which may vary in cost.
- Are the lens, material, and extras billable to the patient’s insurance plan?
- What will the net cost be to the patient after any insurance or discounts?
Figure 6 lists the cost of an uncoated spherical single-vision lens by the respective lens material. The data used for the visual was confidential and collected from a private optical laboratory.
Figure 6: Created by Carissa Dunphy, ABOC
Key features of commonly used lens materials
| Lens Material | Features |
|---|---|
| CR-39 | The most commonly used material, most lens designs available, least expensive, good optics, tints well, not full UV protection, not recommended for >+/-4.00D |
| Crown glass | Great optics, durable and scratch resistant, the original “King” and oldest material used, limited lens design choices, poor tintability, not recommended for impact resistance, thick and heavy |
| Trivex | Great optics, most impact resistant material, lightweight, UV protection, newest material, mid-range price point |
| Polycarbonate | Impact resistant, UV protection, very affordable, nearly all lens designs available, poor tintability, lowest Abbe value |
| 1.60 Mid-Index | Thin and light, UV protection, mid-range price point |
| 1.67 High-Index | Thin and light, UV protection, very good lens design availability, expensive, lowest Abbe value |
| 1.74 Ultra-High Index | Ultra thin and light, UV protection, fair lens design availability, less anti-reflective coating options, expensive, high reflectivity |
Weighing all the factors with your patient
By no means should you go over all of these options in this much detail with each patient. Most of the time, the flow of the interaction will go about the same, but the following are some other common patient-related factors to consider: a patient has had bad vision for so long they know what to expect, “regulars” trust you implicitly and tell you to do whatever you think is best, a patient has room for improved lens design and coating options from their last pair, the patient wants the least out of pocket cost, and the patient wants to go over every option possible and decide for themself.
Ǫuick recap of options to consider with your patient when selecting frames:
- Recommendation for material based on prescription and needs
- Abbe value
- Index of refraction
- Specific gravity
- Is a safety lens needed?
- Frame choice
- Additional lens options
- Cost to patient
Every patient has unique wants and needs, and it’s up to you to find the right solution. Don’t assume you know what they want or what their budget is. In some cases, a patient will have a high prescription and decline all recommended options. Other times you will have an end budget, and the patient would prefer to have thicker lenses to be able to afford the anti-reflective coating, or you’ll need to weigh the options between material weight and Abbe value.
Frame and lens selection pearls
If you have a spectacle history for the patient, see what has worked or not worked for them before. If you don’t have a spectacle history, this may warrant taking their current glasses to the workbench and performing some lens research to see what data you can get from any inscriptions. History or not, always get all manufacturer-recommended measurements per lens design. Do not skip measuring or rely on previous measurements, although these are sometimes valuable in rare cases for comparing or matching.
When you see any of these, you should strongly recommend a specific material:
- +/-4.00D total power of prescription
- Frame with a high wrap
- Oversized frame
- Previous material sensitivity
- Safety lens needed
Call patients back
If you’re uncertain about a product, before you promise your patient the world, get a good understanding of what is important to them and tell them that you will speak with your lab and call them back soon. Sometimes what you think might be a quick question turns into a goose chase. In the hopes of saving you some trouble, below are a few memorable things I’ve learned over the years by asking my lab, then calling the patient back with a positive “here’s what I’m going to make for you.”
Important notes for frame and lens selection:
- When used, know the tintability of different materials.
- Not all photochromic colors are available on all materials.
- One specific brand of anti-reflective coating cannot always be applied to all monomers.
- Polarized photochromics have very few lens designs and materials to select from.
- A blue light filter is rare in CR-39 but available in most other materials.
- Some prescriptions are out of range for some lens designs.
- Some lens designs and/or prescriptions have lens blanks that will not cut out with some frames.
Make sure to download the Abbe Value Chart and Quick Reference Guide
Conclusion
To summarize, all the factors we reviewed will net a significant difference to the wearer and should be considered when selecting lens material. These are the key points to remember when considering which lens material to select: Abbe value, index of refraction, the weight of the material, total cost after extras and insurance, and lastly, ask yourself if the selected frame will work and if all of these options are available together.
As the optician, it’s up to you to ask your patient the right questions and select the best products from within your options. Do your due diligence, and you’re bound to deliver a fantastic pair of glasses!
