When it comes to choosing your glasses, it may feel like the options are endless. You’ve painstakingly looked through a myriad of different frames and settled on the ones that best suit your aesthetic—only to be offered an array of lens coatings.
You don’t need to feel overwhelmed by all the options presented. Knowing the basics of popular lens coatings can help you feel prepared before your next optometrist appointment.
Lens coating options can include:
So what’s the best lens coating for glasses? The best lens coating can depend on your vision needs and individual preferences. Our eyewear experts on staff will be more than happy to explain the benefits of each lens coatings at your next visit.
Let’s break down some of the most common and useful coatings that can be applied to your lenses.
Anti-reflective coating helps you both see and be seen better. Without this coating, the light that reflects off your glasses can render it difficult for those looking at you to see your eyes. Cosmetic and communication concerns aside, the reflection of light can also interfere with vision.
AR coatings are applied to both sides of your lenses, so light is also prevented from bouncing back into your eye. These coatings allow you to take in as much light as possible without the distractions of glares.
Anti-reflective coating can be particularly beneficial if you spend lots of time on the computer or driving at night.
If you’ve ever walked into a warm building from a chilly winter day, you know that the subsequent fogging of your glasses can make you feel like you’re dropped a sheet over your eyes. If you weren’t longing for the visual experience of a half-finished ghost costume on Halloween, you might want to invest in an anti-fog coating on your lenses.
Fogged-up glasses aren’t only a byproduct of winter days, either. Condensation can occur when you’re exerting yourself during exercise or while wearing a face mask, as many of us have discovered since 2020.
Anti-fog coatings work by introducing a coating that gets in the way of the water droplets forming together to create condensation.
Blue light is a part of the visible light spectrum. It consists of short, fast-moving waves and sits near the ultraviolet rays on the light spectrum.
The sun is by far the biggest producer of blue light, but our digital devices and LED lighting emit blue light, too. Blue light may have a part to play in digital eye strain.
While blue light serves the purpose of keeping us alert during the day, too much blue light exposure can interfere with our circadian rhythms, making it difficult to fall asleep.
A blue light filtering applied to your glasses can shield your eyes from an excess of blue light and may even help you have a good night’s sleep with ease.
UV protection isn’t just for sunglasses. UV rays can harm your eyes, increasing the risk of cataracts, age-related macular degeneration, and eye cancer.
There are 3 types of UV Rays:
Look for a UV coating that offers 100% protection from UVA and UVB rays, so your eyes are protected at all times throughout the day.
Accidents happen, and there’s little that’s more frustrating than fumbling your glasses, only to pick them up and find a scratch right across your field of vision. Nearly anything can scratch lenses—even using the incorrect cloth to clean them.
It should be noted that nothing can make your glasses absolutely scratch-proof. It’s always necessary to take good care of your glasses by using the appropriate cleaning cloth and storing them in a protective case when they’re not on your face.
While scratches likely won’t impact your vision long-term, they can be distracting and degrade the quality of your lenses. Use scratch-resistant coating to bolster your lens’s lifespan.
When choosing the right lens coating for your lifestyle, you don’t have to go it alone. The friendly staff at Eye Wellness is on hand to help you make the best choice for your vision. Ask about lens coatings when purchasing your next pair of glasses.
Introduction
Corrective spherocylindrical lenses are commonly used to treat refractive errors such as myopia, hyperopia, presbyopia, and astigmatism. Both lenses and prisms are also frequently used to improve eye alignment and treat diplopia in strabismus. Eyeglasses also serve an important role in protecting the eyes from physical trauma and harmful radiation. Lenses can be produced using a variety of materials and designed with several optical profiles to optimize use in specific applications. Critical lens properties include refractive index, Abbe number (chromatic dispersion), specific gravity, and ultraviolet absorption.[1]
Lens Material Properties
Index of Refraction
Figure 1. Indices of refraction of various materials and lenses.[2]. Indices of refraction of various materials and lenses.
Refraction is defined as the change in direction of a wave of light as it passes from one medium to another.[2] The amount that the light changes direction depends on the indices of refraction of each medium, and is described quantitatively by Snell's law (Equation 1). The index of refraction of a material describes how fast light travels through a material and is defined by the speed of light in a vacuum (3 x 108 meters per second) divided by the speed of a given wavelength of light in the medium (Equation 2).[2] The indices of refraction of a few materials and lenses are listed in Figure 1.
Equation 1. Snell's Law relates the indices of refraction (n) of two media to the directions of propagation in terms of the angles to the normal.[2]Snell's Law relates the indices of refraction () of two media to the directions of propagation in terms of the angles to the normal.
Equation 2. The index of refraction (n) defined as the speed of light in a vacuum (c) divided by the speed of light in the medium (v).[2]The index of refraction () defined as the speed of light in a vacuum () divided by the speed of light in the medium ().
A common misperception is that lens materials with a higher index of refraction have a higher physical density (mass over volume). In reality, it is not the physical density but rather the optical density of the material that determines the speed of a light wave through a medium. The optical density, or absorbance, of a material is a logarithmic ratio of light intensity entering versus exiting a particular material. The change in intensity is due to the tendency of atoms to dissipate some of the energy through vibration as light passes through it.[3]
Figure 2. Chromatic aberration results in a single beam of light to have a different focal point for each wavelength.[4]. Chromatic aberration results in a single beam of light to have a different focal point for each wavelength.
Chromatic Dispersion
Chromatic dispersion, also referred to as chromatic aberration, is a phenomenon where the speed and direction of light propagation in an optical medium depends on the wavelength of light. The interaction of the particular wavelength of light with electrons in the medium gives rise to this phenomenon.[5] When a single beam composed of multiple wavelengths of light passes through a lens, each wavelength of light will change direction and speed to different extents. As a result, the index of refraction for a given medium will vary based on the wavelength of light going through it.[6] In Figure 2, a single lens demonstrates chromatic aberration as different wavelengths of light have different focal lengths after passing through the lens.[4] Each wavelength of light will thus be focused at a different distance away from the lens. This can be visualized on images at bright to dark boundaries as “fringes” of color.[6][7]
Abbe Number
Figure 3. Calculation of the Abbe number to describe the amount of chromatic dispersion for a medium.[8]. Calculation of the Abbe number to describe the amount of chromatic dispersion for a medium.
In the late 1800s, German physicist Ernst Abbe, a colleague of Carl Zeiss, created the Abbe number, a measure of the magnitude of chromatic dispersion for the visible spectrum.[9] Also known as the V-number, the Abbe number combines the refractive indices of a material at three wavelengths (486.1 nm blue from hydrogen, 589.2 nm yellow from sodium, and 656.3 nm red from hydrogen) to approximate the amount of chromatic aberration a material will have when visible light passes through it (Figure 3).[8] The Abbe number is inversely related to the amount of chromatic aberration. A high Abbe number indicates low chromatic aberration, and a low Abbe number indicates high chromatic aberration.
Abbe Diagram
The Abbe diagram is created to compare various glass types based on Abbe number and refractive index for a certain wavelength.[1] As demonstrated in Figure 4, there is a general trend of increased dispersion (a lower Abbe number) in high-index glasses.[10] Using the Abbe diagram, glasses are generally categorized as crown glass (last letter “K”) or flint glass (last letter “F”) based on their Abbe number.[1]
Figure 4. The Abbe diagram compares various glass types by plotting its refractive index at a wavelength of 587.6nm and Abbe number. The red dots denote each glass type, and the blue labels subcategorize the glass types.[10]. The Abbe diagram compares various glass types by plotting its refractive index at a wavelength of 587.6nm and Abbe number. The red dots denote each glass type, and the blue labels subcategorize the glass types.
Crown vs Flint Glass
Crown glasses are optical glasses with low chromatic aberration (Abbe number above 55 or 50) which tend to have low refractive index.[11] Many optical components such as lenses, mirror substrates, optical windows, and prisms are made from crown glasses. They are particularly useful for imaging applications as they minimize chromatic aberrations.[11]
Flint glasses are optical glasses with high chromatic aberration (Abbe number below 50) which tend to have a high refractive index (typically greater than 1.55).[12] Corrective glass lenses are often made from flint glass as the higher refractive index allows for a lighter weight. Dispersive prisms are also often constructed from flint glass to achieve a high angular dispersion.[12]
Chromatic Dispersion Correction
Chromatic aberration can be corrected by combining the crown and flint glass elements to make a compound lens. This is achieved by apposing the two lenses and shaping them so that the chromatic aberration of one is neutralized by the other. The ultimate goal is to reduce the chromatic focal shift—the distance between focal points of dispersed wavelengths by a lens—between different wavelengths on the visible spectrum.[13]
This compound lens can be optimized at a distance of either two wavelengths (achromatic lens), three wavelengths (apochromatic lens), or four wavelengths (superachromatic lens).[14] This point of optimization where chromatic aberration is minimized is called the circle of least confusion.[15] The achromatic lens is the most commonly used commercial lens, referred to as a doublet, and can drastically reduce the chromatic focal shift.[13]
Specific Gravity
Specific gravity is a ratio of a material’s density to the density of water at 4 degrees Celsius. Density and specific gravity can both compare substances in terms to the amount of mass per unit volume. Density is simply mass per unit volume, however specific gravity is a ratio and thus has no units. A lens with a low specific gravity means that an identically sized lens of a high specific gravity would be heavier. This property is relevant in designing lenses as a lighter lens is preferable for eyeglass wearers.[16]
Ultraviolet Absorption
The human eye does not perceive ultraviolet radiation, however the effects of ultraviolet exposure can be harmful to the eye. Transmission of ultraviolet radiation through ophthalmic lenses may be reduced or blocked depending on the lens material.[17]
Ultraviolet light can be broadly categorized into UVA (400-320 nm), UVB (320-290 nm), and UVC (290 nm to 10 nm). Ultraviolet light makes up 5% of all solar radiation falling on the Earth; among the 5%, 90% is composed of UVA and 10% of UVB. Nearly all UVC is absorbed by the ozone layer of the atmosphere. UVC exposure typically occurs from manufactured resources. Ultraviolet light can cause damage to tissue by disrupting molecular bonds and incapacitating their function. These molecules can also be damaged by action of free radicals produced as a byproduct of ultraviolet light in the presence of oxygen and a photosensitizing pigment.[16]
In order to protect the eyes from this radiation, lenses with ultraviolet-absorbing capabilities can be used. Intraocular lens implants, for instance, have chromophores that absorb ultraviolet light. Nearly all dark sunglasses absorb most incident ultraviolet light. Certain coated clear-glass lenses and clear plastic lenses made of CR-39 or polycarbonate also have strong ultraviolet-absorbing capabilities.[16]
Common Eyeglass Lens Materials
Standard glass
Figure 5. Summary of common lenses and their properties.[18]. Summary of common lenses and their properties.
Glass had been the material most widely used for ocular lenses until the 1970s. Glass provides superior optical quality and has the most scratch-resistance surface, however it has several limitations including heavy weight, increased thickness, and low impact resistance. Glass lenses must be treated to comply with the American National Standards Institute impact resistance standards.[17] Chemical or thermal tempering can increase the shatter resistance, however this effect is lost if the lens is scratched or worked on with any tool after tempering. Individuals with myopia who desire thin glasses may opt for high-index glass, however the highest-index glass lenses cannot be tempered and require patients to sign a waiver accepting the risk of breakage. Additionally, high-index glass does not block ultraviolet light without a coating.[16]
Standard plastic
Plastic lenses gained popularity in the 1970s and have the benefits of weighing half as much as glass lenses due to their lower specific gravity and having high optical quality.[17] CR-39 which stands for Columbia resin #39 is the most commonly used plastic polymer lens material. The lenses block 80% of ultraviolet light without treatment, and can be tinted and coated to provide further ultraviolet light blocking. Plastic lenses tend to have a lower index of refraction, which require thicker lenses. The lens surface is also softer and thus easier to scratch, however scratch-resistant coatings are available to create a harder surface. CR-39 lenses in particular do not have the shatter resistance of polycarbonate or Trivex, increasing risk to wearers.[16]
Polycarbonate
High index polycarbonate lenses were popularized in the 1980s due to light weight, thin profile, superior impact resistance, and ultraviolet protection. These lenses are often recommended for children, young adults, individuals with active lifestyles, and as safety eyewear. They are very durable and can be up to 30% thinner than regular glass or plastic lenses.[17] Disadvantages include high chromatic aberration indicated by its low Abbe number, which results in color fringing most noticeable in strong prescriptions. Additionally, polycarbonate is the most easily scratched plastic, thus requiring a scratch-resistant coating.[16]
Trivex
Trivex was introduced in 2001 and is a highly impact-resistant material with a low specific gravity delivering strong optical quality and minimal chromatic aberration indicated by its high Abbe number. Trivex is also able to block nearly all ultraviolet light. A disadvantage of Trivex lenses is its low index of refraction, thus requiring thicker lenses for higher powers. At the ±3.00 Diopter prescription range, this material allows for a comparably thin lens. Trivex is the lightest material available and meets high-velocity impact standards. A scratch-resistant coating is required for this lens.[16]
High-index materials
High-index materials are defined by a refractive index of 1.60 or higher, and can be either glass or plastic. The main utility of high-index lenses is for high-power prescriptions to create thin and cosmetically attractive lenses.[16] The weight, optical quality, and impact resistance of high-index lenses vary based on the material used. For high-index glass, the specific gravity tends to run high, which means that these lenses are often heavy compared to other materials. None of the high-index materials pass the American National Standards Institute’s impact resistance standards.[16]
References
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Zhang, John X. J., and Kazunori Hoshino. Molecular Sensors and Nanodevices: Principles, Designs and Applications in Biomedical Engineering. 2nd ed., Academic Press, an Imprint of Elsevier, 2019.
Mellish, Bob. "Chromatic aberration lens diagram". 6 March 2006. https://upload.wikimedia.org/wikipedia/commons/a/aa/Chromatic_aberration_lens_diagram.svg
Paschotta, Rüdiger. Encyclopedia of Laser Physics and Technology: Chromatic Dispersion. Wiley-VCH, 2008.
Elert, Glenn. "Aberration". The Physics Hypertextbook, 2020. https://physics.info/aberration/
Kruger, P. B.; Mathews, S; Aggarwala, K. R.; Sanchez, N (1993). Chromatic aberration and ocular focus: Fincham revisited. Vision Research. 33 (10): 1397–411.
Günther, Norbert. “Abbe, Ernst”. Dictionary of Scientific Biography. Charles Scribner’s Sons. New York, NY, 1970.
Mellish, Bob. "Abbe diagram". 28 May 2010. https://upload.wikimedia.org/wikipedia/commons/b/bd/Abbe-diagram_2.svg
Paschotta, Rüdiger. Encyclopedia of Laser Physics and Technology: Crown Glasses. Wiley-VCH, 2008.
Paschotta, Rüdiger. Encyclopedia of Laser Physics and Technology: Flint Glasses. Wiley-VCH, 2008.
Wang, L; Oku, H; Ishikawa, M. “An adaptive achromatic doublet design by double variable focus lenses”. Novel Optical Systems Design and Optimization XVII (2014). doi:10.1117/12.2061203.
Cmglee. "Comparison chromatic focus shift plots". 10 July 2013. https://upload.wikimedia.org/wikipedia/commons/2/23/Comparison_chromatic_focus_shift_plots.svg
Hosken, R. W. (2007). "Circle of least confusion of a spherical reflector". Applied Optics. 46 (16): 3107–17.
American Academy of Ophthalmology. “Section 3: Clinical Optics” Basic and Clinical Science Course: 2017 - 2018(2017).
Wohlever, Richard. The Indispensable Dispensing Guide: The Eyecare Professional's Basic Dispensing Handbook. Optical Laboratories Association, 2004.
Meister, D; Sheedy, J. Introduction to Ophthalmic Optics. Carl Zeiss Vision, 2008.