Lens coatings might seem like a minor detail, but they play a crucial role in the performance and clarity of glasses. Often underestimated, JUSTIN CHIANG explains why they shouldn’t be.
Let’s start with multicoats. As the name suggests, it comprises multiple layers of different materials, each with a specific function. These layers can manipulate light in various ways, such as reflecting certain wavelengths or enhancing transmission. Multicoats enhance lens properties like light transmission, reflection, and durability. Although multicoats are much more complicated and used in many industries for many
purposes, in the optical industry, “multicoat” often refers to anti-reflective (AR) coatings, although these are just one type.
How AR coatings work
Reflections occur when light hits a surface and bounces back. AR coatings use a “thin film” to manipulate light interference. There are two types of interference:
• Constructive interference – this occurs when materials with similar refractive indices are layered together. In this phenomenon, the crests and troughs of light waves align, amplifying each other, which is exemplified in mirror coatings.
• Destructive interference – this is when two wavelengths with the same amplitude are exactly half a wavelength out of sync. The crest of one wave will match up with the trough of another wave, and the two will cancel each other out.
An anti-reflective coating is made up of a stack of ultra-thin layers, some highly refractive and some low refractive index materials. These alternating layers create the desired destructive interference, eliminating reflections and allowing more light to pass through to the eye.
The importance of AR coatings
Why are AR coatings necessary? When light encounters a lens, a portion of it reflects off both the front and back surfaces. This reflection reduces the amount of light that ultimately reaches our eyes. This reduction can range from 7.7% to as much as 17%, especially in high index material, and that’s before considering the light lost due to the lens material and its thickness. Furthermore, this reflection leads to a phenomenon known as ‘ghosting’ or ‘halos’. Ghosting manifests as a faint, secondary image slightly offset from the primary one, while halos appear as bright circles or rings surrounding light sources. Therefore, AR coatings are essential to maximise light transmission and minimise these visual distractions.
“As our understanding of light and optics deepens, we can expect even more sophisticated coatings with enhanced visual performance and protection.”
Beyond the visual enhancements, multicoats act as a crucial barrier between the lens and environment. Since lens materials and hardcoats are organic, the inorganic multicoat layer serves to block moisture and chemicals, preventing them from deteriorating the lens material. For example, a tinted lens with only a hardcoat will show colour fading and changes much faster than one with a multicoat. Although some wearers may dislike the multicoat bloom colour on sunglasses, some manufacturers do offer a front vacuum deposition coating without bloom, along with a back surface anti-reflective coating.
The benefits for wearers
While the light lost due to reflection might seem minimal under normal lighting, it becomes critical in low-light conditions, such as night driving. Individuals over 65 require significantly more light than 18-year-olds, making AR coatings essential for them.
Modern AR coatings typically have a reflectance of around 0.8%, but advanced coatings can reduce this to as low as 0.19%. This reduction ensures less than 1% of light is reflected from the lens surface, allowing 99% to reach the eye. Consequently, wearer comfort is improved, visual clarity is enhanced, colour contrast is increased, and eye strain is reduced. Moreover, the aesthetic of the glasses is improved, giving the lenses a nearly invisible appearance.
The manufacturing process
The coating design can affect the durability, such as scratch resistance. Despite variations in coating formulas among lens manufacturers, their production necessitates a clean, dust-free environment. Consequently, environmental control is as crucial to coating quality as the coating design itself. Factors such as double-door entry systems within the factory, cleanroom access protocols, dust-free room maintenance, and precise temperature and humidity regulation can lead to drastically different results, even when using identical equipment and formulas. This also explains why certain manufacturers produce better coatings than others.
Caring for multicoated lenses
Despite their benefits, AR coatings can experience issues such as scratching, peeling, and cracking. These often arise from improper care or exposure to extreme conditions. It’s essential to clean them properly and avoid exposing them to high heat. It is the duty of optical industry professionals to educate customers to enhance and optimise the coating’s benefits.
Lens coating technology continues to evolve, with advancements in materials and processes. As our understanding of light and optics deepens, we can expect even more sophisticated coatings with enhanced visual performance and protection.
About the author: Justin Chiang is the general manager at Tokai Optical Australia. He’s a trained optical dispenser with more than 20 years’ industry experience working for corporate and independent groups.
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