In everyday language, anti-reflection (AR) coatings are often described as ‘anti-glare’, which can be misleading. The glare that bothers patients is largely produced when visible light from bright sources is scattered within the ocular media and degraded further by low and higher order aberrations. AR coatings do not directly correct these internal ocular effects. Instead, their primary purpose is to reduce artefacts generated by reflections at the lens surfaces themselves: ghost images, veiling reflections, loss of contrast, and the cosmetic ‘mirror’ that hides the wearer’s eyes. By minimising these surface and internal lens reflections, AR coatings convert more of the incident light into useful, image-forming light. As a secondary benefit, they reduce certain forms of glare but not always in the way the name ‘anti-glare’ might imply.

WHERE IT ALL BEGINS

The visible light spectrum spans roughly 380–750 nm (Figure 1), with the exact limits varying slightly between individuals. As wavelength increases from shorter (violet, highest photon energy) to longer (red, lowest photon energy), we perceive a smooth progression of colours across this range. Within this band, the visual system can discriminate thousands of distinct colours, a capacity that not only supports survival and everyday tasks (such as detecting contrast and identifying objects) but also allows us to enjoy phenomena like dramatic sunsets and complex works of art.

The wavelength composition of light produces the perception of colour, which is incredibly useful for detecting or distinguishing objects. Hundreds of thousands of different colour sensations are possible due to the presence of just three types of cones in the human retina and colour opponent neural pathways to the visual cortex.1

Although ultraviolet (UV) light is not the focus here, it is worth noting that the UV region is typically defined as beginning just below about 380 nm. In practice, many ophthalmic lens materials and coatings are designed to block up to UV400, typically indicating that they are protecting up to around 400 nm. This approach offers robust UV protection while avoiding the more noticeable yellow brown discolouration that can occur in materials engineered to cut off at even longer wavelengths, such as 410 nm.

When discussing the visible spectrum, it is helpful to distinguish between useful and non-useful visible light (i.e., light that does not help with the task at hand and may be described as stray light, veiling glare or light that reduces image contrast or visual performance). In the context of spectacle lenses, useful light is the portion of the visible spectrum that passes through the lens along the intended optical path and contributes to a high contrast image on the retina. By contrast, light that is scattered, reflected, or enters the eye via unintended paths does not add meaningful visual information and does not improve vision. Instead, it tends to reduce visual performance and comfort.

From a spectacle lens perspective, light near the peak of the eye’s photopic sensitivity curve (around 550–560 nm) is particularly useful because it generates a strong visual response for relatively little radiant power. Wavelengths further from this peak contribute less to perceived brightness per unit energy, but they remain essential for accurate colour discrimination and natural colour vision.2,3

Figure 1. Electromagnetic spectrum, highlighting the visible spectrum.

In some contexts, short wavelength blue light is described as ‘not visually useful’. This is not strictly true in all situations, but the idea arises from how haze and stray light are influenced by Rayleigh scattering. Rayleigh scattering refers to the scattering of light by particles much smaller than the wavelength. Its strength increases as wavelength decreases, so shorter (blue violet) wavelengths are scattered more than longer ones. This same wavelength-dependant scattering is why distant objects often look bluish and why short wavelength light tends to contribute more to stray light than to sharp image formation.4,5

This preferential scattering can reduce the amount of blue light that contributes to clean, high contrast image information, making the lowest wavelength visible light less efficient for forming a sharp image. This discussion is separate from any potential effects of blue light on ocular health or circadian rhythms, which are distinct topics in their own right.

ORIGIN OF PROBLEMS WITH NON-USEFUL LIGHT

When we look at a spectacle lens from an optical point of view, some light is reflected at every transition between air and the lens material, on both the front and back surfaces. These surface reflections can also to produce ghost images, which appear as secondary, out-of-focus copies of bright objects superimposed on the main image.

Figure 2. Light incident on a lens has multiple reflections from both surfaces.

Figure 3. Light scatter created through ‘non-useful’ light (left) versus scatter reduction (right). Illustrative only.

In practice, several different reflection phenomena can occur in and around a lens (Figure 2). The most clinically relevant include:6

Fresnel reflections. Fundamental in the origin of reflections is talking about what fraction of light is reflected vs what is transmitted by a specific ray at a certain angle and through what material. These can often be referred to as interface reflections. When reflections occur between two media of different indices, this is governed by Fresnel equations. This quantifies the approximate amount of reflection (for normal incidence) as follows:

R = ((n1 -n2 )/(n1 +n2 ))2

R refers to reflectance, n1 and n2 refer to the differing indices. In this case the indices under scrutiny would be the refractive index of the material (n2 ) used and the air (n1 ).

Specular reflection. This type of reflection describes where the reflected ray goes, and how light bounces from a surface. Due to the smooth polished surface of a lens, it acts like a specular reflector or a weak mirror. This means it follows the same laws of reflection in that the angle of incidence equals the angle of reflection. So, light arriving at the surface from a given direction leaves in a single well-defined direction. This results in a visible image of a bright source, such as lights or windows. From the front surface, this obscures the wearer’s eyes and from the back surface, it produces images visible to the patient.

Ghost images. This type of reflection is produced when light is bouncing within the lens itself, between the spectacle lens and the cornea and even between internal interfaces (for example, bifocal segment boundaries). These multiple reflections can create a ‘ghost image’, streaks, or flares of light. These can very often be noticed more in low lighting (mesopic) conditions and particularly from point sources of light such as streetlights or headlights. These would still fall under the category of specular reflection at their origin. The main difference is that the overall path they travel becomes much more complex and so the image will appear displaced and often out of focus.

Total internal reflections (TIR). If an internal light ray within the lens hits the surface (when travelling from higher to a lower index medium) at angles exceeding the critical angle (dictated by the critical angle expression: sinθc = n2 /n1 the light is completely reflected back into the lens. Total internally reflected light can then remain trapped in the lens until it emerges elsewhere. This can contribute to internal ghost paths and can be increasingly noted in higher index materials and for thick lenses.

Although the discussion above is not an exhaustive catalogue of every reflection phenomenon that can occur at a lens surface, it is intended to illustrate the main problems, the underlying optics. It also highlights the kinds of visual effects patients experience when lenses are left uncoated.

By deliberately reducing or eliminating these optical artefacts at the lens surface, we can improve overall image clarity. When a greater proportion of incident light is transmitted as useful, image forming light and fewer unwanted reflections reach the eye, patients experience better contrast, fewer distractions from ghost images and veiling reflections, and generally more comfortable vision (Figure 3). In a patient-centred clinical setting, the focus on enhancing the quality of the visual outcome for each individual should sit at the heart of our decisions around lens and coating selection.

DESTRUCTIVE INTERFERENCE

It is initially important to understand that the predominant properties of anti-flection coatings are created through destructive interference.

So, if we now understand the origin of the artefact, how do we work to remove or minimise the impact it has? For us to do this, we have to interfere with the transmission of light. We must alter what is happening to the wavelength and the path it is taking (Figure 4).

Light can be described as an electromagnetic wave with oscillating electric and magnetic fields. When two or more coherent light waves overlap, their electric field amplitudes superpose. This superposition is called interference and can lead to an increase or decrease in the observed intensity, depending on the relative phase of the waves. If they arrive in phase (field maxima and minima align), they interfere constructively, producing a higher resultant field amplitude and thus, a brighter region.

If they arrive out of phase, they interfere destructively, which can reduce the resultant amplitude and produce a dimmer region. In this context, we can think about how a thin film controls the colour of reflected light; for example, the specific colour reflected by a mirror-type interference coating, or the faint residual colour seen in an AR coating. If they arrive out of phase by half a wavelength, they interfere destructively and partially, or completely cancel each other out, producing a darker region (or in this context, increased light transmission for a specific wavelength).7

Figure 4. Demonstrating waves in and out of phase.

In ophthalmic optics, this principle is directly exploited by thin film coatings. In the production of mirror coatings, we directly create constructive interference to reflect light back for specified wavelengths. AR coatings are designed so that reflections from the front and back of the coating layer have a path difference of about half a wavelength (i.e., they are half a wavelength out of phase) in the coating material. At a chosen design wavelength, this makes the reflected waves cancel (destructive interference), greatly reducing surface reflections while allowing most of the light to transmit into the lens and contribute to the retinal image.

To minimise the effects of the reflection from any given wavelength, the light must go through a process of destructive interference.

Now, if we can be successful in moving the wave out of phase, we can then realise transmission through the lens. This minimises the artifact, allowing light to pass through the lenses, which provides increased clarity due to a higher quality of visual information. If we relate this back to what was previously discussed, a thin film coating can therefore be used to suppress non-useful reflected light and boost the useful transmitted light. This can be more easily described using formula T= 1 - R, by reducing the intensity of reflected light (R), the amount of transmitted light (T) increases by the same amount (neglecting absorption).

For simple single layer AR coatings to achieve this at one chosen ‘design’ wavelength, the coating is made so that its optical thickness is one quarter of that wavelength. In symbols, this is written as ncoat =λ/4, which gives t = λ/(4ncoat )6

Figure 5. The wavelength phase shift in single layer anti-reflection coatings.

At this thickness, the two reflected waves (from air-coating-lens interfaces) travel paths that differ by half a wavelength, so they are 180° out of phase and interfere destructively (Figure 5).

THE STACK

Single layer AR coatings are now largely of historical interest in ophthalmic practice. As coating technology has advanced, reducing reflections across the full visible range has relied increasingly on carefully designed multilayer ‘stacks’ rather than a single film. By combining multiple layers of different refractive indices and thicknesses (Figure 6), these modern stacks can be tuned to provide high transmission of useful visible light over a broad spectrum, while keeping residual reflections very low across the wavelengths that are most important for vision.

In practical spectacle AR coating, several thin layers of different refractive index are stacked on each lens surface. Each layer is typically close to a quarter of a wavelength thick (in optical thickness), but with different indices and sometimes, half-wavelength layers included. This multilayer ‘stack’ is designed so that numerous partial reflections from each interface cancel one another out over a broad range of visible wavelengths and angles.8

The result is a broadband AR coating, with very low residual reflection across most of the visual spectrum instead of at just one wavelength.

Lens manufacturers typically guard the exact composition and design of the transparent oxides and fluorides used in their proprietary thin film stacks, as these combinations are an important part of their intellectual property. In practice, this confidentiality tends to apply most strongly to the high index layers that give each stack its distinctive performance, whereas the low index materials used as spacer and matching layers (such as magnesium fluoride or silicon dioxide) are more widely described in the literature.

Materials such as magnesium fluoride (MgF2 ) (n ≈ 1.38) can often be used for single layer AR stacks due to their ability to bridge air to higher index substrates and its excellent transmission. Silicone dioxide (SiO2 ) (n ≈ 1.46) is another low index layer due to its chemical stability, mechanical strength, and increased durability, which makes it exceptionally useful in spectacle lenses.9

For high index materials, reports suggest that titanium oxides (TiO2 ), tantalum pentoxide (Ta2 O5 ), zirconium dioxide (ZrO2 ), and aluminium oxide (Al2 O3 ), may appear in functional stacks for different suppliers depending on index, durability, and process compatibility.10 While there is overlap in the materials used across the industry, some manufacturers use proprietary materials, and detailed material choices may be confidential.

In discussions with Dr Arne Urban and Dr Elisabeth Rosier, thin film specialists at Rodenstock, several additional factors emerged as critical when designing an ‘ideal’ AR coating stack.

Index matching and layer stack precision. A key theme from these discussions was the need to match the optical and mechanical properties of the coating stack to the underlying lens material, and to control every layer in the stack with high precision.

Figure 6. Anti-reflective stack with alternating high and low index layers.

Figure 7. Rodenstock Solitaire X-tra Clean molecular structure.

Figure 8. Spectral sensitivity.

Index matching. Dr Urban noted that the performance of an AR coating depends critically on the refractive index relationship between the substrate and coating layers. Designs must therefore be adjusted for different lens materials (e.g. CR 39 vs high index plastics) so that the stack still delivers the intended low reflection behaviour.

Layer thickness control. He also emphasised that very tight control of layer thickness is essential. Small deviations can shift the reflection spectrum, alter the residual reflex colour, or increase overall reflectance, undermining both the optical performance and the cosmetic appearance of the coating.

Interface management. Another important point was the interfaces between the AR stack are the underlying hard lacquer and the substrate. If the chemical and mechanical properties at these boundaries are not well matched, stresses and incompatibilities can lead to issues such as microcracking or delamination over time.

Optimisation of stack order. Finally, Dr Rosier and Dr Urban highlighted that the order, thickness, and refractive index of the individual layers are carefully optimised as a system. The arrangement of high and low index layers is not arbitrary; it is the result of detailed calculation and modelling to achieve the desired anti-reflective effect, durability, and colour behaviour.

In recent years, AR stacks have been further refined by adding outer hydrophobic and oleophobic layers. Ultra-thin, easy-to-clean topcoats are applied to improve cleanability and resistance to smudging, creating a smooth, molecularly ordered surface that makes the lens effectively more slippery (Figure 7). However, the coating must strike a careful balance: it needs to be slick enough to repel water, oils, and dirt, yet not so lubricious that it compromises the ability to securely block and edge the lens during glazing and fitting.

COLOUR TUNING AND RESIDUAL REFLECTION

Why do AR coatings show a particular residual colour? There will always be optical design principles and visual considerations that drive the choice of residual reflex colour.

From a technical standpoint, the residual colour is set by the shape of the coating’s reflection spectrum. The positions of the minima and maxima in this reflection curve are controlled by the number of layers, their refractive indices, and their optical thicknesses. By adjusting the design of the multilayer stack, manufacturers can shift the wavelength range where reflection is lowest and, consequently, the wavelength range where a small residual reflection remains visible as a faint colour.

According to Dr Urban, it is also possible to tailor achromatic AR coatings, so that the residual reflection colour is white. “This is, for example, a constant/flat reflection curve over the entire visible spectrum,” he explained. “The difficulty is that any small deviation in the reflection curve results in a residual colour (i.e., where a maximum or minimum appears in the curve). Thus, it’s difficult to reproducibly manufacture such AR coatings, but technically it is possible.”

Physiologically, the human eye is most sensitive in the green region of the spectrum, where small amounts of reflected light are most noticeable (Figure 8). For this reason, modern AR designs aim to minimise reflection in the green band and deliberately push any unavoidable residual reflection toward the blue region. A faint blue reflex is less disturbing to visual performance and is often preferred cosmetically, giving lenses a cleaner, more transparent appearance while preserving optimal vision. In other words, we push the unavoidable reflection away from the green band where the eye is most sensitive.

CAN WE ERADICATE REFLECTION COMPLETELY?

In summary, from a purely mathematical and physical standpoint, the answer is “yes”. However, when feasibility, durability, and large-scale manufacturing are considered, the practical answer, for now, remains “no”. That said, several promising developments in the pipeline are worth keeping in mind for the future.

Modern research into AR coatings has been inspired by the structure of nocturnal moth eyes and the use of sub-wavelength patterns. The outer surface of a moth’s eye is covered with a regular array of tiny conical bumps, each only about 200–300 nm high and spaced closer than the wavelength of visible light. Because these nanostructures are so small, incoming light cannot ‘see’ the individual bumps; instead, it experiences a gradual transition in refractive index from air into the eye, rather than a sudden step. This smooth index gradient dramatically reduces Fresnel reflections over a broad range of wavelengths and angles, helping the moth see in low light without giving itself away with eye reflections. Engineers mimic this ‘moth eye’ effect by fabricating similar sub-wavelength surface textures on glass or plastic, creating biomimetic coatings that show very low reflectance and high light transmission. At present, however, the process required to create such sub-wavelength structures reliably over a large, curved lens surface and ensure adequate long-term resistance to abrasion and cleaning remains a little too complex and costly compared to the vacuum-controlled evaporation-deposited multi-layered AR coatings that currently drive the market.

Fundamentally, lens design is about deciding what happens to light: how much is bent, absorbed, or reflected. In practice, we spend most of our time on refraction and transmission, yet reflection silently shapes a great deal of what patients see, and what we see when we look at them. Modern AR coatings are a sophisticated answer to this “third component”: they turn more of the incident spectrum into useful, image-forming light and strip away ghosts, veiling reflections and cosmetic mirror effects. As we have seen, doing this well is not just a matter of adding a green layer or a blue reflex. It requires careful management of Fresnel reflections, interference, material indices, layer order, surface chemistry, and sub-wavelength structure. For now, multilayer thin film stacks remain the practical workhorses, while moth eye biomimetic approaches point to where the field may go next. As clinicians, the key is to recognise that coatings aren’t a decorative extra, they’re an integral part of the optical system. That understanding is what lets us recommend lenses that maximise useful light and minimise artefacts for each individual in the consulting room.

References available at mieducation.com.au.