AR-Reflective Coating

Introduction to Anti-Reflective Coatings
An anti-reflection coating (AR coating) is a dielectric thin-film coating applied to an optical surface in order to reduce the reflectance (also often called reflectivity) of that surface due to Fresnel reflections – at least in a certain wavelength range. In typical imaging systems, this improves the efficiency since less light is lost due to reflection. In complex systems such as telescopes and microscopes the reduction in reflections also improves the contrast of the image by elimination of stray light. This is especially important in planetary astronomy. In other applications, the primary benefit is the elimination of the reflection itself, such as a coating to reduce the glint from a covert viewer's binoculars or telescopic sight, optical systems like camera objectives, optical windows, displays and so on.

Many coatings consist of transparent thin film structures with alternating layers of contrasting refractive index. Layer thicknesses are chosen to produce destructive interference in the beams reflected from the interfaces, and constructive interference in the corresponding transmitted beams. This makes the structure's performance change with wavelength and incident angle, so that color effects often appear at oblique angles. A wavelength range must be specified when designing or ordering such coatings, but good performance can often be achieved for a relatively wide range of frequencies: usually a choice of IR, visible, or UV is offered.

In most cases, the basic principle of operation is that reflected waves from different optical interfaces largely cancel each other by destructive interference.
Note that there are also anti-glare surfaces, which suppress reflections in a completely different way: by diffuse scattering from a microscopically rough surface. Such surfaces are suitable e.g. for some viewing ports, but normally not for laser applications, and should be carefully distinguished from anti-reflection surfaces.

VY Optoelectronics Co.,Ltd. offers all transmissive optics with a variety of anti-reflection (AR) coating options that vastly improve the efficiency of the optic by increasing transmission, enhancing contrast, and eliminating ghost images. Most AR coatings are also very durable, with resistance to both physical and environmental damage. For these reasons, the vast majority of transmissive optics include some form of anti-reflection coating. When specifying an AR coating to suit your specific application, you must first be fully aware of the full spectral range of your system. While an AR coating can significantly improve the performance of an optical system, using the coating at wavelengths outside the design wavelength range could potentially decrease the performance of the system.

Why Choose an Anti-Reflection Coating?
Due to Fresnel reflection, as light passes from air through an uncoated glass substrate approximately 4% of the light will be reflected at each interface. This results in a total transmission of only 92% of the incident light, which can be extremely detrimental in many applications (Figure 1). Excess reflected light reduces throughput and can lead to laser-induced damage in laser applications. Anti-reflection (AR) coatings are applied to optical surfaces to increase the throughput of a system and reduce hazards caused by reflections that travel backwards through the system and create ghost images. Back reflections also destabilize laser systems by allowing unwanted light to enter the laser cavity. AR coatings are especially important for systems containing multiple transmitting optical elements. Many low-light systems incorporate AR coated optics to allow for efficient use of light.

Broadband Anti-Reflection (BBAR) Coating Options 
Broadband anti-reflection (BBAR) coatings are designed to improve transmission over a much wider waveband. They are commonly used with broad spectrum light sources and lasers with multiple-harmonic generation. BBAR coatings typically do not
achieve reflectivity values quite as low as V-coats, but are more versatile because of their wider transmission band. In addition to being applied to transmissive optical components including lenses and windows, AR coatings are also used on laser crystals and nonlinear crystals to minimize reflections.

VY Optoelectronics Co.,Ltd. offers all lenses with an optional single-layer, dielectric anti-reflection (AR) coating to reduce surface reflections. In addition, custom single-layer, multi-layer, V, and 2V coatings are available for both our off-the-shelf and large volume custom orders. View Custom Optical Lens Coatings for information.


1) λ/4 MgF2: The simplest AR coating used is λ/4 MgF2 centered at 550nm (with an index of refraction of 1.38 at 550nm). MgF2 coating is ideal for broadband use though it gives varied results depending upon the glass type involved.
2) VIS 0° and VIS 45°: VIS 0° (for 0° angle of incidence) and VIS 45° (for 45° angle of incidence) provide optimized transmission for 425 – 675nm, reducing average reflection to 0.4% and 0.75% respectively. VIS 0° AR coating is preferred over MgF2 for visible applications.
3) VIS-NIR: Our visible/near-infrared broadband anti-reflection coating is specially optimized to yield maximum transmission (>99%) in the near infrared.
4) Telecom-NIR: Our telecom/near-infrared is a specialized broadband AR coatings for popular telecommunications wavelengths from 1200 – 1600nm.
5) UV-AR and UV-VIS: Ultraviolet coatings are applied to our UV fused silica lenses and UV fused silica windows to increase their coating performance in the ultraviolet region.
6) NIR I and NIR II: Our near-infrared I and near-infrared II broadband AR coatings offer exceptional performance in near-infrared wavelengths of common fiber optics, laser diode modules and LED lights.
7) SWIR: Shortwave infrared broadband AR coating for applications from 900 - 1700nm.
Figure 5, Figure 6, and Table 2 show EO’s standard BBAR coating options.

Figure 5: EO's standard AR coating for the visible spectrum

Figure 6: EO's Standard AR coating for the near infrared (NIR) spectrum cover 400-1600nm, but custom coatings can be designed out past 2um.

Table2: Reflectivity specificaitons for EO's standard BBAR coatings

Standard Broadband Anti-Reflection Coatings
Coating Description Specifictions
lambda/4 MgF2@550nm  Ravg<=1.75%@400-700nm
UV-AR[250-425nm]  Ravg<=1.0%@250-425nm
Ravg<=0.75%@250-425nm
Ravg<=0.5%@370-420nm
Laser UV-VIS[250-532nm]
UV-VIS[250-700nm]
 Ravg<=1.25%@250-532nm
Ravg<=1.0%@350-450nm
Ravg<=1.5%@250-700nm
VIS-EXT[350-700nm]  Ravg<=0.5%@350-700nm
VIS-NIR[400-1000nm]  Ravg<=0.25%@880nm
Ravg<=1.25%@400-870nm
Ravg<=1.25%@890-1000nm
Laser VIS-NIR[500-1090nm]  Ravg<=1%@500-1090nm
VIS 0 degree [425-675nm]
VIS 45 degree [425-675nm]
 Ravg<=0.4%@425-675nm
Ravg<=0.75%@425-675nm
YAG-BBAR[500-1100nm]  Ravg<=0.25%@532nm
Ravg<=0.25%@1064nm
Ravg<=1.0%@500-1100nm
NIR I[600-1050nm]  Ravg<=0.5%@600-1050nm
NIR II[750-1550nm]  Ravg<=1.5%@750-800nm
 Ravg<=1.0%@800-1550nm
 Ravg<=0.7%@750-1550nm
SWIR[900-1700nm]  Ravg<=1.0%@900-1700nm
Ravg<=1.5%@900-1700nm
Laser NIR[1030-1550nm]  Ravg<=0.7%@1030-1550nm
2μm BBAR[1900-2100nm]  Ravg<=0.5%@1900-2100nm
 Ravg<=0.25%@2000-2100nm
 
Table2: Reflectivity specificaitons for EO's standard BBAR coatings

Single-layer Anti-reflection Coatings
In the simplest case, an anti-reflection thin-film coating designed for normal incidence consists of a single quarter-wave layer of a material the refractive index of which is close to the geometric mean value of the refractive indices of the two adjacent media. In that situation, two reflections of equal magnitude arise at the two interfaces, and these cancel each other by destructive interference.
Magnesium Fluoride (MgF2) is often used as a Broadband AR (BBAR) coating suitable for visible light applications such as the crown glass example discussed previously. Magnesium fluoride has refractive index of 1.38 which is close to the anti-reflective ideal index of refraction of 1.23.
If Magnesium Fluoride is applied to the surface of crown glass at a thickness of 0.145μm – approximately one-quarter of a wavelength of green light in the middle of the visible band – the amount of light reflected drops from 4% to around 1%. The performance is even better for glasses with an index of refraction near 1.9.

AR lenses for eyeglasses, cameras, and other visible light optical applications use a Magnesium Fluoride coating. It is ideal for these applications because the coating is hard and relatively easy to apply; however, with improvements in manufacturing techniques, many of these applications have transitioned to multi-layer coatings (see next section). Magnesium Fluoride also has mild resistance to abrasion, good resistance to humidity and can be cleaned with mild solvents.

There are other fluoropolymers with indices of refraction closer to the ideal refractive index of 1.23, but they are harder to apply and less durable. They are better suited for highly specialized applications, but can be applied to plastic substrates such as polycarbonates.

For applications targeting wavelength ranges outside of the visible spectrum, other dielectric coating materials may be used as a single-layer coating. Silicon Nitride (Si3N4) and Titanium Dioxide (TiO2) are common AR coatings for solar cell photovoltaics operating in the near-infrared region (NI

The limitations of this approach are twofold:
•    It is not always possible to find a coating material with suitable refractive index, particularly in cases where the bulk medium has a relatively low refractive index (e.g. in case of plastic optics).
•    A single-layer coating works only in a limited bandwidth (wavelength range).

Multilayer AR Coatings
Multi-layer coatings are a common way to improve the optical performance of an AR coating. As the name implies, a multi-layer coating uses several layers of a thin film coating to successively reduce the reflected light. With a multi-layer coating, it is possible to reduce reflection to less than 0.1% of the incident light.

A multi-layer coating works on the same principles as demonstrated earlier with the air to crown glass example. In this case, there is a reflection between air and the coating (air-coating), at each interface between coating layers (coating-coating) and again between the coating and the substrate (coating-substrate). The material and thickness of each layer of the coating are designed to maximize the destructive interference of the reflected light to maximize transmission.

Although there are no specific combinations of layers, it is common to alternate between higher and lower indices of refraction. For a two-layer AR coating, first, a coating with an index of refraction of 2.3 is applied to the glass. The composite results in an index of refraction of 1.9. If a layer of Magnesium Fluoride is then applied on top of that higher index coating, the result is a near-ideal index of refraction of 1.23 (see Figure 3). The thickness of each layer is a function of the target wavelength.

Typical index of refraction curves of VY Optoelectronics Co.,LTd. Broadband AR (BBAR) Coating for visible spectrum (left) and near-infrared (NIR) spectrum (right) for light at a normal AOI.


 

Limitations of Anti-Reflective Coatings
The index of refraction depends on the Angle of Incidence (or AoI) while Fresnel’s equation is valid only for a normal angle of incidence. Suffice to say, a larger AoI will result in a higher index of refraction.
Many optical applications operate across a spectrum wavelength ranges, including infrared (700nm to 1mm), visible (400nm to 700nm), and ultraviolet (100nm to 400nm). The exception is lasers which are tuned to a narrow band of wavelength ranges.

Properties of Anti-Reflective Coatings
AR coatings are thin-film coatings applied to a substrate. Due to the mechanical and chemical differences between the thin film and the substrate, the durability of AR coatings is highly dependent on the bond between them as well as the bond between layers of coatings in multilayer coatings. As such, AR coatings are most susceptible to abrasion and adhesive pulls that peel away the coating, solvents that damage the bond, and thermal cycling that stresses the bond.

The hardness, strength, and durability of the coating itself plays a significant role in the longevity of an AR coating. The degree to which an AR coating is scratch and solvent resistant depends on the coating material. For example, an SiO2 coating has a slightly higher hardness than an MgF2 coating and this impacts how well they resist scratches and impact.

The most common damage to AR coatings on consumer products is scratching; however, with proper care and cleaning, these coatings can last several years.

One particular case of damage for AR coatings is the laser-induced damage threshold (LIDT) in laser applications. The beam intensity of high-powered lasers can damage coatings. The threshold is dependent on several factors including wavelength, pulse duration/repetition, spot size, angle of incidence and spatial effects. For this reason, it is essential to characterize laser applications to select coatings with high LIDTs.

AR coatings are an excellent way to reduce light reflection and increase light transmission for optical materials. They can be designed for specific applications to work over a broad range of wavelengths or, in the case of “V” coatings, they can be designed for a very narrow and specific target wavelength. AR coatings have applications in everyday items like eyeglasses and high tech applications like infrared imaging systems.

Applications


Anti-reflective coatings are often used in camera lenses, giving lens elements distinctive colours.
Anti-reflective coatings are used in a wide variety of applications where light passes through an optical surface, and low loss or low reflection is desired. Examples include anti-glare coatings on corrective lenses and camera lens elements, and antireflective coatings on solar cells.


Photolithography


Antireflective coatings (ARC) are often used in microelectronic photolithography to help reduce image distortions associated with reflections off the surface of the substrate. Different types of antireflective coatings are applied either before (Bottom ARC, or BARC) or after the photoresist, and help reduce standing waves, thin-film interference, and specular reflections
Anti-reflective coatings are often used in camera lenses, giving lens elements distinctive colours.

 

People also ask


How much does AR coating cost?
The average price for AR coating is anywhere from $50 to $150 in addition to the price of the lenses. It is important to note that Eyewear Insight offers this coating for free on all glasses.

Is anti reflective coating worth it?
AR coatings virtually eliminate all reflections from the front and back surfaces of your lenses. Without bothersome reflections, more light is able to pass through your lenses which optimizes your vision. ... Most people agree that anti-reflective coatings on their glasses are definitely worth the added cost.

How do you know if your glasses have AR coating?
Look at the backside surface of your glasses, if they reflect the light in a color like green, gold, purple or blue, then you have the coating. If the colors reflected are the same color as the original light, then your lenses do not have AR coating.

Is anti glare and anti-reflective the same?
Anti-glare is typically visible to the human eye while anti-reflective coatings typically exhibit a soft blue/green hue when deposited and is essentially invisible to the human eye minus a soft color.
 

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