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Mirrors: Coating Choice Makes a Difference | Materials

Jul. 08, 2024

Mirrors: Coating Choice Makes a Difference | Materials

A mirror is an important element in many optical systems. Its basic function is to redirect light, often with the purpose of making an optical system more compact. This article discusses the kinds of thin-film coatings that can be used for mirrors. The choice of coating depends on the application, including the spectral range of interest, the optical wavefront quality desired and the cost limitations.

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For more information, please visit Aluminum coated mirrors.

The basic difference between the household mirror and the optical mirror is that one is coated on the back surface and the other is coated on the front. For optical applications, a front-surface mirror must be used. This means that the reflective surface is subject to environmental degradation, even though it is usually in an enclosed environment and not exposed to the harsh conditions of the household mirror. An important part of mirror technology is providing a durable front-surface mirror that is stable and can be cleaned.

A mirror&#;s substrate surface should be flat and smooth. The flatness is usually specified in terms of how many wavelengths of light the surface deviates from being a perfect plane. For many applications, the glass can be flat to a few wavelengths of visible light. For the most stringent applications, the surface must be flat to a quarter of a wavelength or less. The surface quality of a mirror, or its smoothness, is measured in terms of scratches and digs that are still present after polishing. A scratch/dig specification of 80/50 is fairly routine, while a specification of 20/10 is much better, but more expensive.

For some applications, a mirror&#;s ability to conduct heat is important. In these cases, metal substrates are often used because metal is much more conductive than glass. Optical-quality metal surfaces can be fabricated by polishing or single-point diamond turning. The most common metals used are copper and aluminum. Although beryllium is highly toxic, it is used when especially light weight, stiff mirrors are required. In the case of metal substrates, the coating improves the reflectance and makes the surface more durable and resistant to scratches.

Metal mirror coatings

The simplest and most common mirror coating is a thin layer of metal. A 100-nm layer of aluminum or silver makes an excellent reflector for the visible spectrum. Aluminum reflects about 90 percent of the light across the visible spectrum, while silver reflects about 95 percent. The reflectance of a metal mirror can be calculated from the index of refraction n and the extinction coefficient k of the metal. The reflectance of a metal surface in air is given by:

An extensive list of n and k values over a wide range of wavelengths and for many metals is available.1,2,3 Table 1 contains an abbreviated list, with data given for ultraviolet (0.2 and 0.3 μm), visible (0.4 to 0.7 μm) and infrared wavelengths (1 to 10 μm). In general, metals with k>>n are shiny, while those with k &#; n &#; 3 are gray. Thus, silver with n = 0.13 and k = 2.92 at 0.5 μm is shiny, while tungsten with n = 3.4 and k = 2.69 is not. As the wavelength increases into the IR region, n and k increase, leading to high reflectance in this spectral region.

TABLE 1.
n AND k FOR SELECTED METALS

Wavelength (µm):

0.2

0.3

0.4

0.5


0.6


0.7


1.0

2.0

4.0

10.0

Aluminum* n:
k:

0.12
2.30

0.28
3.61

0.49
4.86

0.77
6.08

1.20
7.26

1.83
8.31

1.35
9.58

2.15
20.7

6.43
39.8

25.3
89.8

Beryllium n:
k:

0.84
2.52

2.42
3.09

2.89
3.13

3.25
3.17

3.43
3.18

3.47
3.25

3.28
3.87

2.44
7.61

2.38
16.7

8.3
41.0

Chromium n:
k:

0.89
1.69

0.98
2.67

1.50
3.59

2.61
4.45

3.43
4.37

3.84
4.37

4.50
4.28

4.01
6.31

3.08
13.7

14.2
27.5

Copper n:
k:

1.01
1.50

1.39
1.67

1.18
2.21

1.13
2.56

0.40
2.95

0.21
4.16

0.33
6.60

0.85
10.6

2.41
21.5

11.6
49.1

Gold n:
k:

1.43
1.22

1.80
1.92

1.66
1.96

0.85
1.90

0.22
2.97

0.16
3.95

0.26
6.82

0.85
12.6

2.60
24.6

12.4
55.0

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Molybdenum n:
k:

0.81
2.50

2.86
3.70

3.03
3.22

3.41
3.74

3.68
3.47

3.82
3.56

2.58
4.02

1.38
10.4

2.32
23.0

12.6
56.7

Nickel n:

1.00
1.54

1.74
2.00

1.61
2.36

1.68
2.96

1.88
3.54

2.18
4.05

2.81
5.00

3.78
8.17

4.15
14.6

6.83
37.0

Platinum n:
k:

1.24
1.34

1.46
2.17

1.72
2.84

1.97
3.44

2.25
3.97

2.54
4.49

3.44
5.79

5.27
6.72

3.74
15.5

10.4
38.0

Rhodium n:
k:

0.78
1.85

0.84
3.00

1.41
4.20

1.88
4.68

2.07
5.37

2.33
6.11

3.41
7.83

3.83
13.1

5.71
25.1

14.4
57.3

Silver n:
k:

1.07
1.24

1.51
0.96

0.17
1.95

0.13
2.92

0.12
3.73

0.14
4.52

0.21
6.76

0.65
12.2

2.30
24.3

13.3
54.0

Tungsten n:
k:

1.47
3.24

2.98
2.36

3.39
2.41

3.40
2.69

3.56
2.85

3.84
2.88

3.04
3.44

1.28
7.52

1.77
17.6

9.5
45.0

*Aluminum has a reflectance dip at 0.8 µm: for λ = 0.8 µm, n = 2.80 and k = 8.45
for λ = 0.9 µm, n = 2.06 and k = 8.30

SOURCE: Handbook of Optical Constants of Solids

Across the visible spectrum, silver is the most reflective (Figure 1). For UV applications, such as astronomical telescope mirrors, silver is unacceptable and aluminum is the best choice. Unfortunately, aluminum suffers in the region from 0.8 to 1.0 μm, where the reflectance dips well below 90 percent. In an optical system with several mirrors, this can be detrimental to performance. For a reflectance of 85 percent, a system with five mirrors would have a throughput of only 44 percent.

Figure 1. The reflectance of several shiny metals vs. wavelength from 0.2 to 1.2 μm. The reflectance values are calculated using equation 1 with data from references 1 and 2.
Copper and gold are useful only in the red and IR spectral regions. For situations involving higher durability, less shiny metals are adequate. For example, rhodium is used for dental mirrors and chromium is used for rearview mirrors in cars.

A solution to degradation is overcoating the mirror with a dielectric material that is harder than the metal surface. A common overcoat material for visible mirrors is silicon monoxide (SiO). A mirror with a simple dielectric overcoat is called a protected metal mirror. Environmentally stable silver mirrors can be made by including various overcoat layers.4

A more complicated coating can be used to increase the reflectance of a metal mirror. Such a coating would consist of several dielectric layers with alternating high and low indices of refraction. The first layer usually has a low index and the last layer, a high index. This type of mirror is called an enhanced metal mirror or an enhanced reflector. With four layers, the reflectance can be enhanced several percent over a limited spectral region (Figure 2).

Figure 2. The reflectance of an enhanced aluminum mirror vs. wavelength compared with the reflectance of an uncoated aluminum surface. The enhanced mirror includes four alternating layers of silicon dioxide and titanium dioxide.

All-dielectric mirror coatings

Mirrors can be made by depositing a stack of alternate high- and low-index dielectric layers on a glass substrate. If one wishes to make a mirror for a given wavelength of light, usually denoted λ0, the thickness of each layer is chosen so that the product of the thickness and the index of refraction of the layer is λ0/4. This is called a λ/4 stack reflector. The first and last layers of the stack are of the high-index material. Increasing the number of layers can increase the reflectance at λ0, but the spectral width of the high-reflectance region is limited. If the λ/4 stack reflector consists of p+1 high-index layers with refractive index nH and p low-index layers with index nL on a substrate with refractive index nS, the maximum reflectance is given by:

where the effective index nE of the λ/4 stack is given by:

For the 11-layer stack whose reflectance is shown in Figure 3, the following values were used: nH = 2.5, nL = 1.46 and nS = 1.52 with p = 5. The wavelength λedge at each edge of the high-reflectance region defines the width of the reflectance band, as indicated in Figure 3. The two values of λedge are given by:

where

For the reflectance curve in Figure 3, the calculated edge wavelengths are 0.727 and 1.034 μm. Because of the limited width of the high-reflectance region, λ/4 stack mirrors have specific applications. The most common is their use as laser reflectors, either as a part of the laser cavity itself or for the optics that direct the laser beam through the optical system. For example, the 0.85-μm reflector might be used with the laser diode in a CD player. The typical laser reflector usually has between 21 and 27 layers and a maximum reflectance of more than 99.9 percent.

Figure 3. The reflectance of a λ/4 stack, all-dielectric mirror vs. wavelength. The mirror consists of 11 alternating layers of titanium dioxide and silicon dioxide. The reflectance band is centered at λ0 = 0.85 μm and the width of the high-reflectance region is between the two wavelengths marked λedge given by equations 4 and 5.
A λ/4 stack reflector also can be used to filter out or remove a selected portion of the spectrum from an optical system. Such a filter is called a dichroic filter because it separates light of two spectral regions.

A broadband all-dielectric mirror can be made by combining two or more λ/4 stack reflectors with central wavelengths close enough together that the edges of the reflectance bands overlap. Such a mirror is durable and nonconductive and can have more than 99 percent reflectance over the entire visible spectrum. A mirror with such performance might involve as many as 100 layers. The buyer is left with the trade-off between the higher cost of a 100-layer mirror with 99+ percent reflectance and a much less expensive three- or four-layer enhanced aluminum mirror with about 97 percent reflectance.

A final word about optical figure: In cases where optical figure or flatness is important, choose a mirror with fewer layers. The figure of a surface that has been polished to 1/10 of a wavelength will not be adversely affected by a -Å-thick aluminum layer that has thickness variations of several percent. However, a 100-layer coating with thickness variations of 2 percent across the surface (a typical coating uniformity tolerance) would distort the wavefront of the reflected beam by several wavelengths. Amateur astronomers who polish their own telescope mirrors &#; often to a surface accuracy of λ/10 &#; must be careful that the figure is not ruined by an aluminum coating with a protective layer that has poor uniformity.

References

1. Handbook of Optical Constants of Solids. Edward D. Palik, ed. (). Academic Press.

2. Handbook of Optical Constants of Solids II. Edward D. Palik, ed. (). Academic Press.

3. Handbook of Optical Constants of Solids III. Edward D. Palik, ed. (). Academic Press.

4. Wolfe, Jesse D., Ronald E. Laird, C.K. Carniglia and J.P. Lehan (). Durable silver-based antireflection coatings and enhanced mirrors. OPTICAL INTERFERENCE COATINGS, Technical Digest Series, 17:115-117.

Guide to Silver Coating Mirrors vs. Aluminum Mirrors

Metal plating is the most common form of mirror coating. From gold and zinc to copper and platinum, many metals can be used to coat mirrors. Two of the most popular are silver and aluminum. Though each plating process has its differences, both aluminum and silver coating can offer unique benefits that improve the functionality of optical mirrors.

Optical vs. Household Mirrors

The differences between household mirrors and optical mirrors are key to accomplishing their intended goals. Though each of these mirrors is intended to reflect the light that is directed at its surface, their purposes are vastly different. Household mirrors are used to allow individuals to examine their appearance and may also be used as room decorations. Optical mirrors, however, must be much more versatile due to their utility in industrial and manufacturing applications. For instance, optical mirror systems have been used by NASA for the Hubble Space Telescope.

The defining difference between optical mirrors and household mirrors is the location of their coating. Household mirrors are coated on the back while mirrors used in optical applications feature front-surface mirrors. Metals are the most commonly used mirror coatings. Because of their reflectivity, layers of aluminum and silver are often used. Silver is the most reflective across the visible spectrum, reflecting 95 percent of light. Aluminum is slightly less reflective yet still can reflect 90 percent of light. Both coatings are excellent for use in many applications.

Depending on the application in which your optical mirror will function, you&#;ll want to select a plating that provides the most benefits. To help you feel confident in your decision, we have identified the advantages of each coating so that you can determine which is best for you.

Determining Optical Mirror Performance

The choice of coating may enhance or diminish the emissivity or reflectivity of the mirror. When choosing a coating for your optical mirror, you want to ensure the proper plating for the intended application. Determining the most applicable selection involves understanding how the properties of the various metal finishings will impact the mirror&#;s performance. Two crucial aspects of a mirror&#;s performance are its reflectivity, how much light it reflects, and emissivity, the energy radiated from its surface.

Reflectivity is inversely proportional to emissivity, which means that the more emissive a mirror is, the less reflective it is. A coated optical mirror can become more emissive if its coating tarnishes, oxidizes or is rough rather than smooth.

Various coatings are used to increase the reflection properties of an optical system. The number of coating layers, as well as each layer&#;s thickness, directly contributes to the rate of interference it provides. The placement of these coatings is just as significant as the metal used. When coatings are applied correctly, light transmissions can combine for a greater amplitude resulting from constructive interference. However, if a coating is applied at an unintended incident angle, it creates destructive interference and can render the coating itself completely ineffective.

Let&#;s discuss the beneficial properties of aluminum and silver, why they&#;re used for coating optical mirrors and which industries have found successful applications for each.

Benefits of Silver Coating Mirrors

Silver is an excellent metal to use for plating. Though tin is often assumed to be a low-cost substitute, its properties do not give it an advantage in tensile strength, heat absorption, corrosion resistance or conductivity. Silver is also used a more affordable alternative to gold plating. In astronomical optical mirror application, silver coatings provide benefits to primary, secondary and tertiary mirrors because of its high reflectance and lasting durability.

Compared to other plated metals, silver has the most applications and is found throughout nearly every common industry. Classified as a noble metal, silver possesses many inherent benefits that make it a versatile plating material. For years, the process of silvering was applied to coat the surface of glass to create mirrors. Silver coatings are ideal for observing all wavelengths of light, offering high reflectivity and low emissivity that is especially useful for an infrared application.

Silver offers high performance in nearly all of its properties. It has the highest known electrical and thermal conductivity, as well as high malleability and ductility. Silver is mined throughout the world and because of its favorable properties, is used for many diverse applications including silver plating. As the least expensive precious metal, silver is used in the automotive, telecommunications, electronics and solar power industries as a substitute for other metals like gold and palladium.

Though bare silver is considered a poor solution due to its ability to tarnish and its inability to reliably adhere to surfaces like glass, a protected silver mirror coating can be applied. These coatings can be highly reflective dielectric layers that resist tarnishing and improve adhesion. Although it is possible that these protected silver coatings may be susceptible to damage from ultraviolet light, choosing specific dielectric overcoats of higher thicknesses may prevent this degradation.

As a coating for optical mirrors, silver may outperform traditional aluminum coatings. Researchers studying the Gemini telescopes have determined that a silver mirror coating is effective for observing infrared and visible spectrum wavelengths, however, the properties of silver may tend to over-absorb ultraviolet light. Although aluminum may be a better solution, hybrid coatings that combine silver and aluminum are being explored.

Enhanced protective silver coatings are being explored to provide maximum reflectivity while improving the lifespan of the silver base. One prospective process that would create stronger corrosion barriers is through plasma-enhanced atomic layer deposition, using aluminum-oxide as the top layer that acts as a barrier to help protect against corrosion and maintain reflectivity. Results of testing show that these added layers successfully protect against corrosion and moisture. These plasma-enhanced barriers can be applied to optical mirrors that require high reflectivity and absorbency.

Benefits of Aluminum Mirrors

As a resource that can be found throughout the globe, the properties of aluminum have made it one of the most versatile non-ferrous metals that can be used in a variety of applications. Though in pure form it can be weak, chemical treating and alloying processes drastically increase its strength. These increased mechanical properties have made it a valuable resource throughout various applications in industries including aerospace, medicine, optics and defense.

The two distinct aluminum alloy classifications are cast and wrought. Casting is the most widely used and is applied to products for the automotive industry. There are seven types of wrought alloys with varying limits of tensile strength and can be used for plating applications. Both the electroplating and electroless plating processes are applicable for aluminum, especially in telecommunications and energy industries.

Aluminum is a popular choice for mirror coatings because of its reflectance. With the highest reflectance of any metal in the ultraviolet and infrared spectral ranges, aluminum only falls behind silver in the visible light and near-infrared ranges. Bare aluminum can be used in optical mirrors for telescopes in space to achieve larger bandwidths of light. Unfortunately, the natural creation of aluminum oxide can reduce the reflectiveness of the aluminum coating. To counteract this, additional protective coatings can be applied over the aluminum.

A natural barrier oxide layer can form over aluminum mirrors, protecting it from further corrosion. Unfortunately, this barrier is not easy to clean and can be altered based upon other environmental factors. Because of this unreliability, protective coatings are often placed over the bare aluminum. This usually involves a dielectric coating that successfully protects the surface from damage. However, this could potentially reduce the reflectance of the aluminum. An atomic layer deposition technique or another deposition method could potentially improve protection while limitedly effecting mirror reflectance.

Though the presence of oxygen may cause aluminum to corrode, even if the aluminum does have an additional protective coating, there is hope that future optical mirrors used in an astronomic application could be positioned far enough from the Earth not to be impacted by these environmental factors. In this case, mirrors with protected aluminum coatings may be well-preserved and function without issue for long periods of time.

Applications That Use Silver Plated Mirrors

Silver coated mirrors can be used in telescopes. For example, the Gemini Observatory has switched from using an aluminum coating on their telescopes to a silver coating. This resulted in a direct improvement in lowering the emissivity of their primary and secondary mirrors and increasing the telescope&#;s sensitivity. As one of the only telescopes to use silver mirrors, the Gemini Observatory is achieving significant infrared optimization. Though environmental factors do lead to the silver tarnishing, they employ an annual multi-layer coating process to protect the surface from tarnishing.

Researchers believe that, as new mirror-coating technologies are developed, they can provide increasing positive benefits when applied to optical mirrors in new telescopes. They even suggest that current telescopes could see a dramatic increase in efficiency and reflectiveness by switching from an aluminum coating to these new silver coatings. This could potentially offer a cost-effective way to allow existing telescopes to reflect more light and increase their application limitations.

The aerospace industry utilizes silver coating technology as well. The Kepler Mission sought to explore the structure and diversity of planetary systems. While building spacecraft for this mission, engineers needed to design and build optical mirrors to enable them to continuously monitor and measure the brightness of stars. The large and lightweight primary mirror their team constructed needed to be sensitive enough to detect planetary movement, so they utilized an enhanced silver coating technology to increase the mirror&#;s responsiveness to light.

Applications That Use Aluminum Mirrors

Like silver coating, an aluminum coating can also be used in telescope optics. The NASA Cosmic Origins Program developed aluminum coatings to maximize reflectivity for their astronomical telescopes. This coating was used particularly to help their scientists study the far ultraviolet part of the light spectrum. The durability of the aluminum coating was effective enough to enable them to achieve a far ultraviolet reflectance of up to 80 percent throughout the year without the coating succumbing to significant deterioration.

The optics system on the Hubble Space Telescope is an excellent example of the application of aluminum coatings. Known as the Optical Telescope Assembly, this telescope&#;s optic system collects infrared, visible and ultraviolet light through the use of two mirrors. Each of these mirrors is coated with aluminum to provide optimal reflectivity. To keep the surface of the aluminum from oxidizing, layers of magnesium fluoride coat the top aluminum layers.

SPC Optical Mirror Plating

Optical mirrors are used across an assortment of industries, ranging from telecommunication and aerospace industries to optics and medical applications. Depending on the application, certain metallic mirror coatings must be used. Comparing the properties of silver and aluminum, each possesses inherent properties that positively impact factors like emissivity, reflectivity and durability. However, their differences also heavily influence application. For instance, though aluminum may offer a higher rate of reflection, silver may offer lower emissivity. Before selecting a coating, you should consult surface treatment experts at Sharretts Platting Company.

At SPC, we understand that the unique needs of your industry dictate that the optical mirrors you use need to deliver optimal performance. To meet the technical demands of your requirements, we use several different plating techniques including autocatalytic, electroplating and electroless methods. Our variety of precious and non-precious metals can be plated for applications ranging from power generation and optics to the medical, oil, gas and aerospace industries. For nearly 100 years, we&#;ve provided our clients with innovative metal finishing solutions to suit their specific needs. As one of the leading plating companies in the industry, SPC is ready to provide you with reliable and affordable silver coating and aluminum plating for your optical mirrors.

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SPC: Industrial Surface Treatment Experts

Sharretts Plating Company is recognized around the world as a leader and innovator in the metal plating industry. As full-service plating business, we dedicate our skill and expertise to enhancing our customer&#;s competitive positions. Our scientists, engineers and production staff continue to develop new ways of enhancing our production process to improve quality, lower costs and reduce lead times. SPC is focused on helping your manufacturing or industrial operation reach new levels of success with our ISO certified metal coating and plating services.

Specializing in metal finishing solutions, SPC is ready to lend our knowledge and expertise to help you determine the best plating for your optical mirrors. We&#;re able to efficiently coat metals, plastics, ceramics and glass. With hundreds of years of combined plating experience, we&#;re equipped to meet the needs of nearly any industry. SPC is the proud plating business leader and innovator that you can count on to satisfy even the most demanding technical requirements.

Choosing the right plating for your application can be simple when you are knowledgeable about the benefits and limitations of each coating option. SPC is here to ensure that all of our customers select the plating solution that will most effectively help them reach their business goals. We provide consulting services that will assess your operation and recommend a reliable and affordable metal finishing solution that is the ideal application to suit your needs. Call Sharretts Plating Company and receive a free quote today!

Are you interested in learning more about Sapphire Optical Components China? Contact us today to secure an expert consultation!

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