JML Optical

Introduction to Coatings

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Almost every lens receives an antireflective coating to maximize transmission or image brightness, and to minimize ghost images. In fact, complex lens designs involving six or more elements could not realize their maximum potential if it were not for antireflective coatings. 

For mirrors, coatings have replaced solid castings of polished metal in all but a few specialized applications.  Mirror coatings perform with better reflectivity than solid metal mirrors, are lighter in weight and cost less to produce.

In the last 15 years uses for optical coatings have expanded beyond their original applications as antireflectors for lenses and reflectors for mirrors. For example, some coatings are used as transmissive electrodes to activate electro-optic materials, and highly durable coatings can improve the resistance of sensitive optical components to harsh environments as well.

Mirror coatings reflect light and antireflective coatings transmit light by reducing reflection. It is easy to forget that optical coatings are components because they always work with lenses, prisms, windows or solid mirror substrates, whose imaging properties occupy most of the systems-design effort. 

From the perspective of development and manufacturing, coatings can be classified as metallic, dielectric or hybrid and as single-layer or multi-layer. Metallic coatings are typically deposited by evaporating a metal, such as aluminum or gold, in a chamber so that the vapor condenses on the substrate. Other methods include ion-beam-assisted deposition, sputtering and electrolytic deposition. Dielectric coatings are made of dielectric materials (electrically non-conductive) such as magnesium fluoride (MgF2). Hybrid coatings consist of dielectric layers deposited on a metallic base layer. Purely dielectric coatings may be single layer or they may be stacked to form multi-layer coatings with improved characteristics. Hybrid coatings are always multi-layered. 

Applications engineers classify coatings as reflective or antireflective and broadband or narrowband. Broadband coatings handle a broad range of wavelengths whereas narrowband coatings are designed for one color, i.e., a narrow range of wavelengths. A high-performance coating is not just one coating but several thin films deposited on top of each other. Any one of the layers might yield modest performance. Working together, however, a reflective stack of layers can achieve very high reflectivity (99.9%) and an antireflective stack can achieve very low reflectivity (0.1%). Each layer is very thin, typically one-quarter to one-half the wavelength of light, or about 10 to 20 millionths of an inch. Design and manufacture of multi-layer coatings are complex and difficult but computers and new vacuum deposition techniques make them cost effective.

Coatings are designed to work under specific conditions of illumination, tilt and environment because their performance changes with wavelength, polarization of light, intensity of light, angle of incidence, humidity and temperature. Simpler single-layer coatings, such as MgF2 or aluminum, exhibit more modest performance and more latitude in application when compared to high-efficiency, multi-layer coatings.

Choosing a Coating

Choice of a coating is most influenced by the reflectivity or transmission required at certain wavelengths.  Altogether there are seven issues involved in the design and manufacture of a high-quality coating.

1. Wavelength

2. Reflectivity or transmission

3. Polarization of light

4. Angle of incidence

5. Substrate material

6. Intensity or power of light, and

7. Environmental conditions.

Wavelength

All coatings exhibit different reflectivity or transmission at different wavelengths. They may be classified as either broadband or narrowband. Broadband coatings handle large regions of the spectrum. For example, a broadband antireflective coating for visible light will reduce the reflective loss at the surface of a glass element for wavelengths between 400nm (violet light) and 700nm (red light). A broadband mirror coating, such as aluminum, can effectively reflect light as short as 350nm in the ultraviolet and as long as 10,000nm in the infrared. Narrowband coatings are designed to work in just one narrow region of the spectrum. V-coats are narrowband antireflective coatings that reduce reflections at a glass surface over a small range of wavelengths. For example, a high-efficiency V-coat designed for helium-neon laser light of 632.8nm might reduce reflective loss to just 0.1%. Its reflectivity might rise to more than that of uncoated glass at 500nm, just 133nm toward the blue.  A narrowband mirror for helium-neon lasers might reflect 99.5% of red 632.8nm radiation but only 80% of blue-green light at 500nm.

By classifying all coatings, reflective (mirrors) or antireflective (transmitters), as either narrowband or broadband, the wide variety of coatings can be organized in a simple, logical format:

A. Broadband

1. Metallic

2. Dielectric

a. Single layer

b. Multi-layer

3. Hybrid (metal and dielectric)

B. Narrowband (multi-layer, dielectric)

The broadband category includes every kind of coating structure: metallic, dielectric and hybrid. Narrowband coatings are limited to multi-layer dielectric structures because they achieve their performance from complex optical interference effects between the coating layers.

Reflectivity or Transmission

The reflectivity required of a coating completes its fundamental specification. Reflectivity usually defines the behavior of both reflective and antireflective coatings. For reflective mirror coatings high reflectivity is desired. The opposite is true for antireflective coatings; low reflectivity characterizes high-efficiency performance. The performance of single-layer coatings is less efficient than that of multi-layer coatings, but single layers are the most forgiving and the least expensive because of their simplicity.

Broadband multi-layer coatings reflect or transmit over a broad range of wavelengths with performance exceeding that of single-layer coatings. When compared at specific wavelengths, a broadband multi-layer coating can outperform a broadband single-layer coating by a factor of ten. For example, a magnesium fluoride (MgF2) single layer antireflective coating might exhibit reflectivity of about 2% at 550nm, whereas a multi-layer coating designed for the same central wavelength of 550nm might exhibit 0.2% reflectivity.

Narrowband multi-layer coatings can be designed to outperform broadband coatings at specific wavelengths. Narrowband antireflective coatings are often called V-coats. The terminology originates in the appearance of graphs that plot reflectivity against wavelength. A V-coat can perform with 0.1% reflectivity at a specific wavelength, but its reflectivity rises quickly for shorter and longer wavelengths. The graph of its performance looks like the letter “V.”

Polarization of Light

We all observe the effects of polarization every day. When materials around us—asphalt roads, window glass, water and vegetation—reflect one kind of polarization better than the other; we say that they cause glare. Glare often results from the efficient reflection of horizontal, s-polarized light. For example, Polaroid® sunglasses absorb the horizontally polarized light and thereby reduce glare from horizontal surfaces.

Light is composed of many colors or wavelengths, and each ray of specific wavelength can, in general, be analytically decomposed into one or two linear polarizations.  Polarization, which refers to the direction of the electric field vector in a ray of light, is a concept that grows out of the electromagnetic wave theory of light.  A “polarized” ray of light is one that maintains a constant state of polarization over time; “unpolarized” light is polarized at any given instant in time, but is always changing.

Engineers loosely refer to polarization as either “vertical” or “horizontal.” The terms are used with reference to the plane of a surface rather than to our usual sense of up and down or sideways. When intersecting a vertical surface, a horizontal polarization can be straight up and down if the ray of light approaches from one side. The precise terms for polarization are s-polarization and p-polarization. The first, s-polarization, is derived from the German word senkrecht, meaning perpendicular, because the electric field vector of s-polarized light is perpendicular to its plane of incidence. The second term, p-polarization, is taken from the German word parallel. This polarization is parallel to, or within, the plane of incidence (Figure 1).

Figure 1

As a rule of thumb, s-polarizations are reflected more efficiently than p-polarizations. This rule leads to unofficial terms for the two polarizations: skipping and plunging. Skipping, or s-polarization, “skips” off a surface with more intensity than plunging, or p-polarization, which “plunges” through the surface. Coatings can be designed for situations in which this rule does not hold but they are special cases.

Angle of Incidence

Angle of incidence is defined as the angle of light impinging upon a surface as measured from the normal or perpendicular line to that surface. Light with an angle of incidence of 0° comes straight down onto the surface; when light approaches at large angles of incidence, it skims over the surface.

Optical coatings are designed for peak performance at a specified angle of incidence. If the angle of incidence is changed during operation, for example by tilting the optical component, then performance will usually degrade. Narrowband coatings are most sensitive to this specification; a 15° tilt can alter reflectivity at its nominal wavelength by a factor of five and shift the wavelength of peak performance by about 10nm. Broadband coatings exhibit slightly more tolerance to tilt but a 30° change in angle of incidence will dramatically alter their performance.

A coating’s sensitivity to angle of incidence presents a challenge to the design of fast systems (small f-numbers, substantial light-gathering power) and wide-angle systems.  In a fast optical system, the converging or diverging cones of light intersect surfaces at many different angles. The rays at the center of a cone may approach a surface at the angle of incidence to which the coating was designed, but the outer rays may intersect at considerably larger or smaller angles. Likewise, a wide-angle system will contain rays whose angles of incidence cover a broad range.

Designers restrict the most sensitive coatings to planar surfaces in collimated beams. By definition, the rays in a collimated beam travel parallel to each other. Therefore, they all intercept a planar surface at the same angle of incidence.

Substrate

The same coating will perform differently when deposited on different substrate materials. This means that the exact formula for the structure and material of a coating, especially a multi-layer coating, will be tailored to the substrate. This variability is due to the fact that different substrates have different refractive indices. When tailored properly, nearly identical performance can be measured for the same class of coatings applied to different substrates.

Intensity or Power of Light

Some coatings are “soft” while others are “hard.” For imaging applications where the intensity of light is rather low, soft coatings withstand the radiant flux.  However, for high-power laser applications, such as welding or surgery, soft coatings would be destroyed by the radiant flux. Hard coatings have been designed for these high-power applications. The basic thin-film design philosophy is the same for soft and hard coatings; both are designed as stacks of thin layers. Their differences lie in the details of their prescriptions such as material composition and techniques of application.

Specifications that define the “softness” or “hardness” of a coating are written in terms of the threshold intensity that will damage the coating. For example, a typical hard infrared coating is rated for pulsed mode at one gigawatt/cm². A 20-nanosecond, Q-switched laser pulse with a peak power density of less than one gigawatt/cm2 should not damage the coating. Coatings are rated for their damage threshold under pulsed and continuous irradiation. Thresholds for damage are higher for pulsed modes of operation.

Environmental Conditions

Optical coatings must be handled with care. The harder coatings, which are resistant to laser damage, tend to resist scratching and abrasion, but even they are softer than many glasses. The softer coatings will be marred by careless or vigorous rubbing. Today’s coatings are much more durable than those used before 1940. Early coatings would stain easily because their porous microscopic structure trapped finger oils. They could not be cleaned once they had been touched.

Varying humidity or temperature can alter the performance of a coating. Those containing water-absorbing layers exhibit sensitivity to changes in relative humidity because absorbed water changes the layer’s refractive index. Temperature also affects refractive index and even thickness.  In the vast majority of cases, a coating’s sensitivity to the environment is small enough to be ignored. In critical or unusual applications more sensitive coatings are placed on components that can be protected from the environment. For example, a multi-element lens system might feature hard, durable coatings on its outer elements but softer, more sensitive coatings on its internal elements. JML Optical follows ISO 9211:1994 specifications for all of our coatings.

Antireflective Coatings for Efficient Transmission

Antireflective coatings are designed to reduce the amount of light lost to reflection at the surfaces of individual, transmissive elements. The need for the development of antireflective coatings comes from problems with loss of light and ghost images in compound, multi-element lens systems. 

Uncoated glass will reflect some of the light incident on its surface. For a lens element immersed in air, Fresnel’s equation predicts surface reflection for normally incident light:

Fresnel Equation

Reflectivity, R, of uncoated glass is a function of its refractive index, n. The higher the index, the higher its reflectivity.

In a multi-element lens there can be numerous glass-to-air interfaces because many of the elements are separated by air spaces. Lens designers use air spaces as degrees of freedom in the pursuit of a quality lens design.  Uncoated crown glass will reflect about 4% of the light incident on its surface, and uncoated flint glass can reflect as much as 8% because of its higher refractive index. Therefore, one uncoated crown glass lens element will transmit only ~92% of incident light; 4% is reflected away at each of its two surfaces. One uncoated flint element will transmit only ~85%.

For a basic three-element, air-spaced, compound lens with two crown elements and one flint, transmission is limited to 72%. In other words, image brightness is reduced by 28% unless reflection at each glass surface can be reduced. In this basic example of an air-spaced triplet, 28% of the total light that could have helped to brighten the image never reaches the image plane. In an uncoated, air-spaced, six-element lens with four crown glass elements and two flint glass elements, reflective loss is about 50%! Clearly, this significant loss of image brightness is a heavy penalty to pay for the improved resolution of a sophisticated compound lens.

Antireflective coatings can save most of the light that is lost on uncoated glass. For example, if a single-layer coating of magnesium fluoride (MgF2) were used, then reflection at each glass surface would be reduced to about 2%. The hypothetical six-surface lens in the box above would then transmit 89% of the incoming light; only 11% would be lost. A more efficient, multi-layer antireflective coating would reduce losses to 0.5% at each surface.  Then total transmission would be raised to 97%, and only 3% of the light would be lost to reflection. Antireflective coatings circumvent the penalty of low image brightness incurred by an uncoated compound lens.

Ghost images are another penalty associated with uncoated elements that can be minimized with antireflective coatings. The term ghost images comes from the observation that they are superimposed over the primary image but do not obstruct it. Instead they appear as shadowy apparitions floating among the details of the image. Reflections from internal elements can cause ghost images. Most of the time they appear as halos, spots, and star-like patterns.

What is the nature of a reflection that creates a ghost image? There can be a large number of reflections in a compound lens. Reflections that originate from the incoming beams of light may be called primary reflections. They propagate out of the lens toward the object and carry away energy that could have contributed to the brightness of the image. Many other reflections travel through the lens toward the image plane. These are the secondary reflections of the original, primary reflections (Figure 2).

Secondary reflections directed at the image plane encounter some lens elements before they reach the image. They experience refraction as if they emanated from the object; however, since they were created inside the lens itself, they produce strange patterns in the image plane. Ghost images can be controlled in several ways but application of antireflective coatings accomplishes the task most effectively.  When the intensity of the original reflection at each surface is reduced by even a moderate factor, the final intensity of secondary reflections at the image plane is reduced exponentially. Mathematically ghost images still exist in the image plane but their extremely low intensity renders them invisible. For special cases in which absolutely no background intensity or fogging can be tolerated, baffles and special considerations during design can be used to further reduce the intensity of ghost images.

Figure 2

Calculations of Reflective Loss When There Are Multiple Reflections

Here’s how to calculate the triplet example. Transmission, rather than reflective loss, is used for most of the arithmetic. First, convert reflective loss to transmission at each surface. For this simplified example of crown glass, 4% is lost at each glass surface; 96% is transmitted. For one glass element with two transmitting surfaces, total transmission equals 96% of 96%: .96 times .96 gives .922, or 92.2%. One flint element transmits .92 x .92 = 84.6%. For two crown elements with four glass-to-air interfaces and one flint element with two interfaces, as in the example above, six terms are to be multiplied: .96 x .96 x .96 x .96 x .92 x .92 = .719, or approximately 72%. The mathematical shortcut is to express the multiplication in exponential form: (.96)⁴ (.92)²  = .719. The final arithmetic operation involves converting total transmission to total reflective loss: 1 - .719 = .281, or approximately 28%.

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Reflective Coatings for Efficient Mirrors

The first mirrors were made of polished castings of solid tin, bronze, copper, gold, silver or speculum, an alloy of copper and tin. They were usually flat and were used by the ancients as cosmetic mirrors. Today solid-metal mirrors are used only in applications handling very hot beams of light. In these cases, metal’s high thermal conductivity enables efficient cooling. Solid-metal mirrors often contain channels for the circulation of cooling fluids. Copper is commonly used for these mirrors. Its reflecting surface can be enhanced with an overcoat of another metal.

For the vast majority of applications, thin films of metal or dielectric materials are deposited on solid-glass substrates. The advantages over solid metal are in performance and cost.  The reflectivity of metal films for use in the visible spectrum runs from about 75% to 96%. In the infrared gold can achieve more than 98% reflectivity.   Dielectric films designed for broad spectral regions can achieve reflectivities of 99%; when they are designed for a specific wavelength, they can achieve 99.8% reflectivity (for unpolarized light).

Some of the light that is not reflected by these reflective thin films is absorbed by the coating. This “lost” light is converted to heat through interactions with the atomic structure of the coating. Another fraction of the “lost” light is scattered by microscopic surface texture and impurities in the coating. The remaining fraction of “lost” light travels completely through the coating, to be absorbed or scattered by the underlying substrate. Small improvements in a mirror’s reflectivity can have dramatic impact upon the performance of a system. Consider a system of four mirrors designed for an application in white, visible light. If each mirror consists of a simple film of aluminum then its average reflectivity across the visible spectrum is about 90%. Throughput for these four mirrors is (.90)⁴ = 66%. In other words, 34% of the incoming light is lost to the mirrors. That same four-mirror system, installed with highly efficient broadband dielectric coatings offering 99% reflectivity at each surface delivers (.99)⁴ = 96% of the incoming light!

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