Mirror Coatings: More Than Meets the Eye

Don’t let the simplicity and elegance of metal-coated mirrors fool you.

We’ve learned a lot over the years about how to coax more reflectivity and reliability out of these ubiquitous optics. In this tech note, we’ll look at the various types of high performance metal coatings we offer at AccuCoat, the importance of metal purity, and how the use of multiple coatings can improve performance and durability.

Metal mirrors are an often overlooked workhorse of the optical world. This simple, low cost solution to routing light can also provide some measure of light gathering, focusing, or collimation if applied to the appropriate curved surface. Many optical systems have at least one metal mirror – often several – and thus their quality has a substantial impact on both the efficiency and durability of the system.

AccuCoat provides metal mirrors for many laser and lighting applications, ranging from bar-code scanners, range finders, and illumination systems to shutter blades, telescope mirrors, and components for scientific instrumentation and cameras. We work with a variety of metals spanning UV to far IR, many as standard coatings with protective and performance-enhancing overcoats. We also create custom designs as needed for unique wavelength, substrate, geometry, or operating condition needs.

Reflectivity over the desired wavelength range is the primary factor in choosing a metal coating material, though environmental stability plays a factor, particularly when no overcoat can be tolerated. Price is only truly a factor for rare metals like rhodium and platinum. Aluminum is used for most visible and UV applications, despite a dip in reflectivity at 850 nm, while Gold is favored for optics functioning in the near-infrared (NIR) and infrared (IR). Silver falls between the two, and is ideal for applications in the shortwave IR that demand high reflectivity.

Use of high-purity metals in the coating process is critical to reach the maximum theoretical reflectivity of each material. At AccuCoat, we use high-purity gold to ensure peak reflectivity through the IR, while we find aluminum performs very well at 99.999% purity, and silver at 99.99%. Not many coating vendors use 99.999% aluminum – most use an alloy, either to save money, or because they simply don’t know better. Metal coating materials suitable for decorative use on furniture and fixtures, for example, perform very poorly optically. That’s why it’s important to work with a coatings vendor who understands the impact of metal purity on performance, and to ensure that they are starting with high grade materials.

Left bare, most common metal coatings have limited physical or environmental durability. They scratch very easily, making them difficult to clean, and tend to oxidize over time (silver in particular). Dielectric overcoats can be used to protect delicate metal coatings, and even to enhance high reflector performance. These come in two variants:

1. Protected coatings: At AccuCoat, we employ a half-stack of silicon dioxide (SiO₂) to seal the metal surface, allowing it to be cleaned with alcohol, acetone and mild soap solutions. While this typically lowers the reflectivity slightly, it allows us to meet the adhesion, abrasion, temperature and humidity demands of various military and ISO specifications.
2. Enhanced coatings: By adding several more hi/lo layers of dielectric materials, we can not only ensure the same reliability, but also enhance the optical performance of the metal coating to provide better reflectivity at specific wavelengths or across broad bands. For some aluminum and silver coatings, that might mean increasing the reflectivity in the UV or blue regions, respectively, while for others it can mean peaking the reflectivity for a particular laser wavelength. By adjusting our overcoat design, we can shift the spectral profile to compensate for a metal’s natural spectral profile, or balance the needs of different operating wavelength ranges on a single optic.

While versatile, overcoats are not the right solution for every application. Use of an overcoat does result in some polarization splitting, which increases with the number of layers. It can also add some stress to the optic, so in cases where optical figure or flatness is important, it may be best to choose a mirror with fewer layers. As a result, we provide bare metal coatings for some customers, packaged carefully to protect the surface and facilitate easy handling. This is the preferred approach for gold optics used over wide ranges in the IR when maximum reflectivity and minimal polarization splitting is desired.

Dielectric mirrors are a commonly used alternative to metal mirrors; they offer >99% reflectivity at a specific wavelength or over a band, but require many layers to achieve this performance. An enhanced aluminum mirror, by comparison, yields ~97% reflectivity at far lower coating cost – a tradeoff that is well worthwhile in many optical designs. When optical figure or flatness is important, metal mirrors can even be preferable, as they experience far less stress between the coating and substrate than dielectric mirrors. Dielectric mirrors, however, perform well for laser damage threshold (LDT) testing with critical preparation of the optical surface and selection of coating materials.

Metal coatings can also be used over much larger angle of incidence without experiencing significant polarization effects or reducing reflectivity. Even when a dielectric overcoat is used, it can often be optimized for the AOI or angle range to minimize added polarization effects.

Dielectric coatings, in contrast, are the clear winner in caustic and acidic environments. These conditions are an issue for metal coatings, even with the protection of an overcoat, and limit their lifetime.

Neutral density (ND) filters have long been made from chrome-based coatings, including Inconel, chrome, and nichrome (an alloy composed of nickel, chromium, and often iron or other materials). The exact coating recipe used for the ND is determined by the range, spectral flatness, and optical density required, and can be customized to the application.

Reticles and parts that are etched often use chrome or dark chrome coatings, as it’s a very easy material to etch with appropriate methods. Chrome is extremely durable, and handles adhesion and scratch tests very well, making popular for military applications. Dark chrome coatings have a dark blue tint, and we often overcoat them with dielectric to achieve low reflection in the visible spectrum.

We’ve also worked with a number of less common metals requested by customers. Copper coatings, for example, may be used as conductor. Titanium creates coatings that are dark & durable, making it another option for etching. It can even be overcoated with other metals as needed.

Second Surface Mirrors

Metal coatings are often applied to create second surface mirrors with high reflectivity and durability. Gold, silver, and aluminum are all popular as second surface coatings, though we can provide any of our metal coatings – metal, enhanced or dielectric – as second-surface mirrors. We apply the mirror coating first and a protective coating second to achieve the full benefits of the mirror coating’s reflectivity in a prism, corner cube, or on a specialty lens. Once coated and protected, the back side of the second surface mirror can be painted, epoxied, or otherwise modified without impact to durability or performance.

With a variety of metal materials and the capability to coat almost any substrate or geometry, AccuCoat can work with you to provide the metal coating desired for your application. Our experience, knowledge, and commitment to using the highest quality materials will ensure it has the performance you need, so contact us today to discuss your unique requirements.