For a mirror to be highly reflective, its back is evenly coated with a thin layer of metal. The metal you choose will depend on the mirror’s purpose and application. They all have different compositions, different properties, different uses and different prices.
The workhorse of the Replex line: pure, metallic aluminum is used for the backing in the overwhelming majority of our mirrors. The reason for this is the metal’s undeniable list of positive properties. Aluminum is relatively inexpensive and relatively stable, making it perfect for long-lived, durable mirrors for use in outdoor, weather-exposed applications. Aluminum is also highly reflective—typically between 86 and 90 percent. It also provides perfect color rendition to the naked eye, or when analyzed on a spectrometer.
The second-most commonly used metal, gold, is purely cosmetic. To put it plainly, people like gold. People choose gold for its aesthetic quality in high-end applications. When you walk into a casino and look up, you’ll see gold-tinted domes dotting the ceiling. That tint comes from 24-karat gold evaporated and applied over acrylic camera domes, usually also coated with a thin clear layer of silicon monoxide for protection.
Although it is chosen for cosmetic reasons, gold is highly functional. It is chemically stable and it won’t corrode. It is, however, very soft and therefore not suitable for use on mirrors that demand rugged durability. Did we mention that gold is also expensive?
Often erroneously referred to as “chrome” because it has a similar look, stainless steel is used most frequently after gold. We use a grade of stainless steel that is incredibly durable and chemically very stable. Customers order it when they want mirrors that are less reflective (perhaps 50 percent reflectivity) and darker. Adjustable rearview mirrors in cars, for example, can be flipped to high reflectivity in the daytime and lower reflectivity at night to reduce headlight glare from behind. Likewise some applications require lower reflectivity and stainless steel provides that.
Because stainless steel is chemically stable, we’ll sometimes take the uncommon step of evaporating stainless steel on the front of the mirror, not the back. These mirrors are called first-surface mirrors, and they can be found in aviation applications. They are currently being used in helicopters, where our customers required that no coating be applied beyond the metal for optical mirrors.
Silver is rarely used on plastic mirrors, and when it is, it’s usually for the purposes of research and development, not production. Silver isn’t very stable and it tarnishes quickly. Since we can’t protect it for long-term applications, it isn’t used for practical purposes. However many glass mirrors are backed with silver. But in those cases, the metal is applied with a wet chemical process instead of evaporation.
Bronze, Brass, Copper and Other Alloys
These metals and alloys are used almost exclusively for customers who need to match something that is already in place. If a customer has brass railings, for example, they may request brass mirrors be made to match.
These metals have different properties, different prices and are for use in different applications. The mirror’s use will almost always determine the appropriate composition of its metal reflective coating.