Guide to Mirror Coatings

Metallic Mirror Coatings for Maximum Broadband Reflection

For applications requiring broadband reflection, optical engineers have choices when designing mirror coatings deposited onto a glass or metal substrate. While there are various metals to choose from, the biggest overall advantages of metal coatings remain the same: uniform reflectivity over a wide spectral range and minimizing reflectivity losses with optimized coating designs and processes.

Metallic Coatings

In standard mirrors and prism configurations, light bounces off the back of the glass. To achieve highly reflective precision coatings, thin film optical coaters typically utilize a bonding layer to adhere the metal to the front surface prior to depositing the metallic coating. Internal reflectors do not typically use a bonding layer to avoid compromising transmittance and reflectivity requirements.

Metallic coatings are often protected and enhanced by adding a dielectric material such as amorphous SiO2 (silica) or Si3N4 (silicon nitride), which protects the coating against oxidation (tarnish) and scratches. Such enhanced or protected mirror coatings are more abrasion-resistant but tend to be more sensitive than dielectric mirrors. This implies that greater care is required in cleaning metal-coated optics. Metallic mirrors are also less sensitive to humidity and corrosive gases.

Mirror Coating Types

The majority of metal coatings are made of aluminum, silver, and gold. Less common materials include beryllium, copper, and nickel/chrome alloys. Metallic coatings are used in the visible and IR part of the spectrum up to 20 μm. Metal-coated mirrors can also work for extremely long infrared wavelengths. In that region, it is difficult to work with dielectric mirrors due to their strong absorption. Coatings are designed specifically to meet end-use requirements in the specified wavelength range.

Aluminum Mirrors

Protected aluminum mirrors, those with a SiO2 coating for example, are used for many broadband applications in the VIS and UV spectral region, reaching reflection levels well above 90% in the visible but often below 90% in the UV (similar to bare aluminum). Compared with silver mirrors, such enhanced aluminum mirrors have a lower tendency for tarnishing when exposed to humidity.

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