Why replace gold and silver?
Gold reflects excellently in the infrared range, while silver has the highest reflection of all metals in the visible spectrum. However, both materials have disadvantages:
- Cost: Precious metals are expensive and subject to significant price fluctuations.
- Oxidation and aging: Silver tends to oxidize, while gold is chemically stable but mechanically soft.
- Processing: The coating requires special vacuum technologies and adhesion promoters and is often challenging.
Dielectric layers as an alternative
Dielectric coatings consist of several thin layers of transparent materials with different refractive indices, such as TiO₂ (titanium dioxide), SiO₂ (silicon dioxide), or Al₂O₃ (aluminum oxide). Through targeted interference of light waves, reflection properties can be created that rival those of (precious) metals, without their disadvantages.
Advantages:
- High reflection in defined wavelength ranges (visible, near IR, mid IR)
- High environmental stability (no oxidation)
- Mechanical robustness
- Cost efficiency thanks to standard materials and established coating technologies
Use in the infrared range
Modern dielectric coatings offer an attractive solution, particularly in the near-infrared spectrum (e.g., 1–3 µm), which is crucial for many industrial and safety-related applications. Precise layer designs enable high reflectance levels to be achieved, comparable to gold, but significantly cheaper and more robust.
Example from automation technology: Sorting systems
In industrial automation technology, scanner mirrors with dielectric coatings are often used in optical sorting systems. These systems recognize and sort objects, such as food, recycling materials, or packaging, based on their shape, color, or material properties.
A typical example is the sorting of plastic parts by material type using near-infrared spectroscopy (NIR). Here, a laser beam is directed onto the conveyor belt via rotating or tiltable scanner mirrors. The reflected radiation is analyzed to identify the material. The quality and efficiency of the mirror coating is crucial for the accuracy and speed of sorting.
The use of dielectric coatings allows these mirrors to be optimally tuned to the wavelengths used (e.g., in the range from 1.4 to 2.5 µm) while offering a long service life and resistance to dust, moisture, and mechanical stress.
Advantages of aluminum mirrors (aluminum enhanced)
Conventional aluminum mirrors are metal mirrors and offer a high, color-neutral reflectance from UV to IR. They are suitable for broadband reflection of UV, VIS, and IR radiation and can be adjusted to different angles of incidence. They are particularly impressive in terms of their resistance to aging, especially when exposed to UV radiation. Glass (e.g., float glass) or plastics are suitable substrates, as are aluminum and, in some cases, other metals. The properties of aluminum mirrors are positively influenced by additional protective coatings, such as dielectric amplifier coatings (aluminum enhanced/AL enhanced).
Surface mirrors enhanced with aluminum coating and a dielectric protective or amplifier layer can be designed for different spectral ranges. They can be designed as aluminum front surface mirrors with an additional dielectric layer for the visual range, but also with a dielectric protective layer that is specially optimized for the NIR range and has a reflection-enhancing effect. Infrared light energy (SEA-NIR) is also reflected efficiently.
For use in automation technology and sorting systems, it is particularly important that the aluminum-enhanced scanner mirrors offer excellent optical imaging quality without double images or ghosting. Thanks to their high imaging quality, they support the systems in which they are used in efficient and fast recognition, for example in barcode reading or volume measurement. Depending on the coating supplier, they are also available in large formats, as are often required in recycling systems (deposit return), for example.
Efficient alternatives to precious metal coatings
The development of optical coatings has made enormous progress in recent years. Dielectric coating systems offer a high-performance, cost-effective, and durable alternative to traditional precious metal coatings. For manufacturers of scanner mirrors and users in automation technology, aluminum mirrors, especially aluminum enhanced surface mirrors, open up new, efficient possibilities, both technically and economically.
Advantages/strengths of aluminum enhanced:
- Reflection adjustable over a wide spectrum: VIS (visible range), IR to UV
- Reflection can be specifically enhanced
- High oxidation resistance / High stability against moisture and heat
- High hardness and mechanical robustness, more robust than precious metals
- High biocompatibility
- Very high climate resistance to moisture
- Very high availability
- Significantly lower costs than precious metals
