Mirror Coatings for Optics - Reflecting on Reflection
Optical quality mirrors are not your everyday mirror design and are dependent upon the final application, the desired quality, and of course, your project budget. Mirror designs are available as standard off-the-shelf products or as highly custom for material thickness, diameter, and reflection performance. High-reflector coatings which are capable of 99.0% or more are typically used in laser systems as there is very little light lost. The vast global demand for optical mirror coatings used in military, consumer, and medical markets continues to grow every year.
Enhanced aluminum coatings, including a dielectric overcoat, typically reflect 92-95% of the visible light spectrum and are the most common coating for optical mirror production. Thin layers of aluminum are applied to create a flat and highly reflective surface. Protected aluminum coatings may contain a single layer dielectric overcoat and typically reflect 90-92% in the visible region.
Sometimes referred to as metalized platings, the end-use determines whether silver or aluminum coatings are required. Your application will determine the reflectivity requirement as well as the emissivity (the energy radiated from the surface of the mirror).
Aluminum is highly desirable for UV applications when used as a single mirror source. Its high reflectance properties for UV and IR make it a clear choice for telescopes, life science, research, and defense applications.
Silver mirrors, on the other hand, perform better overall in the visible band, as it is the most reflective surface until the light source falls into the UV at 400 µm. Unless protected, bare silver will tarnish over time, which is undesirable as it degrades the mirror’s performance. Observatory telescopes have shown improved performance by changing from aluminum mirrors to silver-coated mirrors. Silver offers less reliability when adhering to glass materials and is more suited for coating metal substrates.
Defending your optic - Protected vs. non-protected coatings
Typically for higher durability, the harder metals such as aluminum coatings fare better. To determine if a protective layer is required, you must consider environmental factors like heat, salt, or humidity. Commonly, MIL-Spec testing and specifications need to be met for specific use cases, even if not a military application. All metal mirror coatings are quite thin and can be easily scratched if not handled carefully or cleaned too aggressively. A protective layer is added to a softer metal for increased durability. Typically, this layer is made of SiO for a visible wavelength mirror and is harder than the coating layer of gold, silver, or aluminum.
Get best-in-class IR with gold coatings. Protected gold polygons and spinners for today’s laser scanning technology, including LiDAR, for use in the IR wavelength range, are a common mirror coating request at AccuCoat. A dielectric overcoating may also be added at the end of the layering process to create a protective barrier to keep the optic clean and less prone to scratching. With a high average reflectance (97-99%), protected gold coatings offer higher performance and are the preferred option when mitigating loss from the light source.
Tips on Ordering Mirror Coatings. The initial questions when you are looking to create a mirror surface optic can be daunting. AccuCoat has given the first time optical mirror buyer a solid ground to start with by using our checklist developed for this purpose, Top Specs for Coatings on Metals.
Items to consider include:
- Durability needs
- Light source specifications
- Clear aperture
- Angle of incidence
- Coating Type - bare, protected or enhanced
- Substrate material
- Design constraints
- Project budget
- The volume needed
For additional substrates tips, see our other checklists for RFQs on glass and plastics substrates.
The seasoned technical professionals at AccuCoat are available to walk you through the selection and design of the right coating for your application. We encourage you to contact our experts for support before your next mirror coating project.