Anti-Reflective Coating

Anti-Reflective Coating

The reflectivity of an optical surface can be reduced by adding one or more non-absorbing dielectric thin film coatings. An anti-reflective coating (aka AR coating) takes advantage of the electromagnetic-wave properties of light to enhance transmittance. AR coatings improve the efficiency of optical instruments, enhance contrast in imaging devices, and reduces scattered light that can interfere with the optical performance of telescopes, cameras, and binoculars, and decreases glare on eyeglasses.

  • A single layer (quarter wave) AR coating of MgF2 will typically reduce reflectivity (glass with a 1.52 index) from 4% to 1.5%
  • A multi-layer AR coating can reduce reflection from 4% to .3% in the visible
  • A “V” design can reduce reflection to .1% across a narrow band of wavelength
  • EMF AR coating can be designed to function in UV, IR and NIR

EMF can coat many optical substrates including IR optics with an anti-reflective coating.

  • Glass
  • Sapphire
  • Germanium
  • Zinc selenide
  • Zinc sulfide
  • Calcium fluoride
  • Chalcogenides

Single-Layer AR Coatings

Magnesium Fluoride (MgF2) is often used as an AR coating suitable for visible light applications such as the crown glass. It has refractive index of 1.38 which is close to the anti-reflective ideal refractive index of 1.23. AR lenses for eyeglasses, cameras, and other visible light optical applications use MgF2 as the AR coating. The coating is hard and relatively easy to apply.

For applications targeting wavelengths outside the visible spectrum, dielectric coating materials like Silicon Nitride (Si3N4) and Titanium Dioxide (TiO2) are used as a single-layer coating to operate in the near-infrared region (NIR) region.

Multi-Layer AR Coatings

A multi-layer coating uses several layers of a thin film coating to successively reduce the reflected light. With a multi-layer coating, it is possible to reduce reflection to less than 0.1% of the incident light. A multi-layer coating can be designed for specific substrate materials, angle of incidence and wavelengths to optimize optical performance. Multi-layer coatings are more costly to manufacture, hard to apply and have to be carefully designed. They are typically reserved for high-performance optical instruments.

Typical BBAR Visible NIR Reflectivity Curves
Typical BBAR Visible NIR Reflectivity Curves
Anti-Reflective V-Coat Curves
Anti-Reflective V-Coat Curves

  • Large format (up to 35″ diameter)
  • Tight tolerance (±5nm at 50% transmission)
  • Glass or plastic substrates
  • 26 vacuum coating chambers
  • Volume production (40m sq. in. of coating capacity)
  • Short lead times
  • Chamber reservation system

Also Read: Primer on AR Coatings