Magnesium fluoride (MgF₂) is commonly used for single-layer antireflection coatings MgF² Coatings is almost ideal because of it’s refractive index (1.38 at 550 nm) and high durability. These coatings are optimized for 550 nm for normal incidence, but as can be seen from the reflectance curves, they are extremely insensitive to wavelength and incidence angle.

Single-layer antireflection coatings are available for almost any angle of incidence and any wavelength between 200 nm in the ultraviolet and 1.6 μm in the infrared. As the 1.6-μm wavelength is approached, the angle of incidence range becomes restricted to near-normal incidence. This is because of practical limitations on physical coating thickness. It is usually inadvisable to request a MgF₂ coating for any wavelength greater than 1.6 μm. Heavier MgF₂ coatings are possible, but they generally exhibit crazing, poor adhesion, and usually produce significantly increased scattering.

Single-layer antireflection coatings for use on very steeply curved or short-radius surfaces should be specified for an angle of incidence approximately half as large as the largest angle of incidence encountered by the surface. Depending on the specific application, determination of the best wavelength for use in a coating specification may require ray and energy tracing of the optical system in its anticipated environment.

The effectiveness of MgF₂ as an antireflection coating is increased dramatically with increasing refractive index of the component material. This means that, for use on high-index materials, there is often little point in using more complex coatings. The reflectance curves shown below MgF₂ on BK7 optical glass.

Single Versus Multilayer Coatings

While MgF₂ does not offer the same performance as multilayer coatings, it is sometimes preferred in certain circumstances. On lenses with very steep surfaces, MgF₂ will actually perform better than multilayer near the edge of the lens because the performance of a coating shifts with the angle of incidence

Single-Layer MgF₂ coatings

                                          Standard / MgF₂ coating                                          IR-shifted, / MgF₂ coating

MgF2 Refractive Index Wavelength, Microns no ne 0,2 1,4231 1,4367 0,5 1,3797 1,3916 1,0 1,3736 1,3852 2,0 1,3686 1,3797 3,0 1,3618 1,3724 4,0 1,3525 1,3622 5,0 1,3400 1,3487 6,0 1,3242 1,3315