Optical-element surface coatings have an important task in the functionality of optical elements and optical assemblies. In most cases they enable the elements or assemblies to perform according to systems requirements. Coating requirements, specifying mostly optical and durability properties, are defined in the drawing of the element to be coated and in a special coating specification.

According to its function

• Reflective coatings (metallic and dielectric),
• Antireflective (AR) coatings (single- or multi-layer, narrow- or broadband),
• Transparent conductive coatings,
• Beamsplitters,
• Filters (high-, low-, and band-pass),
• High-efficiency (HE) coatings,
• High durability (HD) coatings, and
• Diamond-like carbon (DLC) coatings;

According to the spectral wavelength range

• Ultraviolet: ~0.2–0.4 mm,
• Visible: ~0.40–0.75 mm,
• Near infrared (NIR): ~0.75–3.00 mm,
• Mid infrared: ~3–5 mm, and
• Far infrared: ~5–20 mm.

According to the place on the substrate

• Front surface coating, and
• Back surface coating;

According to the coating material

• Aluminum (Al),
• Silver (Ag),
• Gold (Au),
• Cupper (Cu),
• Rhodium (Rh), and
• Platinum (Pt).

Coating materials and their deposition-process parameters are usually proprietary and are not disclosed to the customer. The manufacturer specifies the materials, number, sequence, and thicknesses of the layers, and other parameters of the coating process to achieve the coating requirements stated in the drawing and in the specification. The customer may in some cases specify that the coating will be a single layer or multiple layers.
To produce a reflective coating, for example, the customer may specify the material that will be used for reflection (aluminum, silver, gold, or other materials) and if a protective overcoat layer is needed.