Sometimes metallic coatings are used in other applications, such as beamsplitters, where half the light is transmitted and half is reflected. Also, very thin, semi-transparent, metallic coatings are used as infrared or UV filters. For example, the astronauts' visors are coated with a thin layer of gold.

All of the coatings used in optics are very thin, at most approximately one-half wavelength of light, but often closer to one-tenth wavelength. In some applications the thickness is critical, and it is important that the coating be uniform in thickness across the surface of the mirror or lens.

Metallic Coatings
Metallic coatings are made just thick enough to eliminate light "leaking" through the mirror. Typically, this thickness is about one-tenth wavelength of light, or about 50 nanometres. Adding more thickness causes the surface roughness to increase. A loss of contrast is the result.

The aluminizing of front surface telescope mirrors is carried out by a process known as thermal evaporation. Mirrors to be aluminized are cleaned and assembled in an evacuated chamber where a high vacuum (0.01u) is reached to allow the evaporation of pure aluminum to take place without oxide contamination.

To evaporate aluminum, a tungsten filament is clamped by both ends to electrical feedthroughs. Pre-weighed pure aluminum crimps are then hung off the turns of the coil. When heated in vacuo, the aluminum melts and rapidly vaporizes in all directions. The aluminum vapor condenses onto the mirror's face forming a thin metallic film.

Overcoatings & Protected Aluminum
Metallic coatings can be improved in certain respects. To make an aluminumized mirror more durable, both mechanically and chemically, a thin layer (_ wavelength optical thickness) of silicon monoxide is evaporated onto the aluminum. This is commonly termed "protected aluminum" In return for providing exceptional durability a slight penalty is paid in reduced reflectivity due to small light absorbing effects in the thin film. To minimize these light absorbing effects while still providing maximum protection, the SiO film deposit is of optical-interference thickness of _ wavelength optimized for ~5500A.

Semi-Enhanced Coatings
A second option is to overcoat the aluminum film with a _ wavelength layer of magnesium fluoride (MgF₂) to help preserve the overall reflectivity and at the same time provide some corrosion resistance protection. This is sometimes termed 'semi-enhanced' or 'UV enhanced aluminum.' As an interesting note, the Hubble Space Telescope's mirrors were overcoated with MgF₂, not so much for its protective virtue but more so to preserve the visible and UV reflectivity of non-oxidized aluminum.

Enhanced Coatings
A third option is to enhance the reflectivity by overcoating the metallic film with a _ wavelength thickness of MgF2 followed by a _ wavelength layer of higher index material such as zinc sulphide (ZnS). A constructive interference effect is produced at each interface boundary creating a subtotal of three reflections that are in phase with each other. Each of these boundary reflections add to give an overall reflection that is greater than the reflectivity of the base metal. Typically, the gain for an enhanced aluminized mirror can peak as high as 95% for a denoted portion of the spectrum. A further increase can be made by 'super-enhancing' by adding yet an additional pair of overcoatings.

Anti-Reflection (AR) Coatings
The usual purpose for a coating on a lens is to reduce reflections, which increases contrast and also increases the amount of light that passes through the lens. Conventional single layer anti-reflection coatings (SLAR) comprise a quarter-wave optical thickness of (typically) magnesium fluoride. Such coatings operate in the following way.

The amount of reflection as light passes from one transparent material into another is related to the ratio of the indices of refraction at the interface. Magnesium fluoride is often used because its index of refraction is close to the geometric mean of the index of many glasses and of air. Light passing from air to magnesium fluoride results in most of the light being transmitted, but some being reflected. The transmitted light next strikes the magnesium fluoride-glass interface, where, again, is largely transmitted, but some is reflected. The reflected light passes back through the quarter-wave coating where it destructively interferes with the reflected light from the first reflection. (The second reflection has travelled a total of one half wavelength in the coating layer, so it is a half-wavelength out of phase with the first reflection.) The effect is a cancellation of the reflected light which then results in more light (~97%) being transmitted through the lens.

One problem with single layer AR coatings is that we can see light between 700 and 400 nanometres. So what value do we assign to calculate the quarter wave thickness? Common sense tells us to pick something in the middle of the range, so green-yellow light is traditionally chosen. There will be more red reflected at the low end of the spectrum and blue at the high end, since the coating is not a quarter-wave for either. The result is a residual reflection comprising of mostly blue and red which when combined form a magenta hue. This characteristic colour is most often observed for single layer anti-reflection coatings. The limitation of single layer AR coatings can be further overcome by the use of multi-layer AR coatings. In this case the lens or corrector is coated with a series of different materials, having different indices and thickness, designed to straddle the entire spectrum. One sees many different colours looking off axis into a multi-element, multi-coated lens, the different colours resulting from the small residual reflections off the different layers.

Interference Filters
Interference filters are made by depositing many layers of materials, always with a different index of refraction for adjacent layers, of the same optical thickness. By controlling the number of layers, it is possible to pass only a very narrow band of colours and reflect or absorb all others. The common application of these in astronomy is in solar or nebula filters, which often pass only one emission line in order to increase the contrast of the image.