<P> In some applications, generally those that are cost - sensitive or that require great durability, such as for mounting in a prison cell, mirrors may be made from a single, bulk material such as polished metal . However, metals consist of small crystals (grains) separated by grain boundaries . Thus, crystalline metals do not reflect with perfect uniformity . Other methods like wet - deposition or electroplating produce a non-crystalline coating of amorphous metal (metallic glass). Lacking any grain boundaries, the amorphous coatings have higher reflectivity than crystalline metals of the same type . Electroplating must be performed by first coating the glass with carbon, to make the surface electrically conductive, thus the adhesion is often not as good as with wet - deposition . Both lack the ability to produce perfectly uniform thicknesses with high precision . When high precision or reflectivity is not a requirement, the coating may be placed on the back of the mirror so that the light passes through the glass, and the coating is the second surface it encounters . Therefore, these are called second - surface mirrors, which have the added benefit of high durability, because the glass substrate can protect the coating from damage . </P> <P> For technical applications such as laser mirrors, the reflective coating is typically applied by vacuum deposition . Vacuum deposition provides an effective means of producing a very uniform coating, and controlling the thickness with high precision . In applications where great precision and low losses are required, the coated side of the mirror may be the first material encountered by the light, referred to as a first - surface mirror . This eliminates refraction and double reflections, also called "ghost reflections" (a weak reflection from the surface of the glass, and a stronger one from the reflecting metal), and reduces absorption of light by the mirror . Technical mirrors may use a silver, aluminium, or gold coating (the latter typically for infrared mirrors), and achieve reflectivities of 90--95% when new . A hard, protective, transparent overcoat may be applied to prevent oxidation of the reflective layer and scratching of the soft metal . Applications requiring higher reflectivity or greater durability, where wide bandwidth is not essential, use dielectric coatings, which can achieve reflectivities as high as 99.997% over a limited range of wavelengths . Because the coatings are usually transparent, absorption losses are negligible . Unlike with metals, the reflectivity of the individual dielectric - coatings is a function of Snell's law known as the Fresnel equations, determined by the difference in refractive index between layers . Therefore, the thickness and material of the coatings can be adjusted to be centered on any wavelength . Vacuum deposition can be achieved in a number of ways, including sputtering, evaporation deposition, arc deposition, reactive - gas deposition, and ion plating, among many others . </P> <P> Mirrors can be manufactured to a wide range of engineering tolerances, including reflectivity, surface quality, surface roughness, or transmissivity, depending on the desired application . These tolerances can range from low, such as found in a normal household - mirror, to extremely high, like those used in lasers or telescopes . Increasing the tolerances allows better and more precise imaging or beam transmission over longer distances . In imaging systems this can help reduce anomalies (artifacts), distortion or blur, but at a much higher cost . Where viewing distances are relatively close or high precision is not a concern, lower tolerances can be used to make effective mirrors at affordable costs . </P> <P> The reflectivity of a mirror is determined by the percentage of reflected light per the total of the incident light . The reflectivity may vary with wavelength . All or a portion of the light not reflected is absorbed by the mirror, while in some cases a portion may also transmit through . Although some small portion of the light will be absorbed by the coating, the reflectivity is usually higher for first - surface mirrors, eliminating both reflection and absorption losses from the substrate . The reflectivity is often determined by the type and thickness of the coating . When the thickness of the coating is sufficient to prevent transmission, all of the losses occur due to absorption . Aluminum is harder, less expensive, and more resistant to tarnishing than silver, and will reflect 85 to 90% of the light in the visible to near - ultraviolet range, but is a poor reflector of infrared wavelengths longer than 800 nm . Gold is very soft and easily scratched, costly, yet does not tarnish . Gold is greater than 96% reflective to near and far - infrared light between 800 and 12000 nm, but poorly reflects visible light with wavelengths shorter than 600 nm (yellow). Silver is expensive, soft, and quickly tarnishes, but has the highest reflectivity in the visual to near - infrared of any metal . Silver can reflect up to 98 or 99% of light to wavelengths as long as 2000 nm, but loses nearly all reflectivity at wavelengths shorter than 350 nm . Dielectric mirrors can reflect greater than 99.99% of light, but only for a narrow range of wavelengths, ranging from a bandwidth of only 10 nm to as wide as 100 nm for tunable lasers . However, dielectric coatings can also enhance the reflectivity of metallic coatings and protect them from scratching or tarnishing . Dielectric materials are typically very hard and relatively cheap, however the number of coats needed generally makes it an expensive process . In mirrors with low tolerances, the coating thickness may be reduced to save cost, and simply covered with paint to absorb transmission . </P>

Examples of where two way mirrors are used