Alternatively, the coating can be designed such that the mirror reflects light only in a narrow band of wavelengths, producing an optical filter. Such mirrors are often used as beamsplitters, and as output couplers in lasers. The level of reflectivity can also be tuned to any particular value, for instance to produce a mirror that reflects 90% and transmits 10% of the light that falls on it, over some range of wavelengths. Conversely, the reflectivity can be increased to greater than 99.99%, producing a high-reflector (HR) coating. Reflection coefficients of surfaces can be reduced to less than 0.2%, producing an antireflection (AR) coating. By careful choice of the exact composition, thickness, and number of these layers, it is possible to tailor the reflectivity and transmitivity of the coating to produce almost any desired characteristic. These are constructed from thin layers of materials such as magnesium fluoride, calcium fluoride, and various metal oxides, which are deposited onto the optical substrate.
using materials with a different refractive index to the substrate). The other major type of optical coating is the dielectric coating (i.e. These are sometimes used as " one-way mirrors". Most expensive is gold, which gives excellent (98%-99%) reflectivity throughout the infrared, but limited reflectivity at wavelengths shorter than 550 nm, resulting in the typical gold colour.īy controlling the thickness and density of metal coatings, it is possible to decrease the reflectivity and increase the transmission of the surface, resulting in a half-silvered mirror. More expensive is silver, which has a reflectivity of 95%-99% even into the far infrared, but suffers from decreasing reflectivity (<90%) in the blue and ultraviolet spectral regions. The metal used determines the reflection characteristics of the mirror aluminium is the cheapest and most common coating, and yields a reflectivity of around 88%-92% over the visible spectrum. The simplest optical coatings are thin layers of metals, such as aluminium, which are deposited on glass substrates to make mirror surfaces, a process known as silvering. wavelength curves for aluminium (Al), silver (Ag), and gold (Au) metal mirrors at normal incidence
0 kommentar(er)
