A beam splitter is an optical component which splits the incident light beam into two part-beams. Beam splitter coatings are used at a huge variety of angles of incidence and split the incident light of a specific wavelength or wavelength range into reflected and transmitted fractions respectively. A frequently-used beam splitter coating splits incident light in a percentage ratio of R : T = 50 : 50 or R : T = 30 : 70.
The use of dielectric and thus non-absorptive layers enables beam splitters to be produced with any split ratio desired. Dielectric layers are thermally stable and not particularly susceptible to corrosion. As the function of dielectric layer systems is based on interference effects, changes in spectral characteristics as the angle of incidence changes need to be taken into account. A variety of processes can be used to make the filters. In most cases, an ion-assisted PVD process is selected. In many cases, ion-beam sputtering may also make sense, especially if the splitting characteristics of the filter have a very narrow specification.
Beam splitters can also be produced with the aid of absorption. In such cases, the terms used are generally “neutral splitters”, “neutral density filters” (ND filters) or “attenuators”. Filters of this kind are almost always based on a metal coating.