ABSTRACT
The historical development of thin-layer coatings is described by F. Berthel and the practical necessities by J. Matteucci and P. Baumeister. Surface-silvered mirrors using metal coatings as the reflective surface may have their reflection increased as well as be protected from atmospheric tarnishing by addition of suitable coatings. The transmission and reflection properties of dielectric thin-film coatings vary with the angle of incidence of the input beam, as the optical path length is altered within the individual layers. The coating thickness can only be correct for one wavelength and is usually optimized for green light and hence looks magenta by reflected light. The type and effect of the various possible coatings are used not only to improve transmission but also to balance the spectral transmission of a lens to match others of different design. Consequently, the colour index and colour contribution of a particular lens may be given the desired values.
