Glass#Optical properties
Under optical glass refers to glass for the production of optical components ( such as lenses, prisms and mirrors) for optical systems such as lenses, microscopes or telescopes. Optical glass is chemically different from normal glass not necessary, such as window panes are created from the, but its optical properties using targeted chemical additives set accurately. The optical and mechanical properties are also closely monitored and documented by the manufacturers.
Optical and physicochemical properties
The optical properties, which are crucial for the use of the glass for optical components are observed in the manufacture of the glass within tight tolerances and substantially precisely controlled as, for example, in window glass. Further, they are documented accurately by the manufacturer for each glass type. These include:
- The refractive index and its dependence on the wavelength of light ( dispersion). The latter is mainly characterized by the Abbe number. For a more detailed description of the dispersion partial dispersions and often the coefficient of a dispersion formula as the sell - Meier or the Cauchy equation are given.
- Deviations from the homogeneity of the glass, such as streaks ( band-shaped short-range fluctuations of the refractive index within the glass component in the range of 0.1 mm to 2 mm ), or the content of bubbles and other inclusions.
- Features which are important for the production of optical components and their behavior in use is important, such as the grindability, heat expansion, change in refractive index with the temperature or the sensitivity to chemical agents (glass corrosion).
Species and designation of optical glasses
There are over 250 optical lenses today. They are mainly based on the two main families of crown glass Glass K (English crown ) and flint glass F (English flint ). In addition, in the name of Schott glasses with the addition of S for " heavy " (English dense ), a high refractive index and L for "light " (English light), a low refractive index displayed (eg heavy flint glass SF6, or light flint glass LC5 ). Additional chemical components that are crucial for the optical properties are prefixed with the main types of glass in the description (eg barium crown glass Bak4 N- or S- BAL14 or fluoro - crown glass FK ). Environmentally friendly lead and arsenic -free glass is additionally marked by Schott with N ( eg, N- BK7 ) or S- at Ohara ( eg S- BSL7 ). Exceptions are glasses which are performed under their brand names, such as the glass- ceramic material Zerodur.
Chemical composition and transmission range
The following table gives the chemical composition in weight percent, and the transmission range of some important optical glass types again. The transmission ranges are approximate and may vary slightly in the sub- types of glass. Furthermore, the transmittance of the glass thickness is dependent.
Combination of optical glasses
The glass manufacturers offer a wide range of glass types with different refraction and dispersion properties. The more glasses has a designer to select which he (eg, a lens) can be combined in an optical system, the easier it is to correct the aberrations. For example, can be explained by the use of two ( achromats ), three ( apochromatic ) or more types of glass with different dispersion, minimize lens systems, the chromatic aberration in the visible spectrum or almost completely suppress. The measure of the remaining color error is the secondary spectrum, which is the average length at three wavelengths in relationship. With glass types that have been specifically developed to an unusual dispersion behavior towards, can be completely eliminate secondary spectrum. This mainly includes glass with a high refractive index and high Abbe number and glass with high dispersion in the blue part of the spectrum.
Combination of crown glass and flint glass in an achromatic lens for the correction of chromatic aberration.
Elimination of the secondary spectrum in a Apochromaten by use of multiple optical glass types.