Flint glass
Flint glass is a glass with a high content of lead oxide. Optical flint glass is defined as a glass having an Abbe number less than 50. ( cf. crown glass )
Typical Composition
- Quartz ( SiO2): about 62 %
- Sodium oxide ( Na2O): 6%
- Potassium oxide ( K2O): about 8 %
- Lead (II ) oxide ( PbO): about 24 %
Depending on the lead content, a distinction light flint, flint or heavy flint.
Meanwhile, it is possible to achieve the optical properties of flint glass without the addition of lead. However, these lead-free glasses often have inferior transmission, therefore allows the EC Dangerous Substances Directive RoHS the use of lead-containing optical glasses even for a transitional period.
Properties
Because of the high percentage of lead oxide flint glass has a relatively high density of 3.5 to 4.8 g/cm3. It has a relatively high refractive index and a uniform optical dispersion. Flint optical glass, the refractive index is in the range of 1.5 to 2.0. Optical flint glass is defined as a glass having an Abbe number less than 50.
Due to the strong dispersion of the two different types of glass can be a diverging lens of flint glass with a convex lens of crown glass combined into a achromat, a cemented color-corrected collector lens. The Kronglaslinse is collecting more shaped than the flint glass lens scattering to achieve a collective effect in total. The powers of the two lenses times their specific dispersion results in two contrarian dispersion effects that cancel straight. Thus, different colors with the same refractive power to be collected. This is depending on the structure nevertheless still approached at least two wavelengths of light ( and in certain object distance ), for other wavelengths.
The flint glass used in optics, lenses as a refractive index of n = 1.7 to 1.8 to 1.9. Note that, as the refractive index of the lens is thinner, although, due to the higher density, and heavy. In addition, the imaging quality at very high refractive index materials suffer somewhat, as these ( see color fringing ) the chromatic aberration due to the lower Abbe number accounts for a higher proportion. It comes in the choice of a lens on a balanced relationship between thickness, weight, and imaging characteristics.
An alternative to lenses made of mineral glass represent high index plastic lenses, which, although easier, but more expensive. Plastic lenses are hard coated despite possibly susceptible to scratching, elastic by higher elongation at break, which is less sensitive to fracture, such as when climbing plan. Plastic lenses are today achieving maximum refractive index of n = 1.74, is phototrop discoloring of at least n = 1.67.
Use
Flint optical glass is used in a variety of optical instruments. Examples are telescopes, eyepieces or lenses of cameras. Particularly important is the ability to reduce in composite optical systems along with crown glass certain aberrations (see achromat ).
From flint glass also makes you use and ornaments, which should have a colored glitter. Due to the high dispersion of the light is more easily broken, such as in a prism in the different wavelengths, when it appears by the facets of an object made of flint glass (see, for example, lead crystal; rhinestone).
History
The name of flint glass is derived from the English word for Flint, Flint, ago. Flint nodules that were found in the limestone Southeast England, served by George Ravenscroft in 1662 as a source of high-purity silica. The silica was then required to produce a flint glass, which was the precursor of the English lead crystal.
- Optical glass