Flint glass
Flint glass is a glass with a high content of lead oxide. Optical flint glass is defined as a glass with an Abbe number less than 50. (see crown glass )
Typical composition
- Quartz (SiO 2 ): about 62%
- Sodium Oxide (Na 2 O): about 6%
- Potassium oxide (K 2 O): about 8%
- Lead (II) oxide (PbO): about 24%
Depending on the lead content, a distinction is made between light flint, flint or heavy flint.
It is now possible to achieve the optical properties of flint glass without adding lead. However, these lead-free glasses often have poorer transmission , which is why the EU RoHS directive on hazardous substances has allowed the use of lead-containing optical glasses for a transitional period.
properties
Because of the high proportion of lead oxide, flint glass has a relatively high density of 3.5 to 4.8 g / cm 3 . It has a relatively high index of refraction and uniform optical dispersion . The refractive index of optical flint glasses is in the range from 1.5 to 2.0. Optical flint glass is defined as a glass with an Abbe number less than 50.
Due to the very different dispersion of the two types of glass, a diffusing lens made of flint glass can be combined with a converging lens made of crown glass to form an achromat , a cemented, color-corrected converging lens. The crown glass lens is shaped to be more convergent than the flint glass lens in order to achieve a convergent effect overall. The refractive powers of the two lenses times their specific dispersion result in two opposing dispersion effects that cancel each other out. Thus, different colors with the same refractive power are collected. Depending on the structure, this applies at least to two light wavelengths (and for a certain object distance ), but still approximated for other wavelengths.
The flint lenses used in ophthalmic optics as spectacle lenses have a refractive index of n = 1.7 over 1.8 to 1.9. It should be noted that the higher the refractive index, the thinner the lens, but also heavier due to the higher density. In addition, the image quality also suffers somewhat in the case of very high-index materials, since the chromatic aberration (seeing color fringes) accounts for a higher proportion due to the lower Abbe number. When choosing a lens, it is important to have a good balance between thickness, weight and imaging properties.
An alternative to lenses made of mineral glass are high-index plastic lenses, which are lighter but more expensive. Despite the hardening coating, plastic glasses are more sensitive to scratches, but due to their higher elongation at break they are more elastic, i.e. less sensitive to breakage. Today, plastic glasses have a maximum refractive index of n = 1.76, phototropic glasses of n = 1.74.
use
Optical flint glass is used in a number of optical instruments. Examples are telescopes , eyepieces or lenses from cameras. Particularly important is the property of being able to reduce certain imaging errors in composite optical systems together with crown glass (see achromat ).
Flint glass is also used to make everyday objects and decorative objects that should have a colored sparkle. Due to the high dispersion in relation to crown glasses, white light, such as with a prism , is particularly strongly broken down into the various wavelengths when it shines through the facets of an object made of flint glass (see e.g. lead crystal ; rhinestones ).
history
The name flint glass is derived from the English word for flint , flint . Flint lumps found in the limestone cliffs of south-east England were used by George Ravenscroft as a source of high-purity silicon dioxide around 1662 . Back then, silicon dioxide was needed to produce flint glass, which was the predecessor of the English lead crystal .