Description
All glass used in the production of optical devices belong to this group.
Glass for lenses
By convention, glass for lenses is divided in Crown glass and Flint glass, the first are sodium calcics, the second are lead, characterised by a different index of refraction and optical dispersion. There are other types of glass which are produced using lanthanum, thorium and tantalum oxide.
The biggest telescope mirror is under construction in the United States.
It is a giant plate made of pure glass, devoid of any imperfections like bubbles and lack of homogeneity, is over eight metres in diameter, weighs twenty tonnes and is about 20 centimetres thick. It is said that it is the most difficult and perfect work in glass ever made. It took more than four months to cool the fusion slowly, to room temperature, to avoid even the smallest tension in the mass.
And it will take months of work to level and shine the surface on which a reflective metallic film will be applied.
Glass transmitting UV and IR rays
For objects transmitting ultraviolet rays, quartz glass and glass made of a suitable composition which is extremely pure and is devoid of iron, lead, titanium and cerium is used so as to reduce the transmission of UV rays.
Glass with good transmission of infrared rays are the calcium aluminates and the calcogenuri, which is glass devoid of oxygen, with an arsenic-sulphur-selenium base.
These last ones are used for objects for night vision for both civil and military purposes.
Selective glass
This is glass which can selectively absorb in a particular wavelength band
- glass containing one or more colouring ions which absorb visible rays;
- glass containing either lead glass which absorbs UV rays;
- glass containing boric and phosphoric acid which absorb IR rays.
Glass for lasers
This is glass of the sodium-calcic and borium-silicate type, with a lanthanum and thorium oxide base, spiced with elements like neodymium and ytterbium, which can emit a monochromatic light band (laser effect) when exposed to a light source.