Crown glass (optics)
Crown glass is a type of optical glass that is used in lenses and other optical equipment. They are so called because of the crown-like shape that is given to them after the process of blowing the glass.
Crown glass is normally produced from silicates of alkali-lime and contains about 10% of potassium oxide. The refractive index is low, around 1.52 and the value of dispersion is also low, with Abbe numbers usually around 60.
Crown glass in the middle ages – and today
'Crown glass' is a term used to refer to a type of soda-lime glass that has been used widely in window glass. It happens to be one of the most common glasses that are manufactured in the world. It is produced by a process which involves blowing fluid glass to form a spherical vessel. A crown shape is achieved by the blowing process, which is why this glass is so named. The blowpipe which is used for this purpose is removed, a suitable rod is attached to the molten mass of glass and is spun till the formation of a disc. The outer part of the disc is then cut into several planes. The finish used to be excellent, when this form of glass was popular in the later middle ages. Crown glass had been the preferred choice for window glass until the middle of the 19th Century.
Crown glass today is used to refer to the type of glass used in the manufacture of achromatic optical lenses. It is used along with flint glass in order to get rid of chromatic aberration in lenses. Borosilicate crown glass is the most popular glass used in optical lenses.
Borosilicate crown glass has excellent transmission and low refractive index of around 1.5.
Additives in crown glass for optical purposes
Crown glass is made with certain additives like boric oxide, which is present in borosilicate glasses and is added to the material in order to impart improved optical and mechanical characteristics. It also provides resistance to corrosion due to chemical action or exposure to the environment.
The other important additives include zinc oxide, phosphorus pentoxide, fluorite, and barium oxide.