In general we divide the glass materials that we use into two groups: quartz and optical glass. Quartz consists exclusively of silicon dioxide (SiO₂) and shows some remarkable properties:

• Quartz displays a high UV transmission, in highly purified synthetic quartz down to well below 200 nm.
  
• The thermal expansion of quartz is extremely low, its coefficient of thermal expansion being 6 x 10^-7K^-1 between 20 °C and 300 °C.
  
• Quartz is chemically very resistant and maintains its shape, even at high temperatures up to approximately 1000 °C.
  

Common criteria for all types of optical glass are:

• that the cells made from it are used in the visible range of the spectrum
  
• that the glasses we use are characterised by relatively low values of refractive indexand dispersion
  
• that they display good resistance to chemicals.