4.1 Thermal expansion coefficient (α)
Thermal expansion of optical glass refers to the elongation of the unit length of glass when heated per 1℃ in a certain temperature range. The measurements are made in accordance with GB/T 7962.16. The mean coefficient of thermal expansion from +20℃ to +120℃ and from +100℃ to +300℃ (10-7/K) are displayed in figure 2.
4.2 Transformation temperature (Tg)
Optical glass gradually transforms from its solid state to a ＂plastic＂ state. Its transformation temperature Tg refers to the temperature at which a point intersected by two lines extended, respectively, from low temperature and high temperature zones of this curve when the glass specimen is taken from room temperature to sag temperature (Ts), as shown in figure 2. The measurements are made according to GB/T 7962.16.
4.3 Sag temperature (Ts)
As shown in figure 2, the sag temperature Ts is defined as the temperature at which thermal expansion stops increasing. The measurements are made according to GB/T 7962.16.
4.4 Strain point T10 14.5
The strain point refers to the temperature at which the glass viscosity is T10 14.5dpa•s (or 10 13.5 dPa•s), i.e. at this temperature, the stress of glass will be eliminated within several hours.
4.5 Annealing point T10 13
The annealing point refers to the temperature at which the glass viscosity is T10 13 dPa•s (or 10 12 dPa•s), i.e. at this temperature, the stress of glass will be eliminated within several seconds and it is also the upper limit temperature of glass annealing.