6.1 Tolerance of refractive index and Abbe-number
Tolerance deviation of nd and υd from standard value are divided into three class, as shown in Table 8.
Tolerance deviation of nd (△nd)
Tolerance deviation of υd (△υd)
Notes: Products can be supplied according to the contract on special request.
Refractive index and Abbe-number can be measured according to the method described in GB/T 7962.1. The accuracies of these measurements are ±3×10-5, ±2×10-5for the refractive index and central dispersion respectively.
6.2 Stress Birefringence
6.2.1 Middle stress
The middle stress birefringence of a glass blank is measured by calculating the optical path difference at the middle of the longest side according to the method described in GB/T 7962.5. It is expressed in units of nm/cm (δ). Classes of middle stress are shown in Table 9.
Middle optical path difference δ(nm/cm)
45(strips, coarse annealing)
6.2.2 Edge stress
In accordance with the test methods of GB/T 7962.6, the edge stress birefringence of a glass blank is measured by calculating the optical path differences at a point 5% from the edge (diameter or side length). The maximum optical path difference is expressed in nm/cm, δmax, and divided into 5 classes shown in Table 10.
The edge optical path difference δmax (nm/cm)
Striae are inspected by a striae-scope equipped with a point light source and optical lens system. Striae level is compared with standard reference samples and classified into three class as shown in Table 11.
Feature of striae
No visible striae
Thin and dispersed striae
Slight striae in parallel
Visible striae in parallel
Bubbles are inspected by the method described in GB/T 7962.8. A count is made to obtain the total cross sectional area (mm2) of bubbles per 100 cm3 of glass for bubble diameters Φbubble≥0.05mm and up to 0.5mm. The bubble quality of glass is classified into 4 grades as shown in table 12.
Stones, crystals and other inclusions are treated as bubbles. For oblate bubbles, cross sectional area is obtained from average value of the long and short-axes diameters.
Total cross sectional area (mm2/2500px3) Φbubble≥0.05mm
Permitting number per 1000 cm3
6.5 Coefficient of light absorption
The coefficient of light absorption of glass is measured and calculated according to the method described in GB/T 7962.9. It is expressed by the negative value of the natural logarithm of internal transmission per centimeter distance passed by white light through glass. It is classified into eight grades as shown in Table 13.