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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.

Table 8

Class

Tolerance deviation of nd (△nd)

Class

Tolerance deviation of υd (△υd)

1

±30×10-5

1

±0.5%

2

±50×10-5

2

±0.8%

3

±100×10-5

3

±1.0%

       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.

Table 9

Class

Middle optical path difference δ(nm/cm)

1

2

1a

4

2

6

3

10

4

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.

Table 10

Class

The edge optical path difference δmax (nm/cm)

S1

3

S2

5

S3

10

S4

20

S5

80


6.3 Striae

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.

Table 11

Class

Feature of striae

A

No visible striae

B

Thin and dispersed striae

C

Slight striae in parallel

D

Visible striae in parallel


6.4 Bubbles

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.

Table 12

Grade

Total cross sectional area (mm2/2500px3)    Φbubble≥0.05mm

Permitting number per 1000 cm3    

Φbubble≥0.05mm

1

0.03

5

2

0.05

10

3

0.1

20

4

0.25

40



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.

Table 13


Grade

Max.

Grade

Max.

00

0.001

3

0.008

0

0.002

4

0.010

1

0.004

5

0.015

2

0.006

6

0.030





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