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1.10ptical Homogeneity
1.2.1 Large Aperture Optical Glass
The optical homogeneity is defined as the maximum variation of the refractive index An... within a piece of glass. △nmax is measured by the method specified in GB/T 7962.2-2010,and classified into 6 grades (Table 1):
| Grade | △nmax |
| H00 | 0.5x10-6 |
| H0 | 1x10-6 |
| H1 | 2x10-6 |
| H2 | 5x10-6 |
| H3 | 10x10-6 |
| H4 | 20x10-6 |
1.2 Stress Birefringence
1.2.1 Middle Stress
The middle stress existing in the glass blank is measured by the method specified in GB/T 7962.5-2010. Optical path difference δ in nm/cm is measured along the longest direction of glass blank, and the middle stress reflects optical path difference per unit in the middle section. It is classified into 4 grades (Table 2):
| Grade | δ（nm/cm)The optical path difference at the center of glass δ |
| 1 | 2 |
| 1a | 4 |
| 2 | 6 |
| 3 | 10 |
1.2.2 Edge Stress
It is measured by the method specified in GB/T 7962.5-2010, measuring the optical path difference at a distance of 5% of the diameter or from the longer side of glass blank. Edge stress (nm/cm) is defined by maximum optical path difference δ max per unit thickness and classified into 4grades(Table3):
| Grade | δmax（nm/cm)The optical path difference at the edge of glass δmax |
| S1 | 2 |
| S2 | 5 |
| S3 | 10 |
| S4 | 20 |
1.2.3 Stress Birefringence Requirements for Coarse Annealed Glass
For coarse annealed glass with the thickness smaller than orequalto 20mm, edge stress birefringence shall be smaller than or equal to 100nm/cm. For coarse annealed glass with the thickness greater than 20mm,the edge stress birefringence shall be smaller than or equal to 80nm/cm.
1.4lnclusions
Weight of inclusions in optical glass is measured by the method in GB/T 7962.8-2010, the content of allowed inclusionsin glass is decided by the total cross sectional area of inclusions (diameter Φ >0.03mm) in 100cm' glass and the inclusions are divided into 5 Grades, see Table 4.Cross sectionarea of flat long inclusions is calculated by arithmetic meanvalue of the longest shaft and the shortest shaft as the diameter.
| Grade | Total cross section area of inclusions with the diameter Φ ≥0.03mm in every 100cm3glass |
| A00 | S≤0.03 |
| A0 | 0.03<S≤0.10 |
| A | 0.10-S<0.25 |
| B | 0.25<S≤0.50 |
| C | 0.50<S≤1.0 |
1.5 striae
With the striae-scope,striae isviewed in the direction of maximum visibility.By comparing with a standard sample, the striae is classified into 4 grades.(Table 5):
| Grade | Striae feature |
| A | Invisible to the naked eye |
| B | Fine and scattering striae |
| C | Slight parallel striae |
| D | Obvious parallel striae |
1.6 Coefficient of Optical Absorption
It is measured with spherical photometer by the method specified in GB/T7962.9-2010. There is internal transmittance when the white light passes the path per cm,then the coefficient of optical absorption equals the negative value of the natural logarithm of such internal transmittance.According to thevalue,the quality is divided into 8 grades (Table 6):
| Grade | The max.coefficient of optical absorption |
| 00 | 0.001 |
| 0 | 0.002 |
| 1 | 0.004 |
| 2 | 0.006 |
| 3 | 0.008 |
| 4 | 0.010 |
| 5 | 0.015 |
| 6 | 0.030 |
2.1Blank
2.1.1 Direct-press Blank
Direct -press blank is directly formed from melted glass, and is fine annealed.
2.1.2 Reheated-Press Blank
Reheated-press blank is formed by reheated - press process,and is fine annealed (Table 7):
| Outer diameter Φ （mm） | DP | RP | ||
| Outer diameter tolerance（mm） | Thickness tolerance（mm） | Outer diameter tolerance（mm） | Thickness tolerance（mm） | |
| Φ<15 | ±0.10 | ±0.20 | ±0.10 | ±0.50 |
| 15≤Φ<29 | ±0.10 | ±0.20 | ±0.15 | ±0.35 |
| 29≤Φ<40 | ±0.15 | ±0.20 | ±0.20 | ±0.30 |
| 40≤Φ<60 | ±0.20 | ±0.20 | ±0.30 | ±0.30 |
| 60<Φ<90 | ±0.30 | ±0.40 | ±0.35 | ±0.35 |
| 90≤Φ<150 | ±0.40 | ±0.40 | ±0.40 | ±0.40 |
| Φ≥150 | ±0.50 | ±0.50 | ±0.50 | ±0.50 |
2.2Gobs
It refers to the blank whose cross section is nearly round,and not fine annealed.On request,we can supply Gobs with there quired weight or volume.
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