The differences between refractive index nd, abbe-number υd and standard values for each batch products are generally in the following tolerance ranges:
Refractive index nd: ±30×10-5
Abbe-number υd: ±0.5%
For special request, we can supply according to the following criteria:
Strips：
Refractive index nd：±2×10-5 ~ ±5×10-5
Abbe-number υd：±0.3%
Reheated-press blank：
Refractive index nd：±10×10-5 ~ ±20×10-5
Abbe-number υd：±0.3%
Products cut from round rod：
Refractive index nd：±5×10-5 ~ ±20×10-5
Abbe-number υd：±0.3%
When stress birefringence of glass is expressed by the optical path difference δ per unit in the middle section along the longest direction of glass, middle stress is divided into 4 grades as shown in table 9.
Table 9
Grade | δ(nm/cm) ≤ Middle optical path difference |
1 | 2 |
1a | 4 |
2 | 6 |
3 | 10 |
Remarks: Coarse annealed strips according to the contract. | |
When stress birefringence of glass is measured by the maximum optical path difference δmax at a distance of 5% of the diameter from the edge or per unit thickness from the side length of glass, edge stress is divided into 5 grades as shown in table 10.
Table 10
Grade | δmax(nm/cm) ≤ Edge optical path difference |
S1 | 3 |
S2 | 5 |
S3 | 10 |
S4 | 20 |
S5 | 80 |
Striae are inspected by a striaescope equipped with a point light source and lens system in a parallel path.
Examine the stripe image from direction of the cut surface where striae easily accur by turning round the sample, and then compare with the standard samples. Striae are classified into 4 grades as shown in table 11.
Table 11
Grade | Striae feature |
A | Free of striae |
B | Fine and scattering striae |
C | Parallel and slight striae |
D | Parallel and a bit thick striae |
With the continuous development of manufacturing technology of optical glass, bubbles have been improved obviously, but it is very difficult to manufacture bubble-free optical glass. The diameters and numbers of bubbles in glass are determined by glass types and melting conditions. Bubble grades are established by specifying the total cross sectional area (in mm2) of bubbles (diameter Φ ≥ 0.03 mm) in 100 cm3 glass volume, and are divided into 5 grades as shown in table 12.
Table 12
Grade | 1 | 2 | 3 | 4 | 5 |
Φ≥0.03 mm Total cross sectional area of bubbles（mm2/2500px3） | <0.03 | ≥0.03 <0.1 | ≥0.1 <0.25 | ≥0.25 <0.5 | ≥0.5 |
Stones, crystals and other inclusions are treated as bubbles. The cross sectional area of oblate bubbles is obtained by the average value of the longest axis and shortest axis as the diameter.
The coefficient of light absorption of glass is expressed by the negative value of the natural logarithm of internal transmission per centimeter distance passed by white light through glass. Coefficient of light absorption is calculated by internal transmittance of white light, and is classified into 8 grades as shown in table 13.
Table 13
Grade | Coefficient of light absorption | Grade | Coefficient of light absorption |
00 | 0.001 | 3 | 0.008 |
0 | 0.002 | 4 | 0.010 |
1 | 0.004 | 5 | 0.015 |
2 | 0.006 | 6 | 0.030 |
Optical homogeneity is expressed by the maximum value of the variation in the refractive index △nmax within a single piece of glass. It is measured by resolution method with collimator, according to GB/T7962.2 and is classified into 4 grades as shown in table 14.
Table 14 | |
Grade | △nmax Optical homogeneity |
H1 | ±2×10-6 |
H2 | ±5×10-6 |
H3 | ±10×10-6 |
H4 | ±20×10-6 |
For glass blanks used in extremely high-precision optical systems, variations in the refractive index within a single piece must be controlled within very narrow limits. Glass blanks with a tight index control or very high optical homogeneity control are manufactured by special manufacturing processes and inspected by an interferometer. Please contact us if required.