5.2 Knoop Hardness (Hk)
The indentation hardness of optical glass is determined with the aid of a micro hardness tester. One face of the specimen with the necessary thickness is polished. The diamond indentor is formed rhombic so that the vertically opposite angle from two axes is 172° 30' and 130°. The load time is 15 seconds and the load is 0.98 N {0.1 kgf}. The glass specimen is indented at 5 places. Knoop hardness can be computed with the following equation:
| Knoop hardness |  |
| F | : | Load (N) |
| l | : | Length of longer diagonal line (mm) |
Table 9 shows how the glasses are classified according to Knoop hardness. Please note the Knoop hardness figures have been rounded to the nearest 5 (e. g. a value of 158 is shown as 160).
| Group | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| Knoop Hardness |
< 150 | >= 150 < 250 |
>= 250 < 350 |
>= 350 < 450 |
>= 450 < 550 |
>= 550 < 650 |
>= 650 |
| Table 9 |