Knoop hardness is an indicator of hardness for a substance. The indentation hardness of optical glass is determined with the aid of the 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° respectively. The load time is 15 seconds, the load is 0.98 N (0.1 kgf}. Knoop hardness =1.451 - F/i² F : Load (N) i: Length of longer diagonal line (mm) Below tabel shows how the glasses are classified by the method referred to JOGIS-09. This catalog shows both the class and the Knoop hardness figures have been rounded to the nearest 5 (e.g. value of 158 is shown as 160.)

Class	1	2	3	4	5	6	7
Knoop Hardness	< 150	≥50 < 250	≥250 < 350	≥50 < 450	≥50 < 550	≥550 < 650	≥650