In the Knoop hardness test, a diamond pyramid indenter, which has a rhombic base with included angles of 172°30' and 130°, is pressed onto the specimen under a test force F (kgf). The hardness number (HK) is obtained by dividing the test force F by the projected area, A (mm2), of the identation. The area is calculated from the longer diagonal length, d (mm), of the indentation when the indenter is removed.
The Knoop hardness scale is generally used when shallower depth indentions are required. Knoop hardness can be measured by installing a Knoop indenter on the Macro-Vickers hard- ness testing machine.