Knoop hardness number (HK)

Number related to the applied load and to the projected area of the permanent impression made by a rhombic-based pyramidal diamond indenter having included edge angles of 172º 30' and 130º 0'. The Knoop hardness number is the ratio of the applied load (usually, 0.5 kgf and greater) to the projected area of indentation measured in mm2. The area of indentation is proportional to the length of the long diagonal squared and a constant equal to 0.07028.