This test measures hardness by making an indent in a material and measuring it.
The following is from http://www.instron.us:
A diamond indenter produces an elongated diamond shaped indent. Knoop tests are done at test forces from 10g to 1000g, so a high powered microscope is necessary to measure the indent size.
-The indenter is pressed into the sample by an accurately controlled test force.
-The force is maintained for a specific dwell time, normally 10 - 15 seconds.
-After the dwell time is complete, the indenter is removed leaving an elongated diamond shaped indent in the sample.
-The size of the indent is determined optically by measuring the longest diagonal of the diamond shaped indent.
-The Knoop hardness number is a function of the test force divided by the projected area of the indent. The diagonal is used in the following formula to calculate the Knoop hardness.
HK = Constant x test force / indent diagonal squared
The constant is a function of the indenter geometry and the units of force and diagonal. The Knoop number, which normally ranges from HK 60 to HK1000 for metals, will increase as the sample gets harder. Tables are available to make the calculation simple, while all digital test instruments do it automatically. A typical Knoop hardness is specified as follows:
Where 450 is the calculated hardness and 0.5 is the test force in kg.
ASTM-E384 Hardness Test
Instron Microindentation Tester page