Hardness defines the resistance to local deformation of a material when indented, drilled, sawed, or abraded. It involves a complex combination of properties (elastic modulus, yield strength, strain-hardening capacity). The prevailing deformation mechanism depends upon the material and the type of tester. Hardness is either measured by static penetration of the specimen with a standard indenter at a known force, dynamic rebound of a standard indenter of known mass dropped from a standard height, or scratching with a standard pointed tool under a load. The hardness tester, indenter shape, and force employed strongly influence the hardness numbers.

Hardness is used in identification, classification, and quality control. Hardness tests provide a rapid evaluation of variations in mechanical properties affected by changes in chemical or processing conditions, heat treatment, microstructure, and aging. Since the hardness test usually produces an insignificant permanent change in the specimen, it is considered to be a non-destructive test.

An impact device fires an impact body containing a permanent magnet and the very hard indenter sphere itself towards the surface of the test material. The velocity of the impact body is recorded in three main test phases;

1. The pre-impact phase, where the impact body is accelerated by spring force towards the surface of the test piece.
2. The impact phase, where the impact body and the test piece are in contact. The hard indenter tip deforms the test material elastically and plastically and is deformed itself elastically. After the impact body is fully stopped, elastic recovery of the test material and the impact body takes place and causes the rebound of the impact body.
3. The rebound phase, where the impact body leaves the test piece  with residual energy, not consumed during the impact phase. 

Leeb’s genius idea was to measure the velocity of the impact body contact-free via the induction voltage generated by the moving magnet trough a defined induction coil mounted on the guide tube of the device. The induced voltage is directly proportional to the velocity of the magnet, thus, the impact body containing the magnet. The induced induction signal is recorded in an electronic indicator device and the peak induction voltages are further processed to give Leeb’s hardness number, the L-value.