Engraving with a laser involved substantial ablation of the surface of the workpiece. The full average output of the laser is concentrated in its focus, which results in an extremely high density of output or energy. The interaction with the material is defined in terms of the fluence. This measures the energy density required to produce a change in the material. The high concentration of energy causes the material to heat up very rapidly, resulting in expansion which causes material to be ejected from the gap produced by the interaction. This enables precise removal of material, which in turn produces an extremely robust and long-lasting marking.