Laser marking is achieved by removing material from the substrate or by changing the surface of the substrate. The most important consideration is how well the material being coded absorbs the laser beam. This can determine the type of laser used as different wavelengths can have different absorption characteristics. If the laser beam is transmitted or reflected then coding becomes more difficult or even impossible.

For optimum results the laser beam has to be absorbed in the top few microns of the material surface, so that sufficient energy density is produced to modify the surface by one of the following three processes:

Coating removal: The laser is absorbed by the surface coating and vaporises it to reveal a contrasting substrate. An example of this process is the removal of coloured ink printed on to white paper or card.

Etching: The laser vaporises material from the surface of the substrate without producing any colour change. (This is the process which occurs in the laser marking of PET). The resulting mark looks similar to an embossed print.

Thermochemical: The laser changes the material by heating it to a sufficiently high temperature to break molecular bonds. The new material formed by this process may have a different colour, thus producing a discernible mark.