Abstract: Handpiece formed of a shell with a handgrip connected to a bundle of electrical power supply cables and of heat transfer fluid tubes and a head, accommodating a cartridge fitted with the controlled light source and with a light guide fed from the light source and the outlet surface of which constitutes the surface via which the light signal is emitted towards the surface of the skin that is to be treated. It comprises a housing, made in the shell, arriving under the emission surface of the cartridge and equipped with guide means and clip-fastening members. An interchangeable optical unit carrying the light guide is fitted removably into the housing. This optical unit has a casing accepting the light guide and equipped with guiding and clip-fastening surfaces to collaborate with the guide means and the retractable clip-fastening members belonging to the housing.