In thermodynamics, it is not necessary that the radiative transfer of heat be of pure black-body radiation, nor of incoherent radiation. Of course black-body radiation is incoherent. Thus laser radiation counts in thermodynamics as a one-way component of two-way radiation that is heat transfer. Also, by the [Helmholtz reciprocity] principle, the target system radiates into the laser source system, though of course relatively weakly compared with the laser light. According to Planck, an incoherent monochromatic beam of light transfers entropy and has a temperature. For a transfer to qualify as work, it must be reversible in the surroundings, for example in the concept of a reversible work reservoir. Laser light is not reversible in the surroundings and is therefore a component of transfer of energy as heat, not work.