In the September 2011 issue of Lasers in Medical Science, Esper et al. [1] report that LED irradiation reduces pain after applying elastic separators during orthodontic therapy. The positive outcome is of interest, since until now only lasers have been used for this kind of treatment. The authors compared the effect of an LED to that of a laser and found that, with the parameters used, the LED was superior. Fortunately, the authors give a good account of the parameters used and these give a clue to the poor effect of the laser. In order to compare two phototherapies, the parameters need to be as equal as possible. In this case, they were far from equal. The dose is correctly indicated as 4 J/cm in both groups, but using the dose parameter can lead to several pitfalls. A dose (energy density) is calculated by dividing the applied energy by the size of the irradiated area. If the probe irradiates an area of 1 cm and the energy is 1 J, the dose is 1 J/cm. If the area irradiated by the probe is 0.25 cm, the same 1 J of energy results in a dose of 4 J/cm. In the study by Esper et al. [1], the LED’s power was over three times that of the laser but its irradiated area was almost ten times larger; thus the power density of the LED irradiation was substantially lower. Consequently, in order to reach the 4 J/cm dose, the irradiation time for the 100mW LED must be almost three times that for the 30-mW laser (70 s and 25 s, respectively). During their respective irradiation times, the LED delivers an energy of 7 J, whilst the laser delivers just 0.7 J. Thus, all parameters except the local energy density at the irradiated spots vary considerably. Pain reduction with phototherapy stems partly from a reduction of the inflammatory process [2] and partly from an inhibition of neural signalling [3]. The inflammatory process is best reduced using a low power output and a longer irradiation time [4], whereas acute pain reduction requires higher energies. The energies used in the cited laser studies by Turhani et al. [5] and Youssef et al. [6] were 2.25 and 8 J, respectively, and the best effect reported comes from the latter. Turhani et al. used a single irradiation with a 670-nm laser, while Youssef et al. used an 809-nm laser applied at 0-, 3, 7and 14-day intervals after every activation of the spring which was done every 21 days. However, in the study by Esper et al. only 0.7 J was applied with the laser. The importance of coherence is still controversial, but the reference by Esper et al. to work by Karu is misleading. The investigations by Karu were based upon irradiation of monolayers of cells. Here coherence is of no importance, as both coherent and noncoherent radiation are capable of achieving intensities above the minimum threshold required to produce effects. For bulk tissue, however, the situation is quite different. When a volume of tissue is irradiated with laser light, random interference between coherent ‘waves’ of laser radiation within the same plane of polarization creates regions of polarized light with high intensity [7, 8]. J. Tuner (*) Private dental clinic, Spjutvagen 11, 772 32 Grangesberg, Sweden e-mail: jan.tuner@swipnet.se