Thermal effects can be observed for irradiation times of between a couple of seconds and a few milliseconds and irradiation intensities of above 100 W/cm². These dominate in the long-wavelength part of the visible spectrum and in the IR spectral range. Here, enhanced vibrations of the tissue molecules lead to the heating of the tissue and the generated heat is transferred to the surrounding tissue. Damage can also occur due to the local increase in temperature.
There is a principal difference between thermal and photochemical effects. If, in the case of the thermal effect, the temperature of the tissue remains below a threshold - even for a longer lasting absorption of photons -, no damage is to be expected. On the other hand, in the case of the photochemical effect, the absorption of just one photon may cause damage at a molecular level. These changes are cumulative.