Evidently, light-tissue interaction depends on temporal parameters of the light, whether it is continuous wave (CW) or pulsed. A CW mode means that emitted waves are not intermittent or broken up into damped wave trains, but unless intentionally interrupted, follow one another without any interval of time between them. Pulsed light can be produced as a single pulse of duration tp (pulsewidth), measured in seconds (s), or as successive trains of pulses with some repetition frequency (rate) f, measured in hertz (1/s). Lamps can generate light pulses of duration tp in millisecond (ms) (10-3 s), microsecond (ps) (10-6 s), or nanosecond (ns) (l0-9 s) ranges, and only lasers can generate more shorter pulses, that is, in picosecond (ps) (l0-12 s) and femtosecond (fs) (l0-15 s) ranges with a high repetition rate / up to 100 MHz. A laser with Q-switching produces the so-called giant pulses, as the mode-locked laser produces ultrashort pulses with a high repetition rate. In dependence of technology used, the form of pulses can be different: rectangular, triangle, or Gaussian.
To describe energetic properties of pulsed light, a few more characteristics should be introduced, such as pulse energy Ep, peak power Pp (power within the individual pulse) and average power for a train of pulses. Peak power is calculated as Pp = Ep/tp. Thus, for ultrashort pulses, peak power can be extremely high even for low or moderate light energies, and tissue breakdown can be expected; however the average power of light, calculated as Pave = E x fp, cannot be very high. For example, if a light source generates pulses with an energy of Ep = 0.1 J, at a rate of fp = 1 Hz (1/s) and duration of tp = 10 ns, then Pp = Ep/tp = 107 W, or 10 MW, as the average power is only Pave = Ep x fp= 0.1 W or 100 mW.