Laser light can be represented as periodic waves of energy traveling through space. Wavelength refers to the physical distance between crests of successive waves in the laser beam.
Laser power refers to the rate at which energy is generated by the laser. Laser power of 1 watt means that 1 joule of energy is emitted in one second.
Repetition rate – Frequency (Hz)
Medical lasers are usually operated in a repetitive pulse mode. Laser pulses are emitted periodically at a pulse rate, for example 10 pulses per second. Hertz(Hz) is the most commonly used unit for pulses per second.
Pulse duration (ms)
Pulse duration and pulse width are synonymous terms, which refers to the temporal length of laser pulse; that is, the time during which the laser actually emits energy.
Pulse energy (J)
The radiant energy in a laser pulse. The pulse Energy, unlike the Laser Power, is independent of frequency and repetition rate, because of this Pulse Energy is sometimes used in preference to Laser Power to specify treatment parameters.
Peak Power (W)
Peak power refers to the power level during an individual laser pulse.
Peak Power = Pulse energy/ Pulse duration
For a laser operating in pulsed Mode with energy of 1 Joule and pulse duration of 100 ms, peak power can be calculated as 10 W.
Spot Size (mm)
Laser beam spot size refers to the diameter of the laser beam on the target. By changing the laser beam spot size while keeping the laser pulse energy constant, the energy fluence can be changed substantially and thus the basic mechanism of laser beam interaction with the tissue (heating, ablation, vaporization) influenced.
Fluence refers to the amount of laser energy delivered per unit area. It is also called ‘dose of energy’ or ‘energy density’.
Fluence becomes larger as the spot size becomes smaller if all other parameters are held constant. Fluence is independent of spot size, frequency, and even the time of the treatment. This makes it useful for specifying general treatment characteristics. Other parameters must also be properly adjusted to efficaciously and safely apply laser treatment.
One of the most important optical features of target tissue is its ability to absorb the laser light. The amount of absorbed energy versus the total used energy is called absorption coefficient. Absorption Coefficient = Absorbed energy / Laser energy.
Thermal Relaxation Time (TRT)
TRT can be defined as the time it takes for an object to cool down from 100º C to 50ºC. It is a rule that a smaller object cools faster than a larger object of the same material and shape which means that the smaller target has a shorter thermal relaxation time. This fact is important when the tissue needs to be heated to a desired temperature at a certain fluence setting. If the pulse width is too long, the tissue will start cooling itself via thermal conduction prior to the completion of a pulse causing a negative clinical effect. The second parameter that should be taken into consideration when estimating the TRT is the shape of the target tissue. A sphere (skin cells) having 360 degrees of cooling surface area cools faster than a cylinder (hair follicles). This allows hair follicle to retain its heat while the skin cells can cool itself much more efficiently. For this reason, parameters can be selected to destroy the follicle without causing damage to the skin. For targeting smaller structures, a shorter pulse duration and higher fluence are recommended.