Laser parameters
Typically, lasers are discussed in terms of six parameters:

1. Wavelength (nm, nanometers)
   The laser’s wavelength is determined by the lasing medium, and is chosen based on the targeted chromophore (see below). Although most lasers used in dermatology emit rays contained in the visible light range of the spectrum (400-700 nm), some emit in the infrared range (1,000-11,000 nm). Lasers that emit in the ultra-violet range (150-350 nm, excimer lasers) are also used in dermatology, primarily for the treatment of psoriasis.

1. Pulse duration (ns, μs, ms, nano-, micro-, millisecond)
   The duration of each laser pulse (not relevant for continuous wave lasers). This duration will depend on the thermal relaxation time (see below) of the target.

2. Spot size (mm, millimeters)
   The spot size represents the diameter of the laser beam emitted. This diameter will influence the penetration of the beam in the tissue. Indeed, the larger the spot size, the less the scattering of the photons inside the tissue, leading to a deeper penetration of the laser beam. A larger spot size will also enable a faster treatment time, as each pulse covers a greater area.

3. Fluence (j/cm2, joules per centimeter square)
   Fluence measures the energy delivered per unit area. As the fluence increases, so does the destructive force of the beam. Depending on the laser used, the fluence will vary between 3-150 j/cm2.

4. Irradiance (w/cm2, watts per centimeter square)
   Irradiance measures the rate of energy delivery per unit area, i.e. irradiance describes the intensity of energy delivery. A high irradiance induces fast heating of the chromophore. Q-switched lasers have the highest irradiance (mega- to gigawatt output).

5. Repetition rate (Hz, hertz)
   The repetition rate measures the number of laser pulses emitted per second. A higher repetition rate leads to increased treatment speed.

Laser types
While most often laser types are discussed in terms of what they can treat, it is important to recognize the broader categories of lasers.   

1. Continuous wave (cw) lasers
   These emit a continuous beam and include the CO2 and krypton lasers. Pseudo-continuous wave lasers emit a beam in such close pulses that the effect on tissue is similar to that of a continuous wave laser. These lasers are used to coagulate tissue, as for example in the treatment of moles and warts.
   
   
2. Pulsed lasers
   Lasers that emit a beam in short pulses usually separated by 0.1-1 second. Pulsed lasers are more selective in their destructive effect than continuous wave lasers, and are used in selective photothermolysis.
   
   
3. Q-switched lasers
   Q-switching refers to the process of storing up laser energy in the laser cavity and releasing it in one single very short and extremely powerful pulse. This results in power outputs in the megawatt to gigawatt range, and allows for mechanical (vs. thermal) destruction of the target. Such lasers are often used in the removal of tattoos.

Conclusion
Now that we have discussed lasers from the technical perspective, stay tuned to for the second part of this discussion to better understand which lasers are used to treat which lesions, and how lasers compare and contrast to other light-based devices.

Maiman,T.H.: 1960, Nature, 187, 493.

Part 2

Introduction
As a follow-up to the article in the last newsletter, which discussed lasers from the technical perspective, this second part of the discussion aims to provide you with a better understanding of which lasers are used to treat which lesions, and how lasers compare and contrast to other light-based devices.

Laser Tissue Interaction

Lasers are effective in the treatment of various skin conditions because of their interaction with tissue, which falls in the following categories:

1. Reflection
   The beam is reflected off of tissue, with no clinical effect.

2. Transmission The beam is transmitted through tissue, with no clinical effect.

3. Scattering
   Intra-dermal molecules (for example collagen) induce a scattering of the beam’s photons, thus reducing the efficiency of their progression towards the target.

4. Absorption
   The beam’s photons are absorbed by the chromophore. Once absorbed, the light energy is transformed into thermal or mechanical energy. According to the Grothus-Draper law, which governs all laser light-tissue interactions, light absorption is required for effect on tissue.

In dermatology, thermal, more rarely mechanical, energy is responsible for the laser’s destructive properties. Thermal or mechanical energy is used to destroy the target that contains the chromophore. The speed at which the tissue is heated also has therapeutic properties:

1. Slow heating: this leads to the coagulation of tissue (thermal destruction).
2. Rapid heating: this leads to the vaporization of tissue (thermal destruction).
3. Extremely rapid heating (achieved by Q-switched lasers): this leads to the shattering or explosion of tissue (mechanical destruction).

The desired heating speed will be chosen according to the lesion to be treated and the necessity to protect the surrounding tissue. In addition to heating speed, the peak temperature is also a factor in obtaining the best possible treatment results.

Chromophores

Chromophores are molecules responsible for the color of a mass. These molecules absorb the energy of specific wavelengths. The most common chromophores encountered in dermatology are hemoglobin (which is targeted when vascular lesions are treated, including rosacea, leg veins, and Port Wine Stains), melanin (which is targeted during hair removal treatments, and for the removal of brown spots), tattoo ink (which is targeted during tattoo removal treatments), and water (which is targeted in most rejuvenating laser treatments).

Selective Photothermolysis

The theory of selective photothermolysis was developed by Dr. Rox Anderson at the Wellman Laboratory of the Massachusetts General Hospital in 1983. This theory revolutionized the use of lasers in dermatology by stating that laser light of a specific wavelength can destroy a target containing the adequate chromophore without damaging the surrounding tissue. This theory is based on the concept of thermal relaxation time. The thermal relaxation time of a mass is the time required for it to cool down to the ambient temperature after having been heated (for most targets, this time is determined by their size and shape, varying between 10 nanoseconds for a tattoo ink particle and 100 milliseconds for a hair follicle). Selective photothermolysis suggests that if a mass is heated for a period shorter than its thermal relaxation time, there is not enough time for thermal diffusion to damage the surrounding tissue. Stated another way, if a mass is heated for a period shorter than its thermal relaxation time, the heat and resultant damage (necessary for successful treatment) is confined to the target alone and the surrounding tissue remains unaltered.

Intense Pulsed Light Devices (IPL)
Intense pulsed light devices are not lasers. The two are indeed different from both a technology perspective and from a treatment perspective. The main difference between an intense pulsed light device and a laser is that the light of the former is non-monochromatic. While in the case of lasers, the lasing medium produces a single wavelength, and thus targets a single chromophore, IPL devices function with filters. Non-monochromatic light is created by an energy source, and then filtered. While IPL filters enable the therapist to select the range of wavelength best suited to a specific treatment, they do not enable the therapist to select a single wavelength: for example, a 585 nm IPL filter will filter out shorter wavelenghts, but not longer wavelengths. The therapeutic consequence is that a wider range of conditions can be treated with a single IPL device versus with a single laser (for example, both redness and brown spots can be treated with an IPL). In other words, an IPL is less specific in its effects. Some will argue that this lack of specificity makes IPL devices less effective, and more difficult to use (thus operator error is more likely).

Light Emitting Diodes (LED)
Light Emitting Diodes are neither lasers, nor IPL devices. The main difference between LEDs and lasers is that the light of the former is not collimated (but it is monochromatic); this means that the energy is divergent, and not focalized. The therapeutic consequence is that LEDs overall have a much lower energy output than lasers. LED treatments are still effective, however. Indeed, while the purpose of a laser is to selectively destroy a specific target (hence the need for a high energy output), the purpose of an LED is most often to stimulate (rather than destroy) a specific target, something that can be achieved with a lower energy. The main applications of LEDs are rejuvenation, photo-dynamic therapy, and cellulite reduction.

Conclusion
While as skin care professionals we are not asked to build lasers, improve upon IPLs, or manufacture LEDs, an understanding of the technology and of what is happening “in the box” is essential to providing our clients with accurate information as well as ensuring optimal treatments.

Ada Polla Tray is the President of Alchimie Forever, LLC and was a presenter at the SPSSCS 14th Annual Meeting.