To properly evaluate a laser one should look at three broad areas in context of your needs and expectations. 1) The laser physics and tissue interactions that form the basis of effective hair removal. 2) The characteristics of a specific laser. Both how it relates to physics and tissue interactions, but also factors such as engineering and economics. This is why two similar lasers from the same manufacturer may not meet the same needs due to their different characteristics. 3) The manufacturer or service provider must be
considered as far as reliability, dependability, and even whether they will around the next two years. These are issues that speak to the quality of the relationship that will be established between you and the laser manufacturer or the person who will support your laser.

Unfortunately these issues complicate deciding on a specific laser and are beyond the scope of this article. Instead, I will focus on the issues surrounding physics and tissue interactions. Finally, I assume that the reader has some understanding of basic light and laser physics and hair physiology.

Wavelength

We start with wavelength , the fundamental nature of photonic energy. There are two characteristics of wavelength that are most important. First, the shorter the wavelength the greater the absorption of laser energy by melanin; the longer the wavelength, the less the absorption (by 1,200 nanometers (nm), the absorption of energy by melanin is practically nil). This is critically important because we want to maximize the amount of energy absorbed by the hair, but have to consider the amount of energy that will be absorbed by the skin. The higher the absorption of melanin, the more that melanin in the skin will absorb energy. If enough energy is absorbed by the skin, this leads to leads to collateral damage, something we wish to avoid.

Second, the longer the wavelength the less photons will scatter and the deeper the laser pulse will effectively penetrate. The shorter the wavelength, the more it scatters and the less the laser pulse will effectively penetrate. This would not matter so much if it were not for the simple fact that the main target for laser hair removal resides at the base of the hair shaft, deep in the dermis. Therefore a very short wavelength (400 - 500 nm) does not penetrate deep enough to effectively destroy a hair.

So we have a classic dilemma. A short wavelength, will be highly absorbed by the melanin in the hair but will also be highly absorbed by the melanin in the skin, with a higher risk of collateral damage. In addition the shorter wavelength may not penetrate deep enough to effectively treat the hair. On the other hand, a long wavelength will penetrate deeper, but may not have enough melanin absorption to be effective. But it will be less likely to cause skin damage.

The first effective laser on the market was a ruby laser at 694.6 nm. This is a short wavelength and one which is very effective at treating hair though less effective at treating deeper hairs, such as men’s backs or woman’s faces. More importantly, because it is so highly absorbed by the melanin in the skin, it is difficult to use on anyone with any amount of melanin in their skin (skin types III - VI). For these reasons and some mechanical issues, it has fallen out of favor and is not much sold today.

The next longer wavelength is the alexandrite laser at 755 nm. This wavelength represents a good balance between deeper penetration and yet strong melanin absorption and is probably the most effective wavelength for people with skin types between I to III. But because it is still a shorter wavelength it can cause more problems with darker skin types which limits its effectiveness with skin types IV - VI. There are several manufacturers who make alexandrite lasers today.

Limitations in treating darker skin with the alexandrite led to the development of the diode laser, an even longer laser at 800 nm. This
wavelength allows the treatments of darker skin types (I - V) but at the cost of less absorption. This limitation can be somewhat overcome by increasing the power and we will discuss that in the section on power. There are a number of diodes, from many manufacturers, being sold today.

The final laser to be developed for laser hair removal was the Nd-YAG laser which operates at a long wavelength of 1,064 nm. At this wavelength there is little absorption of melanin, which allows it to be used effectively on the darkest skin types, but because there is less melanin absorption thin fine hairs with less melanin are difficult to treat. Luckily, most individuals with very dark skin have coarse dark hair allowing this laser to be effective.

Though it is often promoted as a laser for all skin types, individuals with lighter skin, who often times have finer hair, may not respond as well due to lower levels of melanin absorption. This is a popular laser, being sold by many manufacturers.

There are many who feel that to effectively treat a broad spectrum of hair removal clients requires having both an alexandrite laser for the lighter skin types with finer hair and an Nd-YAG laser for those with darker skin. On the other hand, there are others who believe that all one needs is a good diode laser or Nd-YAG laser.

Finally there is the intense pulse light (IPL) device. While technically not a laser, it operates on the same principles. But while a laser delivers a monochromatic pulse of light, the IPL delivers the entire spectrum of wavelengths which can be controlled by upper and lower filters to manage the photonic energy. Proponents of IPL believe that with the correct filters, IPLs can be used to treat all skin types. Though IPLs can be effective, the broad spectrum of wavelength is more difficult to control and research has not shown them to be any more effective or safer than lasers. Many proponents of lasers prefer lasers because the single wavelength is more predictable. There are a number of manufacturers who market and sell IPLs which today can be used not just for hair removal but for a number of other applications such as photofacials.

Power

It is not just a matter of having the right wavelength, but also of having enough power reaching the target. Power can be measured in watts, joules, or even BTUs but the best way to think of power is just as the absolute number of photons that reach the target. A few photons will generate low power, while lots of photons will generate high power. Without enough photons reaching the hair, there will not be enough heat generated to destroy the hair follicle.

There are two ways that lasers create energy in the form of photons. In pump chamber lasers such as the ruby, alexandrite and Nd-YAG, energy is pumped into a laser chamber and photons come out. Pump in more energy and more photons come out. So up to a point, the amount of photons put out can be easily increased. Depending on the engineering of the laser, how quickly the photons come out (pulsewidth) is independent of how many come out.

Diode lasers are much different. Diodes are electronic circuits that emit photons as a byproduct of an electrical current. Turn on the current, photons come out. How many photons come out is a function of how many diodes one puts into the laser. Therefore the number of photons can only be increased past a certain point by increasing the number of electronic circuits in the laser. In addition, the photons come out at a constant rate that can not be changed. Therefore to get more power out of a diode laser, it has to be “on”
longer. There are diode lasers that can generate high power, but can only do it by running for a significant period of time, up to half a second.

The problem is that hair, as it absorbs photons and heat, also releases heat. This is an issue for fine hair. The finer the hair, the quicker it releases heat and the shorter the pulse must be to effectively treat that hair. A laser that can not generate a short pulse (5 milliseconds or less) with enough power will not effectively treat fine hair. The only lasers that can provide a short pulse with effective power are the pump chamber lasers. And not all of them are capable of reaching high enough power to be effective due to engineering
decisions and designs.

 

Spot Size

 

How large the spot size a laser can create is significant for two reasons. First, the larger the spot size the quicker the treatment can be completed. But more importantly, at any wavelength, the larger the spot size the deeper the effective penetration of the pulse. This is because the larger spot size has more volume and therefore less scatter, meaning that more of the photons reach the deeper structures such as the hair. There is an direct relationship between increasing the spot size and increasing effectiveness. But increasing
spot size is a significant engineering decision as a small increase in diameter translates into the need for a four fold increase in power. Currently laser manufacturers offer lasers in spot sizes ranging from 7 mm to 18 mm and even larger if one considers the IPL spots.

The Perfect Laser

 

As you can see, there is no perfect laser. Nor could there ever be one. Yet manufacturers will tout the features and benefits of their lasers while telling you how their laser is the best laser since sliced bread. And they may be correct if their laser is what you need. But understanding their claims and how they relate to you means understanding how their laser fits in with the science of hair removal and skin and hair physiology. I hope that this article has given you a better understanding of the issues involved and the knowledge you need to evaluate their claims.

Steven Finder, MD, MBA, MPH