To be able to optimize laser hair removal, we first need to have a look at the science and mechanisms that make laser hair removal possible.
For hair growth to stop you need to destroy the dermal papillae, located at the base of the hair follicles. The dermal papillae play a crucial role in hair growth cycle by inducing follicle development from the epidermis to produce hair fibre.
One method of destroying the dermal papillae is with heat and some forms of light work very well to create this heat. The dermal papillae do not absorbed the light directly. The light is absorbed by pigments in the hair and skin, melanin. These pigments absorb certain wavelengths of light and convert it to heat. The heat then spread outwards from the hair (and skin). Thus, the hair serves as an element.
You can compare this with an element in a water boiler. It takes electricity and convert it to heat. The heat then has to spread over the complete volume of the boiler for long enough to get hot water at the outlet.
The hair itself is not the target, it functions as the element that needs to heat a certain surrounding area over a minimum threshold temperature for a minimum period of time. To achieve this we have to get three parameters right.
The first is wavelength, the easiest of the three. The light needs to be in a wavelength that melanin absorbs. Most equipment come standard in a wavelength proven to be absorbed, between 600 nm and 1100 nm.
The second is energy. But, it is not just energy. You also need to consider the surface you divide the energy over, fluence. It is needed to apply enough energy (light) over a given area to have sufficient conversion to heat. If you do not have enough heat at the spot here the dermal papillae are located, you will not destroy them. Keep in mind that only a small portion of the heat that you create reach the dermal papillae. Most of the heat will be spread over the epidermis and the hair shaft, and dissipate over the length of the follicle.
The biggest problem here is that the same melanin that sits in the hair is also present in the epidermis. However, you do not want to destroy the epidermis or the tissue surrounding the epidermis. Therefore, skin cooling is of utmost importance.
The third parameter is time. The dermal papillae need to be heated for a minimum period over a certain temperature before it will degrade and die. Originally it was thought that the thermal relaxation time (TRT) was the most important influence on the pulse duration. TRT is the time it takes for the hair to lose half of the heat that was generated. The TRT for hair range between 10 ms to 100 ms.
However, lately it has been accepted that the thermal damage time (TDT) is of more importance when it comes to hair removal. TDT is the time required, for the entire target, including the primary chromophore (e.g. melanin) and the surrounding target (e.g. hair follicle), to cool by about 63%. The TDT for hair range between 200 ms and 400 ms.
Thus, general rules of thumb to optimize laser hair removal.
You have to realize that the treatment will have a severe impact on the epidermis. For this reason it is of the utmost importance to ensure sufficient skin cooling during a laser hair removal treatment.
You need to use a fluence that will ensure the desired reaction. A low fluence will have no results and in some cases may stimulate hair growth. The easiest way to find a starting point is to begin with the manufacturer’s recommendation and then increase the fluence in small steps until you reach a level where the client feels it as a “snap”, but it is still well tolerated.
Lastly, you need a long pulse length. Given that the upper range of the TDT is 400 ms, you want to use a pulse length equal to that or longer. Even if you prefer to use the TRT as main reference it still requires a pulse length of 100 ms. General rule is that you use the longest pulse duration available to you for laser hair removal.
Realize that these rules of thumb are not set in stone, but will be sufficient in 95% of cases.