Significance of Wavelength Range for Effective Hair Photo-Epilation

Introduction
The use of light sources as a major new alternative for hair removal, is generating tremendous interest as it provides a non-invasive and safe solution in this high demand consumer market. The potential benefit of photo-epilation over existing methods is faster treatment. Some risks are associated with this procedure due to the application of intense light energy to the skin. For this reason it is important that this mode of hair removal be conducted by properly trained medical practitioners, who understand the clinical applications as well as the dynamics of light-tissue interaction. EpiLight Hair Removal System is designed to effectively eliminate unwanted hair from all parts of the body, while minimizing potential adverse effects. It provides a broad range of treatment options to enable the physician to adjust treatment parameters according to the area of the body being treated, the color of the hair, and the skin type of the patient. Photo-epilation is accomplished by using intense light to damage and destroy hair shaks and hair follicles. The scientific foundation underlying the EpiLight Hair Removal System is the principle of selective photothermolysis in which light is absorbed specifically by the targeted tissue or chromophore with minimal damage to surrounding tissue. The most significant chromophore in hair which absorbs light is melanin, found in melanocytes in the hair shaft, hair bulb, and hair itself. Intense light is converted into heat when it is absorbed in chromophores such as melanin. When light is absorbed in the follicle, hair shaft, or the bulb, it raises their temperature. Once the temperature reaches a high enough level, the targeted hair structures are irreversibly damaged disabling hair regrowth. These conditions are best fulfilled by proper selection of pulsed light parameters used for photo-epilation. The most important parameters are the wavelength range of the light and the pulse sequencing, combined with the size of the area simultaneously illuminated during each application. Light which penetrates the skin is heavily dominated by scattering. Scattering of light involves the deflection of photons from their original direction while their energy is not absorbed by the tissue.
Figure 1 illustrates scattering of light in the tissue and the various paths along which incident photons penetrate the skin. Successful photo-epilation requires that the intense light penetrate deeply enough and with sufficient fluence to damage the hair follicle without damaging surrounding tissue, while taking into account scattering and absorption coefficients. Three criteria must be met for safe and effective photo-epilation:

- Light has to penetrate deeply enough to reach the bulb of the follicle

- Light has to be preferentially absorbed in the follicle

- Light applied to the skin has to penetrate the epidermis without damaging it

Wavelength Range – key for selective treatment
The penetration depth of light into tissue and its absorption is highly dependent on the wavelength range used, since scattering and absorption coefficients of the epidermis, dermis, blood, and hair are all functions of wavelength. Figure 2 highlights the absorption coefficients of the dermis, black hair and blood as a function of wavelength. Light penetration into the dermis is significantly deeper for longer wavelengths. Light absorption in the epidermis is much higher at shorter wavelengths. For example, in the 500 to 570nm wavelength range more than half of the light energy is absorbed by the epidermis. In the 750-1000nm wavelength range most of the energy penetrates into the dermis, with minimal absorption in the epidermis. Since most hair follicles are located in the dermis, this wavelength range is the ideal spectrum to effect appropriate follicle damage.
The blood absorption coefficient is quite high for wavelengths shorter than 600nm. This means that at these wavelengths blood vessels will also absorb light and may be damaged, causing adverse effects such as purpura. To effectively destroy hair follicles, without damaging other tissue, it is important to use wavelengths in the red and near-infrared spectrum. EpiLight Hair Removal System is designed with the flexibility to select and alter the wavelength range used for photo-epilation according to the hair type being treated and the pigmentation level of the individual patient's skin. The optimal wavelength range is selected to ensure deep enough damage to hair of all colors, and minimal damage to surrounding tissue.

Hair Removal from all Parts of the Body
Hair follicles are not located at uniform depths in the dermis. Different types of hair are located at significantly different depths one from another and even similar hair types may differ by as much as 2 or 3mm. For example upper lip or mustache hair follicles are 1 – 2.5 mm deep whereas axillary or pubic hair reaches depths of 3.5 – 4.8 mm. Table 1 highlights this fact. Effectively treating bikini line hair, located at 3.5 - 4.8 mm requires a wavelength which penetrates hair follicles more deeply than that required to remove hair from the upper lip, located at 1 – 2.5 mm. A laser which is set at a fixed wavelength cannot provide this type of flexibility and will not be able to effectively reach hair at all depths. Figure 3 illustrates hair types at their average depths, superimposed upon a graph of light penetration into skin. Effective penetration depth is defined as the depth at which the fluence applied to the surface of the skin is reduced to I/e of its value (35%) on the surface. The graph shows that light penetration depth is different for different wavelengths, and therefore, follicles located at different depths require longer or shorter wavelength ranges to reach them. EpiLight Hair Removal System with its adjustable broad spectral range can reach hair follicles at various depths, thus increasing the probability of damaging hair follicles and preventing regrowth.

Hair Color, Skin Color and Spectral Range
The hair absorption coefficient is a function of hair color. The main absorbing chromophore in dark hair is eumelanin, while in red and blond hair it is phaeomelanin. Dark hair generates a very good "contrast" ratio with the dermis since it has a much higher absorption coefficient. Blond hair has a much lower contrast ratio. Thus, it is much harder to damage lighter hair than darker hair, without damaging surrounding tissue. EpiLight Hair Removal System, eliminates all light hair colors hair as well as dark, as can be seen from the clinical cases on the first page. Another factor that controls the amount of energy absorbed by hair (and the resulting increase in temperature) is the thickness (diameter) of the hair and hair follicle. Figure 4 shows the calculated energy deposited in 3 different hair follicles as a func- tion of wavelength. The three cases shown are: fine shallow blond hair, medium depth and thickness red hair, and coarse deep black hair. The figure illustrates the responses of different hair types as a function of wavelength. It clearly demonstrates that longer wavelengths are more effective for dark and red hair, while the 600 to 700nm wavelength range may be more effective for hair that is thin, shallow and blond. EpiLight Hair Removal System has proven safe and effective for eliminating hair of all colors, including white hair, from all areas of the body. If hair does grow back (or new hair grows after treatment) doctors report that it is usually much finer than the original hair and the hair density is much lower (fewer hairs per same area). The high degree of success of EpiLight~ Hair Removal System treatments results in the user's ability to tailor treatments to fit the specific patient's hair/skin characteristics. Other treatment modalities attempt to destroy the deeper hair follicles by increasing the energy level of the light applied to the epidermis, which can cause severe epidermal changes. EpiLight wavelength ranges reach hair at varying depths and remove hair of all colors. Multiple light pulses as well as delays between pulses, another unique feature of EpiLight Hair Removal System, maximizes selectivity of treatment and minimizes side effects in the surrounding skin. EpiLight's large spot size enables large areas to be covered quickly, reducing treatment time. These versatile features are unique to EpiLight Hair Removal System and are unmatched by any other existing hair removal system.

Black and white hair on upper lip before and 4 months after the beginning treatment.

55 years old female with white hair on chin and above upper lip. Before and three weeks after eight treatments.

Figure 1: schematic representation of photothermal treatment of hair. The light (represented by the arrows on the top) penetrates the epidermis and part of the dermis to reach the hair follicle. Light may be absorbed in the hair follicle or in the epidermis and dernis.

Figure 2: absrption coefficients of dermis, hemoglobin and black hair.

table 1: depth of hair at anagen stage of growth according to body region.

Figure 3: different hair types according to average depth at anagen stage and light penetration into skin across the spectral range of Epilight.

Figure 4: Light absorption by hair as a function of wavelenght, hair color and density.