Page 42 - Chapter 3 - Fundamentals of Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
Figure 12 – How light is attenuated by absorption and scattering in the skin
The epidermal melanin is a kind of optical/thermal barrier and too much melanin will set a limit in performing different types of treatments. In treatments of hair the light that has to reach the specific targets in dermis must pass through epidermis. It is obvious that a portion of the total light energy applied for these treatments will be lost as heat in the epidermal melanin. If the peak temperature in epidermis is higher than the peak temperature in the hair follicle then the treatment should not be performed since it will create unwanted skin burns.
Figure 12 shows how light (blue, green and red) are attenuated, by scattering and absorption, as they penetrate into the skin. All light must pass through the melanin in the basal layer of the epidermis. This will, inevitably, raise the temperature of that layer – which we must mitigate against.
We can also see that the way light traverses the dermis depends on the wavelengths. Blue like tends to be scattered very widely near the skin surface. As a consequence, it cannot penetrate far into the skin. Green and yellow light penetrates deeper because it is not scattered as widely as blue light. Red light undergoes, what is known as, ‘forward-scattering’. It still scatters, but in a narrower beam than the lower wavelengths.
This process is known as ‘anisotropy’ in physics – it describes the magnitude in which light spreads out as it penetrates the skin. It also has a significant effect on ‘back-scattering’ where photons are turned around and can leave the skin altogether. This is discussed in more detail in Section 3.2, ‘Penetration Depth’, since it can have a major effect on treatment outcomes.
Optimal Skin Cooling in Laser Hair Removal
The objective of consistent and depth-controlled skin cooling is to selectively cool down the epidermal skin layer plus about 0.1 mm off upper dermis in a repeatable manner for each light
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