Page 58 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
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 0.5 mm of the upper dermis in a repeatable manner for each light pulse area. The reason why epidermal pre-cooling is so important is that you typically can provide about twice as high a fluence for a given pulse duration/skin type and thereby provide both safer and more effective treatment of hair follicles, compared with no pre-cooling.
The skin cooling should be provided by steering the pre-cooling time just before the light pulse. Note that the cooling depth is only dependent of the effective cooling time while the temperature of the heat sink/cooler does not affect the skin cooling depth. It may be difficult to understand that the depth is not dependent on the temperature of the cooler if you don't have a basic knowledge in thermo-physics, but this is a fact. The cooling affected skin depth is not dependent on the skin colour/tone. The optimal cooling time corresponding to complete epidermal and upper dermal cooling is in the range 90-120 seconds (in skin colour 1 - longer for darker skin colours), see the graph below showing the relation between skin cooling depth and cooling time.
When applying a contact cooling device (as typically found in IPLs and diodes lasers) the tip is usually cold. When this touches the skin surface it immediately begins to cool the epidermis. The longer the tip is applied, the deeper the cooling effect is in the skin. However, most devices do not provide enough cooling power to the tip to maintain the low temperature for any reasonable length of time. As a consequence, we cannot rely on these tips to cool the efficiently.
The graph (Figure 22) shows skin cool cooling depth versus effective cooling time. Skin cooling must always be provided before, during and after the light pulse. Post-cooling will remove the excess heat energy which is trapped in the skin. It is recommended to cool the skin for a minimum of around 3 to 4 minutes after the energy has been deposited. Note that cooling depth is not linear with the cooling time. This topic will be further discussed in a later section.
________________________________________________________________________ 58 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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