Page 43 - Chapter 3 - Fundamentals of Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
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 and blood vessels.
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/type. The optimal cooling time corresponding to complete epidermal cooling is in the range 1-2 second (in skin colour 1- longer for darker skin colours), see the graph below showing the relation between skin cooling depth and cooling time.
So, what is the problem with over-cooling of the skin? The problem is that the tissue target (blood vessels, hair follicles in dermis) also will be cooled down. When the target tissue is cooled you would need to apply a higher fluence in the light pulse to reach the desired temperature i.e. too much skin cooling can be counter-productive!!
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 it. The longer the tip is applied, the deeper the cooling effect is in the skin.
The graph (Figure 13) 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 around 3 to 4 seconds 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.
Figure 13 – The depth of cooling in the skin depends on the applied temperature and how long it is applied
When laser-hair treatments first were developed in mid 1990s no skin cooling was provided which resulted in very high rates of skin burns. It took about 5-10 years before the benefits of skin cooling were fully realised. The common procedure that was then adopted relied on
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