Page 95 - Chapter 3 - An Introduction to Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
Any remaining light energy may well penetrate all the way through the dermis and enter the sub-cutis tissue – the fatty layer. This might be reflected at this boundary, or it may be absorbed in the fat cells.
So, we can see that a number of things might occur with each photon – reflection, scattering, absorption, back-scattering, transmission. This occurs ALL the time, even with ordinary light.
Spot size
Again, more on this topic can be found in Chapter 1.
Spot size is pretty obvious! It is usually the diameter of the spot of light energy when applied to the skin surface. Combined with the applied energy, it determines the fluence – which is critical in achieving good clinical results.
However, the spot size changes as soon as the light hits the skin due to scattering. This causes the beam to expand as the light penetrates deeper into the skin, resulting in a loss of fluence. But, just to confuse things, the back-scattered fluence ‘combines’ with the forward-scattering fluence and adds up (see Chapter 1 for a detailed description on this subject).
Where do the photons go in the skin?
When photons (light) enter the skin they can undergo only two processes – absorption or scattering. Once a photon is absorbed, its energy is ‘gained’ by the absorbing site, usually resulting in a rise in the local temperature.
This may be a melanosome in the epidermal basal layer (see Figure 46 – ‘Epidermal absorption’). This absorption will transfer the photon’s energy into the melanin and raise its temperature (by a very small amount!)
More likely, the photons will scatter throughout the dermis as they ‘bounce’ off many atoms (this can be anywhere between 1 and several million atoms!!).
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