Page 126 - Chapter 3 - Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 2nd Edition
PA did a series of calculations in a Monte-Carlo model and found that a significant amount of the light energy will be turned through more than 90 – and a large chunk of this light will fly straight back out of the skin – we call this ‘back-scattering’. PA found that up to 60% of the light entering the skin may be back-scattered out, depending on the wavelength. (Just to confuse the matter, scattering and back-scattering, are very dependent on the wavelength of the light!)
Of the light that remains in the dermis, some of it will be absorbed by various tissue components – collagen, water, blood, bilirubin, keratin etc.
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. We will describe this further later.
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 (Figure 51). 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 74 – ‘Epidermal absorption’). This absorption will transfer the photon’s energy into the melanin and raise its temperature (by a very small amount!)
________________________________________________________________________ 126 Chapter 3, Ed. 2.0 Laser/IPL Hair Removal
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