Page 93 - Chapter 3 - An Introduction to Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
Figure 45 - As the cell temperatures increases, their rate damage increases – exponentially!
A relatively small increase in fluence (1 or 2 J/cm2) can lead to a very large increase in cell damage. So, we must always be careful when increasing the fluence – it’s not just a straightforward process.
Plus, an increase from 20 to 21 J/cm2 will be much more aggressive than a change from 15 to 16 J/cm2, because that 1oC increase is starting from a higher temperature, which means it has a greater effect on the cells!
Conclusion
The exponential increase in damage to cells is directly due to the temperature increase, which is linear with an increasing fluence. This indicates that we must always be very careful when increasing the fluence in skin, and must compensate for the higher epidermal temperatures with more surface cooling.
Change of the fluence with depth in the skin
When light energy is fired into the skin, it undergoes a number of processes which results in a change in the fluence.
Typically, the skin surface will reflect between 4 and 7% of all light falling on it. When it enters the epidermis, it encounters atoms and molecules, immediately. This causes the photons to scatter, in many directions. As a direct consequence, the beam diameter expands as the light
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