Page 22 - Chapter 3 - Fundamentals of Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
it is too high, then too much collateral damage can occur in the surrounding tissues in the dermis!
We have calculated the ‘optimum’ set of fluences for each of the above lasers and IPL, for follicles at different depths. The fluence must be chosen to efficiently ‘cook’ the follicle germ cells, at the depth they exist.
If the hair colour is not ‘dark’ i.e. the concentration of melanin is not sufficiently high, then more fluence must be used to cook the germ cells. But, this higher fluence might damage the epidermal basal layer too much! For this reason, light-based treatments CANNOT effectively kill lighter- coloured hair such as blonde, grey or white, without inflicting excessive damage to the surrounding skin.
Most people obtain poor results simply because their fluences are too low!
We will discuss how to choose the correct fluence later in this book.
Interestingly, a simple calculation reveals that very little of the fluence we apply to the skin actually heats the hair! Only up to 10% might actually do the job we’re attempting – the remaining energy is either reflected, scattered or absorbed by other tissues. This means that we are always ‘cooking’ the skin, each time we apply the light energy. Consequently, we are highly likely to damage other tissues, which we clearly don’t want. The answer to this problem is proper skin cooling...
Pulsewidth
Pulsewidth (also known as ‘pulse duration’ or ‘pulse length’) is simply how long the light beam is ‘ON’ for. We usually measure this in milliseconds (ms).
We often refer to this as the “cooking” time. If you fry an egg for five minutes, it should turn out fine. The heat energy has been applied to the egg (via the cooker) for a pulsewidth of five minutes.
If, however, the pulsewidth is 20 minutes, then the egg will likely be frazzled, and not very enjoyable!!
This pulsewidth is far too long.
Exactly the same principle applies in skin targets – we are trying to ‘cook’ the germ cells. This difference is simply in the length of the times – with eggs and sausages we use pulsewidths in the minutes range; with germ cells we typically use milliseconds!
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