Page 96 - Chapter 3 - Fundamentals of Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
The graph above (Figure 34) shows the unfiltered output from a Xenon flashlamp, typically found in an IPL system. The wavelength range extends from around 350nm to 1200nm. Between 350 and around 740nm is the visible spectrum – we can see this stuff! Beyond 740nm up to 1200nm is the near infrared light energy, which is invisible but can be felt as heat.
When using IPLs for photothermal treatments, we often filter out those wavelengths we don’t wish to expose to the skin. For example, when treating hair, we want the melanin in the hair shaft to absorb the light energy, but without disrupting the blood vessels. Hence, we choose a filter which blocks all those wavelengths that we don’t want, and allow through those we do.
Figure 35 – Spectral output of a Xenon lamp – filtered with a 600nm cut-on
A 600nm ‘cut on’ filter will stop all wavelengths below 600nm from getting through, so, only those wavelengths longer then 600nm will strike the skin (Figure 75). As a consequence, the filter absorbs all the energy in the wavelength range 350 to 600nm, which can be a significant proportion of the total energy. Therefore, it heats up and can become fragile – hence, IPL filters should always be properly cooled during treatments and carefully handled to prevent damage.
Some IPL devices offer a ‘notch’ filter which will cut out wavelength above and below pre-set levels. For example, some systems offer filters which cut on at 600nm and cut off at 900nm – thereby only allowing through the range 600 to 900nm to the skin. This prevents much of the
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