Page 52 - Chapter 3 - Fundamentals of Laser/IPL Hair Removal
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Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
 Figure 18 – The visible spectrum (for humans!)
It is quite obvious that a white object won't get very warm in sunlight while a black object does get pretty warm. The reason is that all colours of the sunlight will be reflected from the white object while all colour of sunlight will be absorbed when they hit the black surface.
Fundamental optical properties of the skin
Basic knowledge about the optical properties of the skin is compulsory if you should be able to make efficient light-based skin treatments with lasers and IPLs. The two basic phenomena that takes place when skin is exposed to light is scattering and absorption.
How does light interact with skin in general and how deep into the skin can light reach? These questions are central in understanding selective photo thermolysis. As already mentioned above, absorption and following heating may occur if light with "different " colour interacts with the skin chromophores.
The other fundamental skin property is light scattering. The skin consists of many layers of different cell types which divert the light in different directions. This process is called light scattering which occurs in all types of opaque substances for example in clouds, fog, snow, milk etc.
A typical example of light scattering takes place in clouds which consists of small droplets of water. The water droplets split up the rays from the sun, some of the light penetrates through the cloud but a substantial portion of the sun light is reflected backward away from the earth.
As water does not absorb the sun light (at least not the visible portion of sun light) no or very little heat is generated in the cloud.
The vast majority of the skin cells do not contain any absorbing chromophores so the light scatter process in itself does not cause heating but only strong redirection of the light rays that enters the skin. (A substantial portion of the light will actually scatter in the backward direction and thus leave the skin.) From pale skin as much as 60-70 % of red light will be reflected/scattered backwards and thus leave the skin, during treatments!
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