Page 50 - Chapter 3 - Fundamentals of Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
Wavelength
Energy & Energy Density (Fluence) Power & Power Density
Spot Size
Pulse Duration Repetition Rate
These are all discussed in Chapter 1 – Fundamentals of Medical/Aesthetic Lasers and IPLs. You can obtain a copy here. Plus, you can download a pdf of our ‘Brief Guide’.
Selective heating - The impact of object colour vs the colour of light
All materials contain atoms and molecules which will absorb light. However, different atoms/molecules absorb different wavelengths (colours) light. Red objects appear red because of the absorption process – the atoms/molecules in a red object absorbs all the colours which fall on it, except red light! Absorption is, essentially, a subtraction process – when materials absorb light, they subtract out some of the wavelengths, and reject the rest.
If an object absorbs light energy, that energy will usually be converted in to heat, thereby raising the temperature of that object.
However, if the object has a specific colour for example red, then the red portion of the sunlight will be reflected away from the object (that's why we perceive the objects colour as red) while all other colours in the sun spectra will be absorbed and create heat in the object. This leads to the fact that a red object will collect less energy from the sunlight compared to a black object and the temperature rise will be lower but considerably higher in comparison with a white object.
Let's assume now that we expose a red object with red light. In this case all red light will be reflected away from the object and therefore no absorption/heating will occur. If we instead expose the red object with green or blue light then most the energy from these colours will be absorbed and generate heat.
The lesson from these examples is that equal colour of object and light will not generate absorption/heat. In order to achieve selective absorption, the colour of the light must be
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