Page 185 - Chapter 3 - An Introduction to Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
So, it could be argued that ‘laser’ light stops being ‘laser’ when it starts to scatter! It then resembles ordinary light except that it is a single wavelength (colour)...
IPL Light Energy – How Much Actually Reaches the Skin?
IPL systems are fairly simple devices, but they differ significantly from lasers in the way light is both generated and transmitted to the skin surface. The light in any IPL comes from a flashlamp which is excited by an applied voltage. This usually generates a wide range of wavelengths, typically from about 350 to 1400nm. Some of these wavelengths are of no interest to us, so we use filters to block their passage to the skin.
In reality, the filters absorb much of the energy, raising their temperature. We generated a computer model to calculate the amount of energy absorbed by various filters. The results are in the table below. We checked these results against an IPL and found that they agreed to within around 6% - which is pretty good for a mathematical model!
Of the remaining light energy which emerges from the filters, some of this will be lost before reaching the melanin in the hair shaft.
These losses are due to the following:
Filters – each filter absorbs some of the light energy to prevent those wavelengths getting through to the skin. The table on the right shows how much light is absorbed by these filters:
Filter cut-off
Loss of energy
400 nm
12%
500 nm
600 nm
25%
45%
700 nm
49%
At the skin surface, we encounter what are known as ‘Fresnel’ losses:
Between 4 and 7% of the incident light is reflected.
Some of the light energy is then absorbed by the melanosomes in the basal layer of the epidermis. These absorb very strongly at the blue end of the spectrum, and less so at the red end.
ST1 – 2.5% (at 600nm) ST6 – 15.5% (at 600nm)
Scattering occurs as soon as light enters the skin. This is a natural phenomenon. Some of the light energy is backscattered right out of the skin!
ST1 – up to 17% ST6 – up to 10%
Table 44 – Where is light energy ‘lost’ as it penetrates the skin?
________________________________________________________________________ 185 Chapter 3 Laser/IPL Hair Removal
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