Mike Murphy's Blog

WARNING – This post is a bit more scientific than my usual stuff. BUT, it is worth persevering because you will (hopefully) understand more about your laser/IPL treatments, if you do…

When we fire light energy at the skin, to treat hair, blood vessel, pigmentation etc, we know that some of the energy is absorbed by the targets – melanin or haemoglobin. But how does this actually work?

Well, the physics tells us that each of these target chromophores absorb different amounts of light energy depending on the wavelength. The haemoglobin molecules in blood absorb blue light very strongly, and yellow light quite well. Melanin absorbs well above around 600 nm (which is red and infra-red).

Skin Colour

Firstly, we should consider skin colour. People often use the Fitzpatrick Skin Type scale to describe skin colour – this is wrong (I’ve written about this mistake in another post here in July 2021). The Fitzpatrick scale describes the skin’s reaction to ultraviolet light, NOT the skin colour.

A much more scientific way to describe skin colour is to look at the percentage of melanosomes in the epidermis (fraction of melanosomes, f_mel). A very well-respected skin optics researcher in the US, Steve Jacques, devised a good system:

Skin Colour	fmel  %
1 – very pale	1.3
2	6.3
3	11
4	16
5	18
6 – Afro-Caribbean	43

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I have adopted this system with skin colours ranging from very pale (SC1) to ‘Afro-Caribbean’ (SC6). This approach makes much more sense when considering skin treatments,  than the Fitzpatrick scale.

Using this method, and the work by Steve Jacques, it is possible to calculate the absorption characteristics of melanin, for different skin colours.

We use graphs which are determined from lab experiments to find out how this variation looks, across the spectrum. The graph below shows the absorption curves for oxyhaemoglobin (HbO₂) and deoxyhaemoglobin (Hb), and for melanin (SC1 to SC6). (The curves for the haemoglobins are derived from extinction measurements made by another well-respected US scientist, Scott Prahl).

The above curves are familiar with many in this field (except usually only one melanin curve is shown – for which skin colour? I don’t know!!).

So, here we can see how strongly light from 375nm up to 650nm is absorbed by haemoglobin and melanin (in different skin colours). It is clear that blood absorbs more strongly in two regions – between around 380 to 450nm and again between around 530 to 580nm.

The absorption by melanin varies depending on the skin colour! For darker skins, the melanin will absorb more strongly than blood between 450 and 530nm, but this is not the case for the lighter skin colours (SC1 to 3).

However, for all skin colours, melanin absorbs more strongly (preferentially) than blood for all wavelengths above around 600 to 630nm (except the very palest colour, SC1, which needs longer wavelengths to ensure more melanin absorption than in the blood).

However, the above graph is a bit ‘mis-leading’. It shows the curves in a logarithmic scale. Now, human beings are simple creatures – we simply cannot understand exponentials or logarithms when applied to the real world!! Our brains are geared up for purely linear processes – we like straight lines and some curves, but not weird stuff likes logs!!

So, if we redraw the above curves in a linear format, we derive the following curves:

This is quite surprising. The absorption by blood is very much stronger in the blue end of the spectrum (between about 380 to 450nm), and much greater than in melanin. This implies that we should be using these wavelengths to target blood vessels. While this may appear to be a good idea at first, the problem is that these wavelengths cannot penetrate far into the skin! Consequently, very little energy will reach the deeper blood vessels.

Also, the absorption by the blood in the region 530 to 580nm is only a little more preferential than melanin, especially in darker skin colours.

Conclusion

We choose wavelengths in our lasers and IPLs depending on which targets we want to heat. To ensure this, we need to know which wavelengths are preferentially absorbed by those targets.

The graphs above show fairly clearly which wavelengths should be used. Blood vessels should be tackled with light in the range 530 to around 590nm – the haemoglobin will absorb more strongly than melanin in that range.

While hair should be targeted with any wavelength above 600nm (above 700nm for SC1) to minimise any absorption and hence damage, in the blood vessels.

Pigmentation can be successfully hit with lower wavelengths (from 400nm upwards to around 530nm). Even though these are strongly absorbed by blood, very little of this light will reach the blood vessel depths (more to do with scattering and anisotropy than absorption – but that’s another post!!)

This stuff is a bit complicated but is very useful when determining the best wavelengths to use for treatments.

Hope this helps…

Ciao for now,

Mike.