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emerge and probably head off in a new direction altogether. This phenomenon is known as ‘anisotropy’.
• The anisotropy of an atom is determined by its quantum mechanical makeup. This is too bonkers to fully describe here!!
• But, different atoms/molecules have different anisotropies which depends on the incoming wavelength. So, in skin, blue light tends to spread out very widely, whereas red light is more forward directed. We say that red (and near infra-red) light is forward scattering, because of the anisotropy in tissues. This is quite fortuitous since it means that red light will penetrate deeper into the skin than blue (or green or yellow) light.
1. So, let’s pick up our journey on this new photon. We have been absorbed and then spat out as a new photon, during the scattering event above. We are back on our trip, on a new photon.
1. To this point we have been travelling inside the skin for somewhere around 0.33 picoseconds.
1. At this point it is a good idea to introduce the concept of ‘mean free path’ This is the distance a photon may travel without encountering an atom or molecule – in other words, it can travel ‘freely’. In skin tissue, this is typically the distance between scattering events because this is much more likely to occur than absorption events.
1. In skin this distance is about 100 microns (0.1 mm, for visible and near infrared light) – the thickness of the epidermis, typically, which will take around another 0.33 ps.
1. So, this suggests that our photon will encounter an atom or molecule every 100 microns, and will most likely be scattered – unless it happens to be a strong absorber like melanin or haemoglobin (depending on its wavelength!!)
1. Now we encounter another atom. Exactly the same processes will occur as above. We will either pass by it, be totally absorbed or be scattered.
1. For visible and near infra-red wavelengths, the chances of being absorbed in the upper epidermis are rather small. But the epidermis is only about 100 microns thick which is about the same as one mean free pathlength. So, we will be scattered only a few times in here.
1. At a depth of between 0.06 and 0.1 mm, we will enter the basal layer of the skin. This is where the melanocytes live and produce melanin granules.
1. Now, melanin just loves visible light – especially down at the blue end of the spectrum. It just gorges on blue photons. Or, in physics terms, the absorption of blue light by melanin is very high.
Mike’s Blog Posts 270