Mike Murphy's Blog

Let’s take a ride on a photon.

Imagine we can sit on a photon. What would a trip into the skin be like? Well, it might be something like this….

As we leave the laser we will be travelling at nearly the speed of light – that is 30 million kilometres per second! However, as we enter the skin, we will slow down. This is entirely due to the change of local medium we are passing through.

The ‘speed of light’ depends entirely on what is it travelling through. It is fastest in what is known as ‘free space’ – that is deep space where there is virtually nothing – no atoms or molecules!!

But, when light enters other medium, such as air or skin, the local environment changes. In particular, the permittivity and the permeability both change. You will recall that light is an electromagnetic wave (made up from a magnetic wave component and an electric wave component), so, any change in the local electrical and magnetic environment will alter its properties. (Incidentally, it was the great Scottish scientist James Clerk Maxwell who first figured this stuff out).

The main property change is a slowing of its speed. Normally, this is discussed as a change in refractive index – this change is due to the changes in both the permittivity and permeability.

So, we slow down as we enter the skin. Now, if we could ‘see’ what wavelength (colour) we are, we would also see a change in that too. When light slows, its wavelength also shortens. This is because its frequency cannot change – that is determined by the source of the light.

So, if we entered the skin as a ‘red’ photon, we might now see that the colour is closer to green or yellow!

The first layer of skin we will encounter is the stratum corneum. This is very thin (typically around 10 to 40 microns) so we can imagine we will pass through it unaffected (apart from slowing and changing colour!!). However, we will begin to encounter reflections here known as ‘Fresnel Reflections’. This is when light is simply reflected back out of the skin and can account for between 4 and 8% of all light falling onto the skin. This also occurs with laser light – we lose this amount each time we fire a laser pulse into the skin! (Note: not all Fresnel Reflections occur in the stratum corneum – they simply begin there).

The next layer is the top of the epidermis – the stratum granulosum layer (we will ignore the lucidum since this is only found on the palms and the soles). We will undoubtedly encounter something here – an atom or a molecule. There are plenty about, so it’s unlikely we will miss them.

Incidentally, it will have taken between around 0.3 and 3 nanoseconds to have reached this point from the laser (depending on which laser construction you are using – handpiece or articulated arm).

So, we finally ‘meet’ an atom (or molecule). A couple of things might happen:

• We might just pass by with no interaction at all; we will continue our flight;

• We might encounter an electron in an outer shell of the atom. Now, what happens next is determined by the ‘absorption’ or ‘scattering’ probability of this atom. These probabilities are dependent on both the atom and the wavelength of the light. If the absorption probability is high, then the electron will interact with the photon and ‘steal’ its energy. This is where the photon’s journey ends. It has ceased to be. It is now an ex-photon!

• When the electron has absorbed the photon’s energy, its own energy increases. This causes it to ‘jump’ to a higher orbit around the atom’s nucleus (according to the ‘Bohr Model of the Atom’). But, electrons are funny buggers – they don’t like heights!! So, soon after jumping up, it will drop back down to its original orbit. 

• At this point, one of two things may happen:

• If the conditions are right, a new photon will be created using the energy which the electron had gained from the old photon. If this is an ‘elastic’ situation, then the new photon will have exactly the same energy, and hence wavelength, as the old photon. (Energy and wavelength are linked by Planck’s Equation).

• If a different set of conditions exist, then a new photon will not be created. Instead, the energy released by the electron dropping back down, will be released into the local environment as vibrational energy which may be thought of as thermal (heat) energy. If many photons are absorbed by some material, this vibrational energy will be felt as heat as the temperature rises.

• If the original photon’s energy is lost as heat, then that photon is dead!

• However, if a new photon has been created, then we say that a ‘scattering’ event has occurred. Scattering is not ‘photons bouncing off atoms’ – it is, in fact, an absorption event followed by the creation of a new photon.

• Now, something interesting may occur. The direction in which the new photon emerges from the atom is not likely to be the same as its original direction. It will 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.

• 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.

• To this point we have been travelling inside the skin for somewhere around 0.33 picoseconds.

• 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.

• 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.

• 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!!)

• 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.

• 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.

• 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. 

• 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.

• This is simply because melanin is there precisely to minimise the number of blue photons from  entering the dermis. Individual blue photons are highly energetic – they can have much more energy than red photons.

• These highly energetic photons can cause all sorts of damage to DNA and RNA in cells. So, the melanin granules are like a ‘guard’, waiting for those nasty blue photons. They will defend the dermis, as much as they can, to stop entry. It is for this reason the melanocytes produce more melanin when bombarded with blue photons.

• Mere humans refer to this process as ‘tanning’!

• So, we’re back on our nice, low energy, red photon – it’s ‘safe’ compared with the blue nasties.

• In the dermis, the chances are that we will be scattered many, many times. This might be off atoms of water, collagen, blood, hair or nerve tissues or many other possible scattering sites.

• Now, this is an interesting point. Because our little photon is undergoing so many scattering events, its direction is constantly changing. Sometimes, it will change so that it is heading back towards the skin surface; other times it will be heading deeper into the dermis. This is essentially determined by the anisotropy of the epidermis. As a consequence, many photons will escape the skin entirely, never to return. In the skin, it typically takes about 1 mm of travel for the photo’s new direction to be truly ‘random’ – this is due to both the scattering coefficient of the tissue and its anisotropy.

• However, many will continue their journey deep into the skin.

• Since the dermis is so thick (up to 3 or 4 mm in some people) most of the photons entering the skin will likely be absorbed here. But don’t forget, after only 1 mm of travel, many photons can easily be heading in the ‘wrong direction’ back towards the skin’s surface. They will be lost as ‘back-scattered’ light, if they leave the skin.

• My friend, PA Torstensson, did some Monte Carlo calculations on this and found that up to half of all photons of the 1064 nm persuasion might be lost from back-scattering!! See my paper on this.

• This is remarkable, especially when considering laser treatments of the skin. We might be losing half of our energy simply due to scattering.

• Another important issue is that, as you saw above, scattering causes the photons to ‘spread out’ as they penetrate into the skin. This means that the spot diameter increases with depth.

• This is not good!! If we have set our laser to generate a certain fluence at the skin surface, say 5 J/cm², then, at some depth in the skin it will fall to 4 J/cm², then 3 J/cm², then 2 J/cm² and then 1 J/cm². What is fluence?

• Now, if the target we want to destroy (blood vessel, hair follicle) requires a minimum of, say 3 J/cm², then, simply because of scattering, the treatment will stop at some depth. Below this depth, the fluence will be too low to effect the changes we want click here.

• So, it is important to consider the depth of your target when setting your fluence at the skin surface.

• Back to our little, red photon. So, we have encountered many wonderful atoms and molecules and exchanged many energies with them on our trip so far. However, it is important to know that the photon we are currently sitting on has absolutely no connection to the original photon we hitched a ride with (which emerged from the laser)!

• At some point, we will encounter a strong absorber. When this happens, our photon’s energy will be ‘taken’ by an electron and then released into the local environment (lattice) as vibrational (or heat) energy. This will make things a bit toasty there, which is usually what we are trying to achieve.

• The whole trip might last somewhere between 0.33 and 100 picoseconds (or longer), depending on how many scattering events it undergoes. This means, that, if you are using a 5 nanosecond laser for tattoo removal, then the vast majority of the photons you fire into the skin have already been absorbed or lost, before the end of the pulse!! This is still the case for most picosecond lasers too.

• Here endeth the trip!

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So, in summary:

Photons are merely little ‘balls of energy’. We utilise them to transfer energy from the laser into the tissues, to generate heat. 

Many are lost from the tissue via Fresnel Reflections and back-scattering.

Those that remain in the skin disperse throughout it, reducing the fluence as they move deeper.

Eventually, all photons will be absorbed by something, somewhere, after possibly many scattering events.

(By the way, if you want to know how many photons are in a ‘typical’ laser pulse, click here…)

Ciao for now,

Mike.

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