Laser beam interaction with the skin

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Marketing brochures for lasers and other light-based devices usually paints a picture of a light beam that enters the skin and hit the target molecule perfectly, it is then absorbed in its totality and providing the greatest result very time. But is this really true?

No. The reality is that the moment a light beam hits the skin various things happen to that beam. Let’s start with the moment before it hits the skin. Do you have any gels, creams or other fluids and consumable on the skin? If you use a cooling gel you increase the reflection that will occur, you will now have reflection when the light hits the gel and some more when the light hits the skin. On top of that you will cause refraction of the light that otherwise would not have occurred. Keep in mind that your device may have been developed with this in mind and compensate for this to some extent.

What happen next when the light actually hits the skin? This depends greatly on the wavelength, because the wavelength determines the target molecules. Does the skin contain a high density of the target molecules and how deep do they sit in the skin. If you use a laser that targets water, such as a fractional CO2. You will have a very high level of absorption in the outer layer of the skin, but if you have water molecules on top of the skin the energy may as well be absorbed before it reach the skin.

Should you use a wavelength that targets melanin, the reaction is greatly predicted by the melanin density of the epidermis and the hair. Thus, the same beam will have two different reactions when you treat a light-skinned person compared to a dark-skinned person.

The moment the light hits the skin the first thing that happens is that parts of that beam will be reflected in all different directions. The light that goes deeper will now hit various molecules, like fat, blood, water, melanin and collagen to name a few. Each of these molecules will cause a certain reflection associated with that molecule. We call this reflection inside the skin scatter. By this time the light beam’s intensity has greatly decreased.

Not all the light will be reflected. Some of the light is absorbed and it is this absorption that will deliver results, give that the energy is enough.

But what happens to the light that is not absorbed? Any light (energy) that is not absorbed will exit the skin again at some point. Therefore, when you put a bright red light against a skin fold in the dark you will see the scatter of the light within the skin, because the red glow in the skin will be larger than the applicator. And it is possible to see this because of the red light leaving the skin and radiating to your eyes.