Reflection and transmission of light: no results
If light energy is reflected by the tissue or passes through the tissue (transmission), the effect is zero with respect to the tissue. As with a mirror, the light is reflected or simply passes through the tissue without any effect (as with transparent glass). Both cases are undesirable in the case of medical-cosmetic application (in our case permanent hair removal), but must be taken into account when applying the various procedures. About 70% of the light that hits the skin surface in general is reflected directly. The rest is absorbed by the tissue or scattered within the tissue. In hair removal, if the light is directed and a special contact gel is used between the light-emitting handset and the tissue, the reflection can be kept to a minimum and the patient’s treatment more effective.
Scattering of light: no long-term hair removal
Basically, scattering is the change of direction of light (more precisely: photons). The light entering the skin can be deflected and/or reflected back within the lower skin layers by collagen fibres, blood vessels and other structures in the skin. The energy is stored in the tissue and sent back and forth until it is completely absorbed. When scattered, some photons leave the main beam of light before the final absorption. This reduces the effectiveness of the application. The smaller the focal size of the light beam, the greater the likelihood of deflection and scattering. Conversely, it can be said that the larger the focus of the light beam is set, the greater the penetration depth of the light beam and its energy.
Absorption of light: permanent hair removal
During absorption, the energy stored in the photons is converted into heat and given off to one of the chromophores in the skin. A chromophore is a colour carrier that absorbs a certain wavelength of light. When the photon gives up its energy, it ceases to exist. Heat is produced instead. We are all familiar with this transformation of light into heat when we think of the warming rays of the sun. Nothing else happens here: the light energy is absorbed by a chromophore and transformed into heat. The darker the chromophore, the stronger the absorption and the conversion into heat.
Absorption in detail
There are various substances in our skin that are able to absorb light. These are mainly melanin (in the epidermis, hair and hair shaft), the substances oxyhaemoglobin and haemoglobin (in the blood) and water. How strongly the light is absorbed by each chromophore depends on the wavelength of the light used. The wavelength determines how deep the light can penetrate into the tissue. Basically, longer wavelengths (600-1000 nm) can penetrate deeper into the tissue, while wavelengths in the range above 1000 nm are mainly absorbed by the water in the skin.
The light penetrating the tissue therefore only has an effect if it is absorbed and converted into heat. Here, the physiological effect of absorption is also dependent on the temperature reached in the target area. When the light is absorbed, the so-called target chromophore – the colour carrier in the target area of the treatment – is heated. If permanent hair removal is desired, then melanin is the target chromophore, if skin treatment is desired, then haemoglobin in the blood is the target chromophore.
Shortly after absorption, heat loss occurs immediately by releasing the thermal energy into the environment. The heat loss varies depending on the thermal relaxation time (TRT) of the tissue. The TRT is used to describe the amount of time it takes for the fabric to cool back down to the ambient temperature after heating. It is logical that large objects and surfaces cool down more slowly than small ones. Thus, the larger the target object, the longer the “thermal damage time” – i.e. the time required for exposure to destroy a target object.
The heating of the tissue depends on the active heating of the chromophore (colour carrier) by the light energy absorbed by it (conversion into heat energy) and on the passive cooling by heat loss to the surrounding tissue. This process of deliberate damage to tissue by the application of heat (in response to the absorption of light energy) is known as photo-thermolysis.
