Selective photothermolysis

Selective photothermolysis is a physical principle of action that is used in the application of light therapy and is frequently used in the field of cosmetic treatments.

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The goal of selective photothermolysis is to selectively heat and destroy a specific target without unnecessarily attacking the surrounding tissue. For this purpose, the correct tuning of three parameters is crucial for a satisfactory result:

1. the wavelength

The wavelength of the light must be matched to the chromophore. Only if the chromophore of the target area (the target chromophore) can optimally absorb the light energy and convert it into heat can the treatment be successful.

Precise matching of the wavelengths of the emitted light spectrum is important to hit the target chromophore, which is crucial for the treatment.

The longer the wavelength, the deeper the light can penetrate into the tissue. This physical fact affects the selection of the right wavelengths. This is because the penetration depth of the wavelengths must be sufficient to reach the deeper lying hair follicles. However, this effect of greater penetration depth must be carefully considered, because too deep penetration of the waves not only reduces the success of the treatment, but also increases the risk of undesirable side effects.

Furthermore, the wavelengths should allow the best possible absorption by the
melanin and at the same time exclude absorption by other “competing” chromophores – such as hemoglobin and water – as far as possible.

The IPL systems therefore worked with both a 600 nm filter in the lower spectral range, filtering out wavelengths below 600 nm, and a 950 nm water filter in the upper spectrum, absorbing all wavelengths above 950 nm. The light emitted by the IPL system is thus limited to a spectral range between 600 nm and 950 nm for the purpose of hair removal.

With the help of this double filtering of the broadband spectrum, an optimal alignment of the light energy to the target chromophore melanin is achieved with simultaneous maximum possible elimination of unwanted absorption by other color carriers. In plain language, this means that thanks to this sophisticated technique, the IPL light achieves maximum treatment results with minimal risk of side effects.

2. light pulse duration

The next crucial parameter of photothermolysis by IPL is the properly chosen pulse duration of the light flashes during the treatment.

We already know that the light energy is absorbed by the melanin produced in the melanocytes of hair and hair shaft, converted into heat energy, and then directed to the hair follicles.

To damage the hair follicle (and thus permanently prevent regrowth of new hair), the pulse duration of the light flash must be longer than the thermal relaxation time (TRT) of the melanocytes. Only then will the heat actually be transferred to the hair follicles and not previously cooled by the surrounding tissue. On the other hand, the pulse duration must not be longer than the thermal relaxation time of the hair follicle itself, so that not only the hair follicle but also the surrounding tissue is not damaged by the excessively long heat exposure.

We are therefore dealing with a precise coordination between the TRT of the melanocytes (as the lower value of the pulse duration) and the TRT of the hair follicle (as the upper value of the pulse duration). Exactly in between lies the optimal pulse duration for the IPL light flash.

Studies have shown that the hair follicle must be heated up to 70°C for at least 1 ms in order to be destroyed.

If the pulse duration is too short, the hair follicle will only be partially destroyed and new hair will grow back after a recovery phase. If the pulse duration is chosen too long, too much heat will be transferred to the surrounding area and can lead to thermal side effects such as burns.

What makes the exact calculation difficult is the fact that in addition, the diameter of the hair and hair shaft also affects the thermal relaxation time of the hair. The thicker the hair, the longer its thermal relaxation time (TRT). Therefore, it is not possible to set only one pulse duration for the whole treatment, which is suitable for all hair thicknesses. If the hair is thinner, less energy (in this case a shorter pulse duration) is needed to heat the hair.

Therefore, before treatment, the thickness and diameter of the hair to be treated are first measured and these values are entered into the Epilux system. The pulse duration settings are then automatically calculated by the Epilux after the

3. the energy level, fluence (energy density).

Depending on the set pulse duration and wavelength, the decisive effect is achieved by varying the energy density: the hair follicle is brought to a temperature of 70 °C for at least 1 ms.
If the energy level is too high, the risk of undesirable, minor burns is too high; if the energy level is too low, the hair follicle is not completely destroyed but only damaged. A possible consequence of such partial damage can be the regrowth of woolly, white hair (the so-called vellus hair), which on the one hand has lost its color due to the destruction of the pigment melanin, but on the other hand has not lost its ability to regrow.

Furthermore, a too low energy level can synchronize the growth cycle of the hair, which can result in an even stronger hair growth. This synchronization then causes more hairs to enter the anagen growth phase at the same time than before.


Only the precise tuning of these three parameters enables optimal treatment success. The target chromophore can be destroyed with minimal power while protecting the surrounding tissue from damage.

Fields of application of selective photothermolysis

In the removal of vascular lesions, the goal of selective photothermolysis is to
the blood pigment hemoglobin.

When removing pigmentary disorders, the target of selective photothermolysis is
the pigment.

In epilation, the chromophore is the pigment melanin in the hair


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The medical staff of trained specialists, beauticians and alternative practitioners are happy to advise you on the possible treatments. The consultation is non-binding and free of charge. Call us on +49/30/88 92 92 20.

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