“Chromophores” in the skin refer to molecules or substances on the skin surface or within tissues that can absorb light of specific wavelengths.
The light absorption of chromophores changes with variations in wavelength.
Although photorejuvenation (intense pulsed light, IPL) is broad-spectrum light different from lasers, it still follows the principle of “selective photothermolysis”.
Chromophores in the skin absorb photorejuvenation energy, and through “selective photothermolysis”, exert effects via thermal effects.
The main chromophores in the skin include melanin (in the epidermis and hair follicles), hemoglobin (in blood vessels), and water (in the epidermis and dermis). During photorejuvenation therapy, multiple chromophores absorb light, leading to a variety of biological effects. When performing photorejuvenation, doctors must keep in mind the light absorption characteristics of these multiple chromophores. This is because when we attempt to address a specific skin issue (such as fading pigmentation) using photorejuvenation, other chromophores besides the pigment will also absorb light and generate heat. This increases the risk of excessive thermal damage to the skin, and may even lead to burns and hyperpigmentation.
Photorejuvenation differs from lasers. Lasers are light of a single wavelength amplified through stimulated emission (LASER). In contrast, photorejuvenation is “intense pulsed light”, which is intense light emitted in pulses. Unlike lasers, it is not light of a single wavelength; instead, it is broad-spectrum light within a specific wavelength range.
When a filter of a certain wavelength is used, such as a 590nm filter, it filters out the wavelength range below 590nm. Therefore, when using a 590nm filter, in fact, light in the range of 590nm – 1200nm (yellow light, red light, and infrared light) all act on the skin.
[Image: Laser vs. Intense Pulsed Light]
When the light from a filter of a specific wavelength range is absorbed by multiple chromophores in the skin, multiple effects are exerted. For example, when a 590nm filter is used:
- Absorption by melanin achieves the effect of brightening the skin and fading spots.
- Absorption by hemoglobin achieves the effect of reducing redness.
- Absorption by water produces a significant effect of improving skin texture.
Thus, filters of different wavelength ranges each have their own focuses. In fact, even when only one filter is used, the multiple effects of “skin brightening, redness reduction, and skin rejuvenation” can still be achieved.
Intense pulsed light is a segment of broad-spectrum light, and the wavelength range of any filter can be absorbed by multiple chromophores such as melanin, hemoglobin, and water.
Therefore, when selecting filters of different wavelength ranges, a balance must be struck between the therapeutic effect achieved by the absorption of light by the primary chromophore and the adverse reactions caused by the absorption of light by non-primary chromophores.
Filters should not be selected solely based on the absorption peak of the primary chromophore.
The optimal approach is to compare the absorption curves of various chromophores, and identify the filter that allows good absorption by the primary chromophore while minimizing absorption by non-primary chromophores.
- Brightening the skin, fading spots, and enhancing skin radiance (reducing pigmentation).
- Improving skin smoothness, refinement, and increasing skin elasticity (stimulating collagen regeneration).
- Constricting and sealing blood vessels (reducing redness and eliminating spider veins).
- Anti-inflammatory and soothing sensitive skin (via low-energy photomodulation).
- Hair removal.
- Acne treatment (using ACNE mode or a 420nm filter).
In the treatment of vascular lesions (mainly vasodilation) using photorejuvenation, hemoglobin in blood vessels absorbs light, triggering selective photothermolysis. The generated heat diffuses to the vessel walls, and the therapy works by damaging these vessel walls to seal the blood vessels.
Hemoglobin has three absorption peaks, located at 418nm, 542nm, and 577nm respectively. Although the maximum absorption peak is at 418nm, this wavelength has poor penetration ability and can be competitively absorbed by epidermal melanin. This prevents photorejuvenation energy from being more effectively absorbed by hemoglobin. Additionally, the photothermal effect generated by melanin absorbing light increases the risk of adverse reactions.
Therefore, for the treatment of vascular lesions, wavelengths close to the latter two absorption peaks should be selected. At the same time, it should be noted that blood vessels are located in the dermis, requiring wavelengths with deeper penetration.
Consequently, for vascular lesions, a 590nm filter or a Vascular dual-wavelength filter (530 – 650nm, 900 – 1200nm) is usually selected. However, it should also be recognized that filters with wavelengths of 640nm and 695nm have deeper penetration than the 590nm filter, and their effect on deep blood vessels is superior to that of the shorter-wavelength 590nm filter.
[Image: Absorption Curves of Various Chromophores. Note the three absorption peaks of hemoglobin. (Wavelength (nm): 500, 700, 1000; Other labels: 694nm, 1064nm, ϕ, 102, A, Water, FMelanin, 101, 50X, 10°, T6X, 10, 10⁻²)]
In the treatment of vascular lesions (mainly vasodilation) using photorejuvenation, hemoglobin in blood vessels absorbs light, triggering selective photothermolysis. The generated heat diffuses to the vessel walls, and the therapy works by damaging these vessel walls to seal the blood vessels.
Hemoglobin in blood vessels includes oxyhemoglobin and deoxyhemoglobin.
During the process of blood vessel sealing by photorejuvenation (IPL), three steps are required: the conversion of oxyhemoglobin (HbO₂) to deoxyhemoglobin (Hb), followed by the conversion of deoxyhemoglobin (Hb) to methemoglobin (MetHb).
The entire conversion process takes approximately 10ms to complete. Therefore, the total pulse width for blood vessel sealing must be ≥ 10ms. Among this, the conversion of HbO₂ to Hb takes about 4ms, and the conversion of Hb to MetHb takes about 6ms.
Meanwhile, the temperature condition for the conversion must reach and be maintained at 70℃. Therefore, a relatively high energy level is required for blood vessel sealing.
[Image: Figure 12-1-2: Schematic Diagram of the Conversion Process of Oxyhemoglobin and Deoxyhemoglobin to Methemoglobin. (Temperature (˚C): 37, >70; Time (ms): >10, to; Labels: HbO₂, Hb, MetHb)]
The narrow-spectrum IPL (DPL) with a wavelength range of 500 – 600nm has a emission spectrum that well covers the absorption peaks of hemoglobin at 542nm and 577nm. Moreover, with a wavelength range of only 100nm, it reduces the competitive absorption by other chromophores.
As a result, it has a good therapeutic effect on vascular skin lesions. However, melanin also has strong absorption within this spectral range, so caution should be exercised when using it on patients with darker skin tones.
Due to the large size of the photorejuvenation treatment head (light guide crystal), to act on blood vessels more precisely, a small treatment head is usually used during blood vessel sealing operations, and it is best to use a perforated light shield.
[Image: Light Shield (tmp/163428828674_doc.dat_3.jpg)]
It can be clearly stated that in most cases, multiple filters are not required for photorejuvenation therapy. Due to the broad-spectrum nature of photorejuvenation, a filter of any wavelength range can achieve multiple effects such as pigment fading, redness reduction, and skin texture improvement.
Furthermore, also based on the broad-spectrum nature of photorejuvenation, even when a filter focused on addressing a specific skin issue (e.g., targeting melanin) is selected, other chromophores (such as hemoglobin, water, and pigment in hair follicles) will also absorb light and generate photothermal effects. Repeated application of multiple filters on the skin will lead to excessive heat accumulation due to the photothermal effects of chromophores, resulting in excessive thermal damage and subsequent adverse reactions.
Therefore, if a single filter can solve the problem, there is no need to use two. If two filters can solve the problem (e.g., one focusing on pigment fading and the other on skin texture improvement), there is no need to use three. This approach not only ensures therapeutic efficacy but also reduces the occurrence of adverse reactions. Similarly, when using a single filter, there is no need to perform repeated operations multiple times.
Before undergoing photon therapy, it is routine to apply light-guiding gel.
The role of light-guiding gel is not limited to guiding light alone.
To enhance therapeutic efficacy and avoid burns or excessive skin redness and swelling, proper application of light-guiding gel is crucial.
The main functions of light-guiding gel are light guiding and heat dissipation, and it also has other beneficial effects you may not have expected:
① Reduces skin scattering and enhances light-guiding performance.
② The gel has good heat dissipation properties, enabling uniform and effective cooling of the skin.
③ Facilitates the sliding of the treatment head on the skin.
④ Contributes to the cleaning and protection of the treatment head; without this layer of gel, tissues such as the epidermis and hair may adhere to the surface of the treatment head, easily causing damage to it.
From the perspective of light guiding, the gel is required to have high transparency, good uniformity, and no lumps or air bubbles.
From the perspective of heat dissipation, the gel must be applied with sufficient thickness.
Based on the above principles, ultrasonic gel is not suitable for IPL (Intense Pulsed Light) treatment, as it is relatively thin and has poor heat dissipation.
Typically, light-guiding gel needs to be applied with a thickness of at least 1mm. When using high-quality gel, a thicker application not only improves heat dissipation but also does not affect the penetration of photons.
The following are personal experiences (corrections are welcome if there are any omissions or errors):
① When using low energy for maintenance purposes, a thickness of 1mm is sufficient.
② When treating blood vessel occlusion, higher energy is used. To enhance heat dissipation and reduce epidermal reactions, the light-guiding gel should be applied thicker, with a recommended thickness of 2-3mm.
③ For treating pigmentation and skin rejuvenation, it is usually necessary to press the skin firmly with the treatment head, which may squeeze the gel thinner. Therefore, it is recommended to apply the gel with a thickness of 2mm to ensure adequate light guiding and heat dissipation.
Note: The light-guiding gel should be applied evenly without accumulation.
The gel has good heat dissipation properties.
The therapeutic efficacy of photons is proportional to the number of photons that reach the target tissue.
Therefore, enhancing light-guiding performance can improve the therapeutic effect of photons.
① Use high-quality gel with high transparency and good uniformity.
② Apply low-concentration fruit acid products externally one month before treatment to improve skin smoothness and reduce skin scattering.
On the day of photon treatment, a 4.5% glycolic acid solution can be used for chemical peeling (the principle is the same as above).
