Technically, this setting leverages the concept of Thermal Relaxation Time (TRT) to differentiate between skin and hair. Setting the pulse width to 30 milliseconds or longer slows the delivery of laser energy. This extended duration grants the pigment in the epidermis sufficient time to dissipate heat and cool down, while the larger hair follicle retains that heat long enough to be destroyed.
The Core Logic By extending the pulse width, you prioritize thermal protection of the skin over rapid energy delivery. This approach exploits the fact that skin cools down much faster than a hair follicle, allowing you to bypass the skin's natural melanin barrier without causing burns.
The Physics of Pulse Duration on Dark Skin
The Challenge of Melanin Competition
In Fitzpatrick Type VI skin, the concentration of melanin in the epidermis is extremely high. Since hair removal lasers target melanin, the device creates a "competition" for energy absorption between the hair follicle and the surrounding skin. Without adjustment, the laser cannot easily distinguish between the dark hair and the dark skin, leading to a high risk of surface burns.
Pulse Width as a Safety Throttle
Pulse width defines the time span over which the laser energy is released. A short pulse (e.g., 5ms) delivers energy like a sudden explosion, creating a rapid temperature spike. A long pulse (30ms to 100ms) delivers the same amount of energy like a steady stream, resulting in a gentler photothermal effect.
Thermal Relaxation Time (TRT)
The critical technical factor here is Thermal Relaxation Time (TRT). TRT is the time it takes for a target (like skin or hair) to cool down by 50% after being heated. The epidermis has a very short TRT, meaning it sheds heat quickly. Hair follicles, being larger structures, have a longer TRT, meaning they hold onto heat for a longer period.
The 30ms Threshold
By setting the pulse width to 30ms or longer, you exceed the TRT of the epidermis but remain within the effective heating window of the follicle. This allows the skin to "relax" (cool down) during the laser pulse. Consequently, the hair follicle accumulates destructive heat, while the epidermal temperature remains below the threshold for blistering or pigmentation damage.
Optimizing Energy Density (Fluence)
Balancing Power and Time
Using a longer pulse width changes the safety profile of the energy density (fluence) you can use. Because the heating is slower, you can often safely increase the fluence. Supplementary data suggests that with ultra-long pulse widths (up to 100ms), clinicians can increase energy density by approximately 30% (e.g., from 20J/cm² to 30J/cm²) for Type VI skin.
Ensuring Deep Penetration
This technique ensures that the energy is not wasted burning the surface. Instead, the thermal energy diffuses deeper into the dermis. This targets the hair follicle where it resides, rather than the melanin located at the skin's surface.
Understanding the Trade-offs
The Risk of Inefficiency
While longer pulse widths are safer, there is a limit to their utility. If the pulse width is too long relative to the hair's thickness, the follicle may cool down as fast as it is being heated. This results in a treatment that is safe but ineffective, as the follicle never reaches the temperature required for permanent damage.
The Necessity of Wavelength Selection
Pulse width is not the only variable; wavelength is equally critical for Type VI skin. While 30ms helps manage heat, using the correct wavelength (such as the 1064 nm Nd:YAG) is often preferred. This wavelength naturally bypasses epidermal melanin more effectively than shorter wavelengths, working in tandem with long pulse widths to maximize safety.
Making the Right Choice for Your Goal
To achieve the best results on Fitzpatrick Type VI skin, you must balance safety with efficacy.
- If your primary focus is Patient Safety: Prioritize a longer pulse width (30ms to 100ms) to allow the epidermis to cool significantly during energy delivery, minimizing the risk of hyperpigmentation or burns.
- If your primary focus is Treatment Efficacy: Ensure the energy density (fluence) is adjusted upward to compensate for the longer pulse width, ensuring the follicle receives enough total energy to be destroyed.
Ultimately, a long pulse width acts as a temporal shield, allowing the skin to survive the high energy required to treat dark hair on dark skin.
Summary Table:
| Technical Parameter | Fitzpatrick Type VI Target | Mechanism & Impact |
|---|---|---|
| Pulse Width | 30ms to 100ms | Slows energy delivery; allows epidermis to cool via short TRT. |
| Thermal Focus | Hair Follicle | Larger structures retain heat longer, enabling targeted destruction. |
| Energy (Fluence) | Higher (20-30J/cm²) | Safe to increase with longer pulses to maintain efficacy. |
| Wavelength | 1064nm (Nd:YAG) | Preferred for deep penetration and minimal melanin competition. |
| Primary Goal | Epidermal Protection | Prevents burns and hyperpigmentation on dark skin tones. |
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References
- Suzanne W. Yee. Laser Hair Removal in Fitzpatrick Type IV to VI Patients. DOI: 10.1055/s-2005-872415
This article is also based on technical information from Belislaser Knowledge Base .
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