A longer pulse width, such as 20 to 25ms, is critical for safety on dark skin because it allows the laser energy to heat the hair follicle slowly enough that the surrounding skin can cool down simultaneously. By extending the duration of the laser shot, you provide the melanin-rich epidermis with sufficient time to dissipate heat, preventing burns and blisters, while still maintaining enough steady thermal energy to effectively destroy the thick hair follicle.
The Core Mechanism The goal of a longer pulse width is to match the Thermal Relaxation Time (TRT) of the target hair. This ensures the laser destroys the hair follicle by maintaining heat within it, while allowing the surface skin—which cools faster than the hair—to remain safe from thermal damage.
The Physics of Thermal Relaxation
To understand why 20-25ms is the "sweet spot" for dark skin, you must understand how different tissues handle heat.
Matching the Hair's Retention
Thick, coarse hair follicles—common in many patients with darker skin tones—have a longer Thermal Relaxation Time (TRT).
This means they retain heat for a longer period before cooling down. A pulse width of 20ms to 25ms is sustained long enough to fully coagulate and destroy these larger structures.
Protecting the Epidermis
The epidermis (the top layer of skin) is thin and has a much shorter TRT than a thick hair follicle.
When you use a long pulse width, you are delivering energy at a slower rate. This allows the epidermis to dissipate the heat during the laser pulse itself.
If the pulse were too short (high intensity, short duration), the melanin in dark skin would absorb that energy instantly, leading to rapid overheating, burns, or pigmentary changes before the skin could cool itself.
The Synergy of Wavelength and Timing
While the pulse width controls the time, the laser type controls the depth. Both are required for success on dark skin.
Bypassing Surface Melanin
The Nd:YAG laser operates at a 1064 nm wavelength. This specific wavelength has a lower absorption coefficient for melanin compared to Alexandrite or Ruby lasers.
This "colorblindness" allows the beam to penetrate deeper into the dermis to find the hair root, rather than being absorbed by the pigment on the skin's surface.
The Volume Knob Analogy
Think of the wavelength (1064 nm) as the pathway and the pulse width (20-25ms) as the speed of delivery.
Even with the correct pathway (Nd:YAG), driving too fast (short pulse width) can still cause an accident (burns). The long pulse width ensures the energy is delivered gradually, respecting the skin's biological limits.
Common Pitfalls to Avoid
Using the correct settings is a balancing act. It is vital to understand the risks associated with improper pulse configuration.
The Danger of Short Pulses
Using short pulses or Q-switched modes on dark skin is dangerous. These settings deliver energy in nanoseconds or short milliseconds.
This creates an "acoustic" or explosive shockwave effect rather than a thermal heating effect. In dark skin, this almost invariably triggers epidermal injury, leading to immediate blistering or long-term post-inflammatory hyperpigmentation.
The Role of Cooling Systems
A long pulse width alone is often not enough; it must be paired with aggressive epidermal cooling (contact cooling or cryogen spray).
The long pulse gives the skin time to cool, but the active cooling system physically extracts the heat. These two factors work in tandem to keep the epidermis safe while the follicle reaches lethal temperatures.
Making the Right Choice for Your Goal
When configuring an Nd:YAG laser for a patient, your settings should be dictated by the specific risk profile of their skin type.
- If your primary focus is Patient Safety (Fitzpatrick V-VI): Prioritize a longer pulse width (20-50ms) to ensure the epidermis has ample time to dissipate heat, virtually eliminating the risk of burns.
- If your primary focus is Efficacy on Thick Hair: Ensure the pulse width is roughly equal to the hair's Thermal Relaxation Time; typically, coarser hair requires the longer end of the pulse spectrum (20ms+) to ensure destruction.
By extending the pulse width, you turn a potentially volatile treatment into a controlled, gradual heating process that spares the skin while effectively targeting the hair.
Summary Table:
| Feature | Short Pulse Width (<10ms) | Long Pulse Width (20-25ms+) |
|---|---|---|
| Energy Delivery | Rapid, high-intensity burst | Gradual, sustained heating |
| Epidermal Impact | High risk of burns/blisters | Allows time for heat dissipation |
| Hair Target | May not fully coagulate thick hair | Matches TRT for effective destruction |
| Safety Profile | Dangerous for Fitzpatrick IV-VI | Ideal for dark skin tones |
| Clinical Result | Risk of hyperpigmentation | Controlled, safe hair removal |
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References
- Goh Cl. Comparative study on a single treatment response to long pulse Nd:YAG lasers and intense pulse light therapy for hair removal on skin type IV to VI – Is longer wavelengths lasers preferred over shorter wavelengths lights for assisted hair removal. DOI: 10.1080/09546630310004171
This article is also based on technical information from Belislaser Knowledge Base .
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