Long pulse settings are the critical safety barrier that prevents thermal injury when treating dark skin with diode lasers. By extending the energy delivery over a duration of 30 milliseconds (ms) or longer, the system lowers the peak power intensity, allowing the melanin-rich epidermis to dissipate heat harmlessly while maintaining enough cumulative energy to destroy the hair follicle.
Core Takeaway The safety of long pulse settings relies on the principle of Thermal Relaxation Time (TRT). Because hair follicles are larger than skin melanin particles, they cool down much slower; a 30ms pulse exploits this difference, allowing the skin to cool off during the laser shot while the hair follicle retains the heat necessary for destruction.
The Mechanics of Selective Safety
Lowering Peak Power
When treating dark skin (Fitzpatrick types IV-V), the primary risk is that the melanin in the skin will absorb the laser energy just as quickly as the melanin in the hair.
Short pulses (e.g., 3ms) deliver energy in a high-intensity "spike." This rapid delivery overwhelms the skin's ability to cool down, leading to burns or pigmentation changes.
Long pulse settings (30ms) spread that same energy over a longer window. This reduces the immediate "peak power," preventing the rapid overheating of the epidermal layer.
Protecting the Epidermis
The goal is to destroy the hair papilla (the root) without damaging the surface skin.
By extending the pulse width, you provide a time buffer. This allows the epidermis to dissipate a significant amount of heat into the air or cooling gel while the laser is still firing.
This ensures that the temperature of the skin remains below the threshold for injury, even though the total energy delivered is high enough to be effective.
The Principle of Thermal Relaxation Time (TRT)
Volume Dictates Cooling Speed
The scientific concept driving this safety feature is Thermal Relaxation Time (TRT). This is the time it takes for a target to lose 50% of its heat.
Small objects cool down very quickly. Large objects hold heat longer.
Melanin granules in the skin are microscopic, so they have a very short TRT (they lose heat instantly). Hair follicles are much larger and bulkier, so they have a long TRT (they hold onto heat).
The "Thermal Head Start"
A 30ms pulse width is calibrated to match or slightly exceed the TRT of the hair follicle.
Because the pulse is long, the small skin particles lose the heat almost as fast as they receive it. They never reach a critical burn temperature.
However, the larger hair follicle cannot cool down that fast. It accumulates the thermal energy throughout the entire 30ms duration.
This results in the destruction of the hair follicle's germinal layer and inner root sheath, while the surrounding skin remains safe.
Understanding the Trade-offs
The Risk of Pulses Being "Too Short"
Using a pulse width that is shorter than the hair's TRT (e.g., under 10ms on dark skin) is dangerous.
The energy enters the tissue faster than the skin can release it. This causes immediate thermal injury to the epidermis, leading to blistering, hyperpigmentation (dark spots), or hypopigmentation (white spots).
The Risk of Pulses Being "Too Long"
While extending the pulse increases safety, there is an upper limit.
If the pulse width is excessively long—far exceeding the TRT of the hair follicle—the heat may begin to dissipate from the hair into the surrounding tissue before the follicle is destroyed.
This results in ineffective treatment (the hair survives) and can potentially cause non-specific heating of the surrounding dermis, leading to pain or side effects unrelated to pigment.
Making the Right Choice for Your Goal
To maximize safety and efficacy on dark skin, clinical parameters must be precise.
- If your primary focus is Safety on Dark Skin: Prioritize a pulse width between 30ms and 34ms. This ensures the epidermis has adequate time to cool, drastically reducing the risk of burns.
- If your primary focus is Fine Hair Removal: Be cautious with extending the pulse too far. Finer hair has a shorter TRT than coarse hair; if the pulse is too long, the fine hair may cool down before it is destroyed.
- If your primary focus is Coarse Hair Efficacy: Ensure the pulse width is sufficient (15-30ms) to allow heat to fully permeate the large volume of the follicle without leaking into surrounding tissue.
Success in treating dark skin lies in balancing the time it takes to heat the hair against the time it takes to cool the skin.
Summary Table:
| Feature | Short Pulse (e.g., 3ms) | Long Pulse (e.g., 30ms) |
|---|---|---|
| Peak Power | High Intensity Spike | Controlled & Gradual |
| Skin Cooling | Insufficient time to dissipate heat | Skin cools during energy delivery |
| Target Audience | Light Skin (Fitzpatrick I-III) | Dark Skin (Fitzpatrick IV-VI) |
| Safety Profile | High risk of burns on dark skin | High safety; protects the epidermis |
| Hair Type | Ideal for fine, thin hair | Best for coarse and deep follicles |
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References
- Ivani Greppi. Diode laser hair removal of the black patient. DOI: 10.1002/lsm.1031
This article is also based on technical information from Belislaser Knowledge Base .
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