Pulse durations of 50 ms or longer are essential for the irreversible destruction of thick, coarse hair follicles. By extending the energy delivery time, these systems leverage Thermal Damage Time (TDT) theory. This allows sufficient time for lethal heat to conduct from the hair shaft into the surrounding stem cells of the follicle, ensuring permanent damage without overheating the skin surface.
Core Insight: Effective hair removal requires heating the follicle, not just the hair shaft. For thick hair, a pulse duration of 50 ms or longer provides the necessary "soak time" for heat to propagate from the shaft to the germinal center, achieving permanent reduction while bypassing the epidermis.
The Physics of Thermal Damage Time (TDT)
Beyond Simple Heating
To permanently stop hair growth, the laser must destroy the bulge area and the bulb, which are the reproductive centers of the follicle.
However, the laser light is primarily absorbed by the melanin in the hair shaft, not the follicle wall itself.
The Conduction Requirement
Heat must physically conduct from the hot shaft outward to the follicle lining.
Thick structures (like coarse terminal hair) require more time to absorb and transfer this heat than thin structures.
A pulse of 50 ms aligns with the TDT of these thicker hairs, ensuring the heat has enough time to travel to the target stem cells before the laser energy stops.
Protecting the Epidermis
Preventing Temperature Spikes
Short, intense pulses deliver energy very rapidly. In thick hair, this can cause the hair shaft to vaporize or explode before the heat can effectively damage the follicle wall.
This rapid energy release also creates dangerous temperature spikes in the skin.
Leveraging Different Cooling Rates
The epidermis (skin surface) is flat and dissipates heat differently than the cylindrical hair follicle.
By using a long pulse (50 ms+), you allow the epidermis to dissipate heat during the laser pulse.
While the skin cools itself, the bulky hair follicle retains the heat, leading to selective destruction of the hair while preserving the safety of the skin.
The Role of Wavelength and Depth
Deep Penetration
Long-pulsed Nd:YAG systems utilize the 1064 nm wavelength, which is characterized by exceptional tissue penetration (approximately 5 to 7 mm).
Targeting Deep Structures
This depth allows the energy to reach deep-seated structures in the dermis where thick hair follicles reside.
Combined with a long pulse duration, this ensures that even the deepest parts of the follicle reach the thermal threshold required for destruction.
Understanding the Trade-offs
While 50 ms pulses are ideal for thick hair, they are not a universal solution. It is vital to understand when this setting becomes ineffective.
The Limitation for Fine Hair
Fine hairs have a very short Thermal Relaxation Time (TRT). They lose heat almost as fast as they absorb it.
If you use a 50 ms pulse on fine hair, the heat dissipates into the surrounding tissue before it can build up enough to destroy the follicle.
When to Switch to Short Pulses
For fine or light-colored hair, shorter pulse widths (such as 0.3 ms to 10 ms) are required.
These shorter durations confine the heat strictly to the hair structure, creating the high-peak power necessary to damage thin targets that cannot hold heat for long periods.
Making the Right Choice for Your Goal
Selecting the correct pulse duration is a balance between the size of the target and the safety of the skin.
- If your primary focus is Thick/Coarse Hair: Use 50 ms or longer to allow heat to fully conduct into the bulky follicle structure and ensure irreversible damage.
- If your primary focus is Fine/Thin Hair: Use shorter pulses (0.3 ms – 10 ms) to maintain high peak power and confine heat before it dissipates.
- If your primary focus is Darker Skin Safety: Prioritize longer pulses, as they allow the epidermis to cool during energy delivery, reducing the risk of burns and hyperpigmentation.
Match the pulse duration to the hair thickness to transform a temporary singe into permanent elimination.
Summary Table:
| Feature | Long Pulse (≥50 ms) | Short Pulse (0.3 - 10 ms) |
|---|---|---|
| Target Hair Type | Thick, Coarse, Deep-seated | Fine, Thin, Shallow |
| Primary Mechanism | Thermal Damage Time (TDT) | Thermal Relaxation Time (TRT) |
| Heat Distribution | Conducts from shaft to follicle wall | Confined strictly to the hair shaft |
| Skin Safety | High (allows epidermal cooling) | Moderate (higher peak power) |
| Clinical Goal | Permanent destruction of bulky follicles | Damage to thin, fast-cooling targets |
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References
- K. Raff, Ulrich Hohenleutner. Optimizing treatment parameters for hair removal using long-pulsed Nd:YAG-lasers. DOI: 10.1007/s10103-004-0287-9
This article is also based on technical information from Belislaser Knowledge Base .
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