Long-pulse laser hair removal equipment depends on millisecond-level adjustability to align with the principle of selective photothermolysis. Specifically, the pulse width must match the thermal relaxation time (TRT) of the hair follicle—typically between 40 and 100 milliseconds—while significantly exceeding the TRT of the epidermis (approximately 0.3 milliseconds). This timing ensures the laser destroys the follicle through heat accumulation while allowing the skin enough time to cool down and prevent burns.
Core Insight: The millisecond range creates a vital "safety window." It delivers energy slowly enough for the skin to dissipate heat safely, yet fast enough that the hair follicle accumulates fatal heat before it can cool down.
The Physics of Thermal Relaxation
The Concept of Thermal Relaxation Time (TRT)
To destroy a target without damaging surrounding tissue, the laser pulse duration is critical. TRT is the time required for an object to cool down by 50% after being heated.
The Disparity Between Skin and Hair
There is a massive difference in how quickly skin and hair cool down. The epidermis has a very short TRT of roughly 0.3 milliseconds, meaning it sheds heat rapidly.
The Target Window
In contrast, hair follicles are larger structures with a much longer TRT, generally ranging from 40 to 100 milliseconds. Effective equipment must operate within this millisecond range to exploit the gap between the skin's rapid cooling and the follicle's slow cooling.
Mechanism of Action: Why Milliseconds Matter
Heat Accumulation in the Follicle
The goal is to destroy the germinative structure (the hair papilla), not just the hair shaft. A pulse width in the millisecond range allows the hair shaft to absorb energy and conduct that heat outward to the entire follicle structure.
Epidermal Protection
If the pulse were shorter (micro- or nanoseconds), the energy delivery would be too intense for the epidermis to handle. By stretching the pulse into milliseconds, the rate of heat accumulation in the skin remains lower than its rate of heat dissipation, especially when combined with active cooling systems.
Preventing Mechanical Damage
Ultra-short pulses (like Q-switched lasers) cause mechanical, acoustic shockwaves rather than controlled heating. Long-pulse millisecond lasers avoid this "explosive" effect, ensuring the damage is purely thermal and confined to the follicle.
Understanding the Trade-offs
The Risk of Pulses Being Too Short
If the pulse width is significantly shorter than the follicle's TRT, the heat may not have time to conduct fully from the melanin-rich shaft to the germinal cells. This can result in temporary hair loss rather than permanent reduction, or increase the risk of epidermal injury in darker skin types.
The Risk of Pulses Being Too Long
If the pulse width far exceeds the follicle's TRT, heat will begin to diffuse into the surrounding dermal tissue. This lowers the efficiency of the treatment and increases pain, as the energy is wasted on non-target tissue rather than cooking the follicle.
The Complexity of Dark Skin
Patients with darker skin tones have more melanin in their epidermis, acting as a competing target for the laser. Using longer pulse widths (e.g., 30–100ms) is essential here; it slows the heating rate, giving the epidermal melanin time to offload heat while still slowly raising the follicle temperature to destructive levels.
Making the Right Choice for Your Goal
The ability to adjust pulse width within the millisecond range is the primary control for safety and efficacy across different patient profiles.
- If your primary focus is treating thick, coarse hair: Select a longer pulse width (closer to the follicle's upper TRT limit) to allow sufficient time for heat to conduct through the larger diameter of the hair shaft to the follicle wall.
- If your primary focus is treating darker skin types (Fitzpatrick IV-VI): Utilize extended pulse widths to protect the epidermis by allowing it to dissipate heat during the energy delivery, preventing pigmentary changes or burns.
- If your primary focus is treating fine or thin hair: Use a shorter pulse width (lower end of the millisecond range) because smaller structures cool faster and require rapid energy delivery to achieve destruction before the heat escapes.
Mastering millisecond pulse width adjustment allows you to decouple the heating of the hair from the heating of the skin, ensuring maximum efficacy with minimal risk.
Summary Table:
| Pulse Duration Range | Target Structure | Thermal Relaxation Time (TRT) | Clinical Outcome |
|---|---|---|---|
| 0.3 - 1 ms | Epidermis (Skin) | Very Short (~0.3ms) | Heat dissipates quickly; prevents burns |
| 10 - 40 ms | Fine/Thin Hair | Short to Medium | Rapid heat accumulation for small targets |
| 40 - 100 ms | Coarse/Thick Hair | Long (40-100ms) | Deep thermal conduction to the hair papilla |
| > 100 ms | Darker Skin Tones | Extended | Maximum epidermal protection for high-melanin skin |
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References
- Elisabeth Russe, Katharina Russe‐Wilflingseder. Evaluation of Safety and Efficacy of Laser Hair Removal With the Long‐Pulsed 755 nm Wavelength Laser: A Two‐Center Study With 948 Patients. DOI: 10.1002/lsm.23160
This article is also based on technical information from Belislaser Knowledge Base .
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