The specific pulse duration of 15 to 22 milliseconds is selected to optimize the balance between efficacy and safety for dark skin. This range ensures the laser energy is delivered slowly enough to allow the melanin-rich epidermis to cool down, while still allowing heat to accumulate effectively within the hair follicle to destroy the root. This timing strategy prevents the surface burns and pigmentation changes often associated with treating darker skin types.
Core Takeaway: By extending the pulse width beyond the thermal relaxation time of the epidermis, the laser selectively damages the hair follicle without overheating the skin surface. This thermal delay is the fundamental safety mechanism that protects patients with high melanin content from scarring and hyperpigmentation.
The Science of Thermal Relaxation
Understanding Thermal Relaxation Time (TRT)
The 15-22 msec setting is derived from the principle of Thermal Relaxation Time (TRT). This is the time it takes for a targeted tissue to lose 50% of the heat generated by the laser.
Targeting the Follicle
Hair follicles are thicker structures with a longer TRT. A pulse duration in the 15-22 msec range allows heat to build up effectively within the germinative center of the hair root.
Sparing the Epidermis
The epidermis is a thinner structure that cools down much faster than the hair follicle. By using a pulse duration that exceeds the cooling time of the epidermis, the skin surface can dissipate heat safely while the follicle retains it.
Why Dark Skin Requires Precise Timing
The Melanin Competition
Patients with dark skin (Fitzpatrick types IV-VI) have a high concentration of epidermal melanin. This melanin competes with the hair follicle for laser energy absorption.
Preventing Thermal Injury
If the pulse is too short, energy is delivered too rapidly for the epidermal melanin to release the heat. This results in thermal accumulation on the surface, leading to burns, blisters, or pigmentation changes.
The Millisecond Advantage
Unlike Q-switched lasers or short pulses that trigger immediate surface reactions, the long-pulse (millisecond level) Nd:YAG provides a "slow heating" mechanism. This ensures the destruction of the hair root without damaging the surface of dark skin.
Understanding the Trade-offs
Risks of Pulses That Are Too Short
Reducing the pulse duration below 10 msec on dark skin significantly increases the risk of side effects. Without adequate cooling time, the epidermis becomes susceptible to post-inflammatory hyperpigmentation or blistering.
Risks of Pulses That Are Too Long
If the pulse duration is extended excessively beyond the necessary range, the heat may dissipate from the follicle into surrounding nerves and blood vessels before the follicle is destroyed. This can reduce efficacy and increase pain without achieving permanent hair reduction.
The Scanning Mode Exception
It is worth noting that some high-frequency scanning modes utilize shorter pulses (3 to 8 msec). However, these rely on rapid repetition to achieve thermal accumulation over time, rather than the single-pulse thermal impact of the standard 15-22 msec setting.
Making the Right Choice for Your Goal
To ensure the best outcomes, align your parameters with your specific clinical objective:
- If your primary focus is safety on dark skin: Adhere to the 15-22 msec range to strictly match the thermal relaxation needs of the epidermis, minimizing the risk of burns.
- If your primary focus is high-speed coverage: Utilization of shorter pulses (3-8 msec) is permissible only within high-frequency scanning modes that build heat gradually through repetition.
Precise control of pulse duration is the single most critical factor in transforming a potentially high-risk procedure into a safe, effective treatment for patients with melanin-rich skin.
Summary Table:
| Feature | Pulse Duration (15-22 msec) | Effect on Dark Skin (Fitzpatrick IV-VI) |
|---|---|---|
| Epidermis (Skin Surface) | Exceeds Skin's TRT | Allows surface melanin to cool, preventing burns and blisters. |
| Hair Follicle | Matches Follicle's TRT | Heat accumulates in the root to ensure permanent hair reduction. |
| Safety Profile | Slow Heat Delivery | Minimizes risk of post-inflammatory hyperpigmentation (PIH). |
| Clinical Outcome | Balanced Thermal Impact | High efficacy for hair removal with maximum patient safety. |
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References
- Ehiaghe L. Anaba, Babawale Arabambi. Efficacy of long pulse neodymium-doped yttrium aluminium garnet 1064 laser permanent hair reduction treatment in skin types V and VI Nigerian patients. DOI: 10.18203/issn.2455-4529.intjresdermatol20231159
This article is also based on technical information from Belislaser Knowledge Base .
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