The 10 to 15 ms pulse duration is critical because it aligns with the thermal relaxation time of the hair follicle. By maintaining the laser pulse for this specific duration, the device delivers thermal energy slowly and continuously. This ensures the heat destroys the follicle's regenerative cells without causing the explosive surface damage associated with shorter, more aggressive pulses.
Core Insight: Successful laser hair removal relies on "Selective Photothermolysis." The 10-15 ms window is the specific timeframe required to heat the hair follicle to a destruction point while ensuring the surrounding skin has just enough time to dissipate that heat safely.
The Mechanism of Action
Matching Thermal Relaxation Time
The fundamental principle behind this pulse duration is thermal relaxation time (TRT). This is the time it takes for an object (in this case, the hair follicle) to lose 50% of its heat.
For effective treatment, the laser pulse duration must roughly match the TRT of the follicle. A duration of 10 to 15 ms allows the energy to act on the target long enough to cause irreversible damage, rather than dissipating too quickly.
Targeting Germinal Cells
The ultimate goal of hair removal is not just to burn the hair shaft, but to disable the follicle's ability to regenerate.
A pulse in the 10 to 15 ms range ensures the thermal energy penetrates deep enough to reach the germinal cells. These are the reproductive centers of the hair; destroying them is the only way to achieve long-lasting reduction.
Preventing Explosive Surface Damage
The 755nm Alexandrite wavelength is highly absorbed by melanin. If the energy is delivered too rapidly (in very short pulses), the reaction can be explosive.
By stretching the pulse to 10-15 ms, the energy release is controlled. This prevents rapid temperature spikes that could physically damage the epidermis, reducing the risk of immediate mechanical injury to the skin.
Safety and Skin Protection
Allowing Epidermal Cooling
While the laser heats the hair, it inevitably heats the skin's surface (epidermis) as well.
The 10-15 ms duration provides a critical safety buffer. It allows the melanin in the epidermis just enough time to diffuse some heat into the surrounding tissue, lowering the risk of surface burns compared to shorter, high-intensity pulses.
Managing Heat Diffusion
Pulse duration controls heat diffusion. If the pulse is too long (excessively beyond the TRT), heat spreads into the surrounding tissue, causing unnecessary pain and potential side effects.
The 10-15 ms range strikes a balance: it keeps the heat concentrated within the follicle structure long enough to destroy it, but not so long that it causes widespread collateral thermal damage.
Understanding the Trade-offs
The Risk of Pigmented Skin
While 10-15 ms is ideal for lighter skin (Fitzpatrick I-II), it may be too aggressive for darker skin tones (Fitzpatrick IV-VI).
Patients with higher epidermal melanin retain more heat. For these individuals, the pulse duration often needs to be extended beyond 15 ms (up to 30 ms or more) to allow for greater epidermal cooling and prevent hyperpigmentation or burns.
Hair Thickness Variations
The 10-15 ms standard is generally effective, but hair thickness dictates the exact TRT.
Thicker hairs generally have a longer TRT and may require the upper end of this range or even longer pulses to heat thoroughly. Conversely, finer hairs cool down faster and may require the lower end of the pulse duration spectrum to be effectively treated.
Making the Right Choice for Your Goal
When configuring an Alexandrite laser, the pulse duration is your primary lever for balancing safety and aggression.
- If your primary focus is treating light skin (Types I-II): Utilize the 10 to 15 ms range to maximize germinal cell destruction and ensure high efficacy.
- If your primary focus is safety on darker skin (Types IV-V): Extend the pulse duration beyond 15 ms to allow the epidermis sufficient time to cool and prevent pigmentary changes.
- If your primary focus is avoiding surface injury: Ensure you do not drop below the recommended threshold, as extremely short pulses can cause "explosive" vaporization of the target.
Precise synchronization of the pulse duration with the hair's cooling time is the difference between permanent removal and temporary surface burns.
Summary Table:
| Parameter | Range/Value | Impact on Treatment |
|---|---|---|
| Optimal Pulse Duration | 10 - 15 ms | Matches Thermal Relaxation Time (TRT) of hair follicles. |
| Wavelength | 755 nm | High melanin absorption for effective hair targeting. |
| Primary Goal | Germinal Cell Destruction | Disables the follicle's ability to regenerate for long-term results. |
| Safety Mechanism | Epidermal Cooling | Allows heat to diffuse from the skin surface to prevent burns. |
| Skin Type Adjustment | Fitzpatrick I-II (Standard) | For darker skin (IV-VI), pulse duration should exceed 15 ms. |
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References
- Kawthar Shurrab, Manal Asad Nassr. Long-Term Efficacy and Safety of755-nm Alexandrite Laser for Axillary Hair Removal: A Comparative Analysis of Single vs. DualFlash lamp Systems. DOI: 10.2196/76523
This article is also based on technical information from Belislaser Knowledge Base .
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