A pulse width of 20 milliseconds is selected to precisely balance safety and efficacy by targeting the specific cooling rates of hair versus skin. This duration sits in a critical "thermal window": it is long enough to allow the skin to cool down and prevent burns, yet short enough to trap heat inside the hair follicle, ensuring the destruction of growth structures.
The Core Insight A 20ms pulse width relies on the principle of Selective Photothermolysis. It exploits the physical reality that the epidermis cools down much faster (3–10ms) than a hair follicle (40–100ms), allowing the laser to burn the target while sparing the surrounding tissue.
The Mechanics of Thermal Relaxation
To understand why 20 milliseconds is the standard setting, you must understand Thermal Relaxation Time (TRT). This is the time it takes for a target tissue to dissipate 50% of the heat it has absorbed.
The Epidermal Protection Window
The skin's surface (epidermis) is thin and flat. Consequently, it releases heat relatively quickly.
The TRT of the epidermis is approximately 3 to 10 milliseconds.
Because the 20ms pulse is longer than the skin's TRT, the epidermis has sufficient time to transfer the laser's heat to surrounding tissues or the air. This prevents heat from accumulating to the point of causing burns or pigmentary changes on the surface.
The Follicular Destruction Window
Hair follicles are thicker, cylindrical structures located deeper in the dermis. They hold onto heat much longer than the surface skin.
The TRT of a typical hair follicle ranges from 40 to 100 milliseconds.
Because the 20ms pulse is shorter than the follicle's TRT, the hair cannot release the energy as fast as it receives it. The heat accumulates rapidly within the shaft, thermally destroying the vital growth structures—specifically the bulb and the bulge.
The "Goldilocks" Zone
The 20ms setting is the calculated compromise between these two biological clocks.
It falls strictly between the rapid cooling time of the skin and the slow cooling time of the hair.
This ensures the laser energy is "confined" to the follicle for destruction, while simultaneously being "released" by the skin for protection.
Understanding the Trade-offs
While 20ms is a robust standard for long-pulsed Alexandrite lasers, it is not a magic number for every scenario. Understanding the limitations is vital for clinical judgment.
Risks of Shorter Pulse Widths (< 10ms)
Reducing the pulse width significantly below 20ms increases the "snap" or peak power of the laser.
If the pulse becomes shorter than the epidermis's cooling time (e.g., < 3ms), the skin cannot dissipate the heat fast enough. This drastically increases the risk of epidermal burns and post-inflammatory hyperpigmentation, particularly in darker skin tones.
Risks of Longer Pulse Widths (> 40ms)
Extending the pulse width too far can reduce efficacy.
If the pulse approaches or exceeds the TRT of the hair follicle (e.g., > 40-50ms), the follicle begins to cool down during the laser shot. The heat diffuses into the surrounding dermis rather than building up to the temperature required to coagulate the hair bulb, potentially leading to sub-optimal hair reduction.
Making the Right Choice for Your Goal
The 20ms pulse width is a baseline optimization, but patient variables often dictate adjustments.
- If your primary focus is Safety on Darker Skin: You may need to lengthen the pulse width (e.g., 30ms or higher) to give the melanin-rich epidermis even more time to cool, accepting a slight trade-off in follicle heating efficiency.
- If your primary focus is Fine/Thin Hair: You may need to shorten the pulse width slightly, as thinner hair has a shorter TRT and cools down faster than coarse hair.
Ultimately, 20 milliseconds is the strategic standard because it creates a thermal lag that spares the skin while causing irreversible damage to the hair follicle.
Summary Table:
| Feature | Thermal Relaxation Time (TRT) | Interaction with 20ms Pulse | Clinical Outcome |
|---|---|---|---|
| Epidermis (Skin) | 3 - 10 ms | Longer than TRT | Heat dissipates; prevents burns |
| Hair Follicle | 40 - 100 ms | Shorter than TRT | Heat accumulates; destroys follicle |
| Shorter Pulses | < 10 ms | Below Skin TRT | High risk of epidermal damage |
| Longer Pulses | > 40 ms | Above Follicle TRT | Reduced efficacy on hair removal |
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References
- Jenifer R. Lloyd, Mirko Mirkov. Long-Term Evaluation of the Long-Pulsed Alexandrite Laser for the Removal of Bikini Hair at Shortened Treatment Intervals. DOI: 10.1046/j.1524-4725.2000.00013.x
This article is also based on technical information from Belislaser Knowledge Base .
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