Precise pulse duration is the critical control mechanism that allows laser hair removal to be both effective and safe.
The pulse duration (or pulse width) must be matched to the hair follicle's Thermal Relaxation Time (TRT) to achieve selective photothermolysis. Specifically, the laser pulse must be long enough to allow the skin to cool down, yet short enough to trap heat inside the follicle. If the pulse is too long, heat escapes the follicle and burns the surrounding tissue; if it is too short, the energy may cause mechanical shock rather than the sustained heat required to permanently destroy the follicle.
The Core Mechanism The goal is to heat the target (the follicle) faster than it can cool down. By matching the pulse duration to the follicle's cooling speed, you ensure that thermal energy accumulates purely within the hair shaft to destroy it, while simultaneously allowing the surrounding skin enough time to dissipate that same energy safely.
The Principle of Thermal Relaxation Time
Defining the Threshold
Thermal Relaxation Time (TRT) is the time required for an object to lose 50% of its heat energy to surrounding tissues.
Every biological structure has a specific TRT based on its size and composition. Smaller objects cool down rapidly, while larger, denser objects retain heat longer.
The Biological "Window of Opportunity"
Effective treatment relies on exploiting the difference in TRT between the skin and the hair.
The epidermis (skin surface) has a very short TRT, typically between 3 to 10 milliseconds. Conversely, the hair follicle is a much larger structure with a significantly longer TRT, ranging from 40 to 100 milliseconds.
This discrepancy creates a specific time window where you can safely heat the hair without overheating the skin.
Achieving Selective Photothermolysis
Concentrating Energy in the Follicle
To permanently destroy a hair follicle, you must "cook" it.
The pulse width should be roughly equal to or slightly shorter than the follicle's TRT (40–100ms).
This ensures that the laser pumps energy into the follicle faster than the follicle can release it. The result is a rapid buildup of heat that destroys the stem cells, the bulge, and the dermal papilla required for hair regrowth.
Protecting the Surrounding Skin
While the follicle is heating up, the skin must be kept cool.
Because the epidermis has a short TRT (3–10ms), a pulse duration that is longer than 10ms allows the skin to dissipate heat during the laser shot.
By setting a pulse duration longer than the skin's cooling time but shorter than the follicle's cooling time, the skin sheds the heat harmlessly while the follicle absorbs it destructively.
Understanding the Trade-offs
The Risk of Pulses That Are Too Long
If the pulse duration exceeds the TRT of the hair follicle (e.g., significantly longer than 100ms), the containment of heat fails.
The follicle will begin to dissipate heat into the surrounding non-pigmented dermis before the hair is destroyed. This reduces the efficacy of the treatment and drastically increases the risk of collateral damage, such as redness, burns, or pain.
The Inefficiency of Pulses That Are Too Short
Pulse durations that are excessively short (such as nanosecond pulses used in tattoo removal) rely on mechanical shock rather than thermal destruction.
While these short pulses generate energy, they often fail to raise the temperature of the entire follicle unit long enough to cause permanent coagulation. They may produce temporary hair loss or growth delay, but they rarely achieve long-term reduction.
Making the Right Choice for Your Goal
To maximize results while minimizing injury, the pulse duration must be customized based on the specific physics of the patient's hair and skin.
- If your primary focus is Thick, Terminal Hair: Select a longer pulse duration (closer to 100ms), as larger follicles cool slowly and require sustained heating to be fully destroyed.
- If your primary focus is Fine or Thin Hair: Use a shorter pulse duration (closer to 10-30ms), as small follicles dissipate heat rapidly and require a faster energy delivery to achieve destruction.
- If your primary focus is Darker Skin Types: Extend the pulse duration significantly to allow the pigmented epidermis ample time to cool and diffuse heat, preventing surface burns.
Ultimate Success Factor: The perfect pulse duration is a balancing act that delivers heat fast enough to kill the hair, but slow enough to spare the skin.
Summary Table:
| Biological Structure | Thermal Relaxation Time (TRT) | Recommended Pulse Duration | Goal of Precision |
|---|---|---|---|
| Epidermis (Skin) | 3 – 10 milliseconds | Must be shorter than skin cooling | Prevent surface burns and cooling |
| Hair Follicle | 40 – 100 milliseconds | Roughly 40 - 100ms | Trap heat to destroy stem cells |
| Fine/Thin Hair | Very Short | 10 – 30 milliseconds | Rapid energy delivery for small targets |
| Thick/Terminal Hair | Long | Close to 100 milliseconds | Sustained heating for large follicles |
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Ensure your clients receive the most effective, burn-free treatments by choosing equipment that masters the physics of thermal relaxation. Contact us today to upgrade your practice and discover how our expertise can drive your clinic's success.
References
- Wioleta Jankowiak, Weronika Jereczek. Application of laser technology in the removal of unwanted hair. DOI: 10.7241/ourd.2023e.18
This article is also based on technical information from Belislaser Knowledge Base .
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