The critical role of pulse width is its ability to match the thermal characteristics of the target hair follicle to ensure permanent damage. Pulse width settings determine the duration of energy exposure, governing whether heat can sufficiently conduct from the hair shaft to the germinative cells (the root and bulge) without diffusing into and burning the surrounding skin.
Core Takeaway To achieve permanent hair reduction, the pulse width must align with the principle of selective photothermolysis, specifically matching or staying slightly below the Thermal Relaxation Time (TRT) of the follicle. For large follicles, a longer pulse width (120 to 125 ms) is essential to allow heat to accumulate and destroy stem cells while moderating instantaneous power to protect the epidermis.
The Mechanism of Thermal Destruction
The Principle of Selective Photothermolysis
The foundational rule for setting pulse width is the Thermal Relaxation Time (TRT). This is the time it takes for an object to lose 50% of its heat.
For effective destruction, the laser or IPL pulse duration must be less than or equal to the TRT of the hair follicle. This ensures that the target absorbs the energy and heats up before it can cool down.
Conducting Heat to Critical Structures
Destroying the hair shaft alone is insufficient; the energy must travel to the biological "life centers" of the hair.
A properly calibrated long pulse allows thermal energy to conduct from the melanin-rich hair shaft to the bulge and dermal papilla. These areas contain the germinative cells responsible for regeneration. If the pulse is too short, the heat never reaches these peripheral structures.
Optimizing for Follicle Size
Requirements for Large Follicles
Large hair follicles have a greater thermal mass and retain heat longer. According to primary technical guidelines, these require a long pulse width setting of 120 to 125 ms.
This extended duration ensures that thermal energy has sufficient time to accumulate within the large structure. This accumulation is necessary to raise the temperature of the germinative cells to the coagulation point.
The Safety Benefit of Longer Pulses
Using a longer pulse width for large follicles also serves a critical safety function. It prevents the delivery of excessively high instantaneous power.
By spreading the energy delivery over 120ms rather than a shorter burst, the system avoids the rapid, explosive heating that could injure the epidermis (skin surface).
Settings for Average to Fine Hair
While large follicles need 100ms+, average follicles often have a TRT ranging from 20ms to 100ms.
A setting of approximately 20ms is often cited for general targets. This allows for uniform energy release that destroys the follicle while giving the surrounding normal skin tissue—which loses heat faster than the hair—enough time to dissipate residual heat.
Understanding the Trade-offs
The Pitfall of Nanosecond Pulses
Shorter is not always better. Pulses that are extremely short (such as nanosecond pulses) fail to heat the follicle thoroughly.
Instead of thermal destruction, these pulses primarily produce mechanical shock. This may cause a temporary growth delay or "stunning" of the hair, but it rarely results in the permanent inactivation of the follicle provided by millisecond-domain pulses.
Balancing Efficacy and Epidermal Protection
The pulse width creates a window for safety. The goal is to heat the hair follicle faster than it can cool, but slower than the skin can cool.
If the pulse width is significantly longer than the follicle's TRT, the heat will dissipate into the surrounding tissue, reducing efficacy and increasing the risk of pain or burns. Conversely, if it is too short for a large follicle, the shaft may vaporize without destroying the stem cells.
Making the Right Choice for Your Goal
To maximize clinical efficacy while maintaining safety, adjust your pulse width based on the specific target:
- If your primary focus is large, deep hair follicles: Use a long pulse width (120–125 ms) to ensure deep heat conduction to stem cells and prevent surface burns.
- If your primary focus is average or finer hair: Use a shorter pulse width (approx. 20–40 ms) to match the shorter thermal relaxation time of smaller targets.
- If your primary focus is patient safety: Avoid excessively high peak power by lengthening the pulse width, allowing the epidermis to cool while the follicle retains heat.
Final thought: Precision in pulse width is the difference between temporary hair shedding and permanent follicular destruction.
Summary Table:
| Hair Type/Target | Recommended Pulse Width | Key Mechanism | Benefit |
|---|---|---|---|
| Large/Deep Follicles | 120 - 125 ms | Deep heat conduction to stem cells | Prevents epidermal burns & ensures destruction |
| Average/Fine Hair | 20 - 40 ms | Matches shorter TRT of smaller targets | Efficient energy release with skin protection |
| General Safety | > TRT of skin | Selective Photothermolysis | Protects surrounding tissue from heat damage |
| Short Pulses (ns) | Nanoseconds | Mechanical shock/Stunning | Ineffective for permanent hair removal |
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References
- Martin Sochor, J Buchvald. Comparison of hair reduction with three lasers and light sources: Prospective, blinded and controlled study. DOI: 10.3109/14764172.2011.586422
This article is also based on technical information from Belislaser Knowledge Base .
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