The technical rationale for a 20ms pulse width is strictly based on the principle of Thermal Relaxation Time (TRT). By extending the pulse duration to 20ms, the system converts optical energy into thermal energy at a controlled, steady rate. This ensures sufficient heat accumulates to destroy the hair follicle's regenerative structures (the bulb and bulge) without causing the instantaneous temperature spikes that lead to burns on the surrounding scalp.
Core Takeaway: A 20ms pulse width is the "safety lock" that synchronizes the laser's energy delivery with the natural cooling properties of the hair follicle. It allows enough time for deep thermal destruction of the hair root while preventing heat from diffusing laterally into healthy, normal skin tissue.
The Physics of Thermal Relaxation
To understand why 20ms is effective for the scalp, you must understand the relationship between pulse duration and the target's ability to dissipate heat.
Matching the Target's TRT
Every object has a Thermal Relaxation Time (TRT)—the time it takes for an object to lose 50% of its heat.
For effective treatment, the laser pulse width should roughly match the TRT of the hair follicle. A 20ms setting is designed to align with this window, allowing the follicle to absorb and hold lethal heat levels before it can cool down.
Controlled Energy Conversion
Unlike shorter pulses (e.g., 3ms) which deliver energy rapidly, a 20ms pulse converts optical energy to heat at a steady rate.
This pacing is critical. It prevents the rapid vaporization of tissue, instead favoring a "slow-cook" approach that coagulates the follicle structure thoroughly.
Mechanism of Action at 20ms
The 20ms setting is not arbitrary; it is engineered to target specific biological structures while sparing others.
Destruction of the Bulb and Bulge
The primary goal of the treatment is to disable the hair's ability to regenerate.
The 20ms duration provides sufficient time for thermal energy to diffuse deeply throughout the entire follicle apparatus. This ensures that both the hair bulb (the root) and the bulge (the stem cell reservoir) are completely destroyed.
Preventing Epidermal Spikes
The scalp is a large, vascular area often sensitive to trauma.
A 20ms pulse distributes the energy delivery over a longer timeline. This prevents instantaneous spikes in epidermal temperature. By avoiding these rapid heat shocks, the system minimizes the risk of non-specific thermal impact, such as blistering or surface burns.
Understanding the Trade-offs
While 20ms offers a high safety profile, it is important to understand how it contrasts with other pulse widths to ensure it is the right tool for the job.
Short Pulse (3ms) vs. Long Pulse (20ms)
A 3ms pulse is highly aggressive. It keeps energy tightly localized within the follicle because the pulse ends before the heat can move. While effective, this rapid delivery can increase the risk of injury if the skin is compromised or if the operator lacks precision.
A 20ms pulse trades some of that localization for safety. It allows for controlled heat diffusion. This is particularly beneficial when treating large areas like the scalp or when working with compromised tissue (such as cicatricial tissue), where preventing secondary thermal damage is paramount.
The Risk of Excessive Diffusion
If the pulse width were extended significantly beyond 20-30ms without adjusting energy levels, the heat might dissipate too quickly into the surrounding dermis.
This would result in ineffective treatment (failure to kill the follicle) and increased heating of the surrounding skin (pain and potential side effects). 20ms sits in the optimal range: long enough to be safe, but short enough to be lethal to the follicle.
Making the Right Choice for Your Goal
The decision to use 20ms should be driven by the specific clinical endpoint you are trying to achieve.
- If your primary focus is Safety on Large Areas: Use 20ms to prevent temperature spikes and minimize non-specific thermal damage to the normal scalp skin.
- If your primary focus is Deep Follicle Destruction: Use 20ms to ensure heat has time to diffuse thoroughly into the bulb and bulge areas for permanent removal.
- If your primary focus is Treating Compromised Tissue: Use 20ms (or slightly higher) to ensure a gentle, steady energy release that avoids aggravating existing scar tissue.
Summary: Set the pulse width to 20ms to maximize hair removal efficiency through deep thermal accumulation while strictly limiting the risk of lateral heat damage to the patient's scalp.
Summary Table:
| Feature | 3ms Pulse Width | 20ms Pulse Width |
|---|---|---|
| Energy Delivery | Rapid & Aggressive | Steady & Controlled |
| Target Area | Tight Localization | Deep Diffusion (Bulb/Bulge) |
| Scalp Safety | Higher Risk of Burns | Minimized Epidermal Spikes |
| Ideal Use Case | Precision/High Pigment | Large Areas/Sensitive Tissue |
| Thermal Effect | Rapid Vaporization | Coagulation/Slow-Cook |
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References
- Geoffrey Brent, Rob Sheehan-Dare. Laser Hair Removal in Alopecia Areata of the Scalp: A Novel Therapeutic Approach. DOI: 10.1097/dss.0000000000001182
This article is also based on technical information from Belislaser Knowledge Base .
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