The technical principle behind extended pulse width is the strategic exploitation of differential Thermal Relaxation Times (TRT). By delivering energy over a specific duration that exceeds the cooling time of the skin but remains shorter than that of the hair follicle, the device achieves selective destruction. This ensures the epidermis has time to dissipate absorbed heat, while the follicle accumulates enough thermal energy to undergo denaturation.
Core Insight: The effectiveness of extended pulse width relies on a specific "timing window." The laser pulse must be slow enough to let the skin cool down mid-blast, yet fast enough to overwhelm the hair follicle's ability to shed heat.
The Physics of Thermal Relaxation
To understand why extending the pulse width works, you must first understand how biological tissues handle heat.
Defining Thermal Relaxation Time (TRT)
Thermal Relaxation Time is the time required for a target tissue to lose 50% of its heat through diffusion to surrounding areas. Every structure in the skin—whether it is a strand of hair or a patch of epidermal cells—has a unique TRT.
The Critical Difference
The technical foundation of this feature is that epidermal melanin and follicular melanin have different TRTs. The epidermis is a thin surface layer that dissipates heat relatively quickly. In contrast, the hair follicle is a larger, deeper structure that retains heat for a longer period.
How Extended Pulse Width Solves the Problem
The goal of hair removal is selective photothermolysis: destroying the target (hair) without harming the bystander (skin). Extended pulse width achieves this through precise timing.
Protecting the Epidermis
When the pulse width is extended to be longer than the TRT of the epidermis, the skin is allowed to "breathe" thermodynamically. As the laser energy hits the skin, the extended duration allows the epidermis to transfer that heat away before it reaches a damage threshold.
Destroying the Follicle
Simultaneously, this same pulse duration must be shorter than the TRT of the hair follicle. Because the follicle loses heat more slowly, it cannot dissipate the energy as fast as it arrives. Consequently, heat accumulates within the follicle until it reaches the temperature required to destroy the germinative cells.
The Role of Hair Diameter
The specific pulse setting often depends on the hair's thickness, as diameter determines TRT. Thicker hair loses heat more slowly, requiring a longer pulse width (extended duration) to effectively heat the center of the follicle without burning the surrounding tissue.
Understanding the Trade-offs
While extended pulse width improves safety and versatility, it requires precise calibration to avoid adverse effects.
The Risk of Heat Diffusion
If the pulse width is extended too far—beyond the TRT of the follicle—the heat will begin to conduct into the surrounding dermis. This leads to non-selective heating, which causes pain and collateral damage to healthy tissue rather than targeted hair destruction.
Balancing Intensity and Duration
Extending the pulse spreads the energy out over time, which can create a gentler heating profile. However, if the energy density (fluence) is not maintained, the peak temperature in the follicle may never be reached, resulting in ineffective treatment.
Making the Right Choice for Your Goal
The pulse width is not a "set and forget" parameter; it is a variable that dictates the balance between safety and aggression.
- If your primary focus is Patient Safety (Darker Skin): You require a longer pulse width. This gives the melanin-rich epidermis more time to cool down during the shot, preventing surface burns.
- If your primary focus is Treating Thick/Coarse Hair: You require a longer pulse width. Thick structures have long TRTs, so a rapid pulse might only heat the surface of the hair; a longer pulse ensures the heat soaks through to the root.
Ultimately, the correct pulse width is defined by the physical dimensions of the target: it must be long enough to spare the skin, but aggressive enough to cook the follicle.
Summary Table:
| Feature | Thermal Relaxation Time (TRT) | Pulse Width Strategy | Clinical Outcome |
|---|---|---|---|
| Epidermis (Skin) | Short (Fast cooling) | Longer than Skin TRT | Heat dissipates, preventing surface burns |
| Hair Follicle | Long (Slow cooling) | Shorter than Follicle TRT | Heat accumulates, leading to denaturation |
| Thick/Coarse Hair | Very Long | Extended Pulse Width | Ensures heat reaches the deep root structure |
| Darker Skin Tones | Short (High Melanin) | Extended Pulse Width | Maximum safety by allowing epidermal cooling |
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References
- Mark M. Hamilton, Paul J. Carniol. Laser Hair Removal Update. DOI: 10.1055/s-2001-17975
This article is also based on technical information from Belislaser Knowledge Base .
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