Adjustable pulse width functions as the critical timing mechanism that determines how rapidly laser energy is delivered to the hair follicle. An adjustable range of 10 to 30 milliseconds allows the operator to synchronize the laser with the Thermal Relaxation Time (TRT) of the specific hair being treated, ensuring effective destruction of the follicle without damaging the surrounding skin.
Core Takeaway The goal of adjusting pulse width is to deliver heat slowly enough to protect the skin but fast enough to permanently damage the hair. By matching the pulse duration to the hair's thickness and depth, you maximize energy accumulation within the follicle while allowing the epidermis sufficient time to cool down.
The Principle of Thermal Relaxation Time (TRT)
Matching Physics to Biology
The effectiveness of the 10–30 ms range relies on the principle of Thermal Relaxation Time (TRT). TRT is the time it takes for a target tissue to lose 50% of its heat.
Selective Destruction
To permanently destroy a hair follicle, the laser pulse duration should be roughly equal to or slightly longer than the hair's TRT. This ensures the follicle retains enough heat to reach the destruction threshold, rather than letting it dissipate harmlessly.
Optimizing for Hair Thickness and Depth
Treating Thicker Hair (Approaching 30 ms)
Thicker, coarser hair has a longer TRT, meaning it holds onto heat longer. By extending the pulse width toward 30 milliseconds, the system delivers energy steadily over a longer period.
Deep Follicle Penetration
This slower delivery allows thermal energy to accumulate deep within the thick hair shaft. It ensures the root is destroyed without creating a sudden, dangerous spike in temperature at the skin's surface.
Treating Fine Hair (Approaching 10 ms)
Fine or residual hair has a much shorter TRT and loses heat rapidly. To treat this hair effectively, you must use a shorter pulse width, closer to 10 milliseconds.
High Instantaneous Impact
A shorter pulse creates a higher instantaneous thermal impact. This is necessary to "shock" thin hair follicles with sufficient heat before they have a chance to cool down.
Ensuring Epidermal Safety
Avoiding Micro-Injuries
The primary reference highlights that adjustable long-pulse widths prevent "epidermal micro-injuries." These injuries are often caused by sudden energy spikes associated with fixed, short pulse durations that do not account for skin cooling.
Protecting Darker Skin Tones
For patients with darker skin (higher melanin content), longer pulse widths (e.g., 20–30 ms) are essential safety features. They allow the melanin in the epidermis to dissipate heat through thermal conduction, preventing burns while the follicle continues to heat up.
Understanding the Trade-offs
The Risk of Pulses Being Too Short
If the pulse width is set too short (below the optimal range for the hair type), the energy delivery is too aggressive. This can cause immediate epidermal damage, such as burns or hyperpigmentation, because the skin does not have time to dissipate the heat.
The Risk of Pulses Being Too Long
Conversely, if the pulse width is significantly longer than the hair's TRT, the heat will diffuse into the surrounding tissue before the follicle reaches a lethal temperature. This results in ineffective treatment and potential heating of the surrounding dermis, rather than the hair root.
Making the Right Choice for Your Goal
To maximize clinical results and safety, the pulse width must be customized based on the patient's specific anatomy.
- If your primary focus is Fine or Light Hair: Use a shorter pulse width (10–15 ms) to generate the rapid heat accumulation required to destroy thin targets before they cool.
- If your primary focus is Thick Hair or Darker Skin: Use a longer pulse width (20–30 ms) to allow the epidermis to cool safely while slowly cooking the larger follicle.
Success in laser hair removal is defined by finding the precise window where the follicle is destroyed, and the skin remains untouched.
Summary Table:
| Pulse Width Range | Target Hair Type | Target Skin Tone | Thermal Effect |
|---|---|---|---|
| 10 - 15 ms | Fine / Thin Hair | Lighter Tones | High instantaneous impact to 'shock' thin follicles |
| 15 - 20 ms | Medium / Normal Hair | Medium Tones | Balanced heat delivery for standard treatments |
| 20 - 30 ms | Thick / Coarse Hair | Darker Tones | Gradual energy accumulation; maximizes epidermal safety |
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References
- Jaggi Rao, Mitchel P. Goldman. Prospective, Comparative Evaluation of Three Laser Systems Used Individually and in Combination for Axillary Hair Removal. DOI: 10.1097/00042728-200512000-00007
This article is also based on technical information from Belislaser Knowledge Base .
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