Adjustable pulse width acts as the critical control mechanism for balancing safety and efficacy in laser treatments. Specifically, a setting such as 20ms ensures the laser energy delivery matches the thermal relaxation time of the target hair, allowing sufficient heat to build up for follicle destruction without burning the surrounding skin.
Core Takeaway: The duration of the laser pulse determines the intensity and pace of heat delivery. By matching the pulse width to the specific thermal properties of the hair, you maximize destruction of the follicle while providing the epidermis vital time to dissipate heat and prevent thermal injury.
The Physics of Thermal Relaxation
To understand why a specific timing like 20ms is necessary, one must understand the interaction between heat and biological tissue.
Matching the Thermal Relaxation Time (TRT)
Every object, including a hair follicle, has a specific "thermal relaxation time." This is the time it takes for the object to lose 50% of its heat.
The primary goal is to keep the pulse width roughly equal to or slightly longer than the hair's TRT.
Accumulating Heat for Destruction
If the pulse is too short, the energy hits the target too aggressively. By extending the pulse to 20ms, the system ensures heat has sufficient time to accumulate deeply within the hair follicle.
This sustained heating is required to achieve the temperature necessary for permanent structural damage to the follicle.
Protecting the Surrounding Tissue
The secondary function of adjustable pulse width is safety. The skin (epidermis) and the hair follicle cool down at different rates.
Limiting Heat Diffusion
While the follicle is heating up, the surrounding dermal tissue needs protection. A properly calibrated pulse width ensures that the heat remains concentrated in the target (the hair).
This prevents the energy from diffusing outward and causing thermal damage to the healthy cells surrounding the follicle.
Safeguarding the Epidermis
Supplementary data indicates that longer pulse widths allow for a more gradual release of energy. This "slow-cook" approach gives the epidermal pigment time to dissipate heat during the shot.
This capability is critical for preventing heat concentration in the shallow dermis, which is the primary cause of surface burns.
Customization for Anatomy and Skin Tone
Fixed pulse widths cannot accommodate the biological variance found across different patients.
Adapting to Hair Thickness
Thicker hairs generally have longer thermal relaxation times than fine hairs. An adjustable range (e.g., 15ms to 400ms) allows technicians to precisely match the parameter to the physical thickness of the hair.
Treating Darker Skin Tones (Fitzpatrick IV-VI)
Patients with darker skin tones have more melanin in their epidermis, which competes for the laser energy.
Using a longer pulse width enables a gentler heating process. This reduces the risk of burns or hyperpigmentation in Fitzpatrick types IV through VI by avoiding the rapid energy spikes that damage melanin-rich skin.
Understanding the Trade-offs
While adjustable pulse widths offer precision, incorrect settings can compromise the treatment.
The Risk of Pulses Being Too Short
If the pulse width is significantly shorter than the hair's TRT, the energy delivery becomes explosive. This increases the risk of epidermal damage and immediate adverse reactions, particularly in darker skin.
The Risk of Pulses Being Too Long
If the pulse width is excessive relative to the hair's thickness, heat dissipates from the follicle faster than it accumulates. This results in sub-optimal heating, meaning the follicle is warmed but not destroyed, leading to ineffective treatment.
Making the Right Choice for Your Goal
The "correct" pulse width is not a static number; it is a variable that changes based on the patient profile.
- If your primary focus is efficacy on thick hair: Select a pulse width (like 20ms) that matches the longer thermal relaxation time to ensure deep follicle destruction.
- If your primary focus is safety on dark skin: Utilize longer pulse widths to slow the energy delivery, allowing the epidermis to cool while the follicle heats.
- If your primary focus is fine hair: Shorter pulse widths are generally required to heat the target quickly before it cools down.
Success in laser therapy relies on finding the precise window where the follicle is destroyed, and the skin remains untouched.
Summary Table:
| Pulse Width Setting | Impact on Hair Follicle | Effect on Surrounding Skin | Best Used For |
|---|---|---|---|
| Short Pulse | Rapid energy delivery; explosive heat | High risk of epidermal burns | Fine hair on light skin |
| Medium (e.g., 20ms) | Deep heat accumulation; follicle destruction | Controlled heat diffusion | Thick hair / General treatments |
| Long Pulse | Slow, sustained heating | Maximum epidermal cooling/safety | Darker skin (Fitzpatrick IV-VI) |
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References
- JANE G. KHOURY, Mitchel P. Goldman. Comparative Evaluation of Long-Pulse Alexandrite and Long-Pulse Nd:YAG Laser Systems Used Individually and in Combination for Axillary Hair Removal. DOI: 10.1111/j.1524-4725.2007.34125.x
This article is also based on technical information from Belislaser Knowledge Base .
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