Why Is It Necessary For Long-Pulse Laser Systems To Feature Adjustable Pulse Widths, Such As 20Ms?

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)

Elevate Your Clinic’s Results with BELIS Advanced Laser Systems

Precision is the key to balancing patient safety with permanent results. BELIS specializes in professional-grade medical aesthetic equipment designed exclusively for clinics and premium salons. Our advanced laser systems—including Diode, CO2 Fractional, Nd:YAG, and Pico lasers—feature fully adjustable parameters like pulse width to ensure you can treat every skin type and hair density with confidence.

Beyond hair removal, our portfolio includes HIFU, Microneedle RF, and body sculpting solutions (EMSlim, Cryolipolysis), as well as specialized Hydrafacial and skin testing devices. Empower your practice with the technology your clients deserve.

Contact our experts today to find the perfect system for your clinic

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 .