Long pulse widths fundamentally alter how laser energy interacts with biological tissue. By stretching the energy delivery over 15 to 25 milliseconds, the system prevents the immediate, destructive power spikes associated with shorter pulses. This creates a "slow-heating" profile that safeguards the epidermis while effectively raising the temperature of deeper dermal layers.
Core Takeaway: A 15-25 millisecond pulse width decouples total energy from peak power. It allows for the delivery of sufficient energy to stimulate deep collagen regeneration without the instantaneous intensity that causes surface burns.
The Mechanism of Safety and Efficacy
Preventing Thermal Shock
The primary safety risk in laser therapy is instantaneous ultra-high power.
When energy is delivered too quickly, it causes rapid vaporization or burning of the outer skin layer (epidermis).
A pulse width of 15-25ms mitigates this by releasing energy in a gentle, sustained manner rather than a sharp spike.
Epidermal Preservation
Because the power is distributed over a longer duration, the surface of the skin is spared from extreme thermal stress.
This creates a safety margin that preserves the integrity of the epidermis.
It allows practitioners to deliver therapeutic levels of energy without risking superficial scarring or pigmentation changes.
Deep Dermal Accumulation
While the surface remains protected, the sustained pulse ensures that heat accumulates effectively within the deeper dermal tissue.
This acts as a bulk heating mechanism.
The gradual buildup of thermal energy is critical for reaching the specific temperatures required to trigger biological responses deep in the skin.
Collagen Stimulation
The ultimate goal of this thermal accumulation is collagen contraction and regeneration.
By maintaining the target temperature for the duration of the long pulse, the laser maximizes the stimulation of fibroblasts.
This results in tighter skin and improved texture over time, driven by the body's natural healing response.
The Principles of Thermal Relaxation
The Time-Dispersion Effect
Effectiveness relies on the principle of time dispersion.
Energy must be delivered slowly enough to protect the surface, but fast enough to heat the target before it cools down.
The 15-25ms range is optimized to allow heat to soak into the bulk tissue of the dermis continuously.
Matching the Target
Laser therapy is governed by the concept of Thermal Relaxation Time (TRT).
Different targets require different pulse durations to match their cooling properties.
While shorter pulses (microsecond level) are needed for tiny targets like fine vessels, 15-25ms pulses are specifically tuned for larger tissue volumes involved in collagen remodeling.
Understanding the Trade-offs
Specificity vs. Volume
The 15-25ms pulse width is a "volume heating" tool, not a precision scalpel.
It is excellent for generalized tightening and bulk heating.
However, it may be less effective for ablating very fine, small vessels, which cool down faster than the pulse delivers energy.
Heat Diffusion Risks
While longer pulses protect the epidermis, they allow time for heat to conduct into surrounding tissues.
If the pulse is too long relative to the target, heat may diffuse into healthy tissue rather than staying confined to the target area.
Precise calibration within the 15-25ms window is required to balance sufficient heating with tissue safety.
Making the Right Choice for Your Goal
Selecting the correct pulse width is a decision based entirely on the biological target you intend to treat.
- If your primary focus is Skin Tightening and Remodeling: Utilize the 15-25ms pulse width to safely accumulate heat in the dermis and maximize collagen regeneration without surface damage.
- If your primary focus is Fine Vessel Ablation: Opt for shorter pulse widths, as these prevent excessive heat diffusion and match the faster thermal relaxation time of small capillaries.
- If your primary focus is Hair Removal: Consider pulse widths near 10ms, which match the thermal relaxation time of the hair follicle to ensure destruction of the germinal center.
Success in laser therapy lies in matching the speed of energy delivery to the thermal characteristics of the tissue you wish to change.
Summary Table:
| Feature | 15-25ms Long Pulse Width | Impact on Treatment |
|---|---|---|
| Energy Delivery | Sustained, slow-heating profile | Prevents surface burns and thermal shock |
| Primary Target | Deep dermal tissue / Bulk heating | Maximizes collagen contraction and regrowth |
| Safety Margin | High epidermal preservation | Minimizes risk of scarring and pigmentation |
| Clinical Focus | Skin tightening and remodeling | Superior results for skin texture and laxity |
| Mechanism | Thermal energy accumulation | Stimulates fibroblasts via controlled heat soak |
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References
- Omneia Alaa Selim, Tarek Amin. Long-Pulsed Nd:YAG Laser as a Therapeutic Option for Striae Distensae. DOI: 10.33545/26649411.2025.v8.i2a.237
This article is also based on technical information from Belislaser Knowledge Base .
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