A long pulse width, such as 400ms, fundamentally shifts the mechanism of action from immediate, intense destruction to a gradual, controlled accumulation of heat.
While short pulse widths (e.g., 3ms) rely on aggressive bursts of energy to destroy hair before heat can escape, a long pulse width allows for a gentle, sustained temperature rise. This extended duration aligns with the Thermal Damage Time (TDT) of the tissue, granting the surrounding epidermis time to dissipate heat and cool down while the follicle is destroyed. This results in significantly reduced side effects and a more uniform distribution of heat within the follicle compared to short-pulse technology.
The Core Takeaway Short pulse widths rely on confinement, striking the target fast before heat spreads. Long pulse widths rely on management, delivering energy slowly enough to spare the skin, but efficiently enough to uniformly destroy the follicle.
The Mechanics of Heat Delivery
Short Pulse: The Instantaneous Spike
Short pulse technology, often operating around 3ms, is designed based on the theory of Thermal Relaxation Time (TRT).
The goal is to deliver a high-intensity burst of energy faster than the hair follicle can cool down.
Because the energy release is nearly instantaneous, it remains highly localized within the follicle. This creates an intense spike in heat that destroys the target tissue rapidly before it can diffuse into the surrounding dermis.
Long Pulse: The Gradual Accumulation
A long pulse width, such as 400ms, operates on the principle of Thermal Damage Time (TDT) rather than just relaxation time.
Instead of a thermal "explosion," the laser provides a continuous, lower-intensity stream of energy.
This approach results in a gradual, uniform heating of the entire hair follicle structure. It avoids the violent peak temperatures associated with shorter pulses, aiming instead for a cumulative heating effect that ensures total follicle destruction.
Thermal Damage Control and Safety
Protecting the Epidermis
The most critical advantage of a long pulse width is the protection of the skin surface (epidermis).
During a 400ms pulse, the epidermis has sufficient time to dissipate heat into the air or into a cooling gel.
In contrast, short pulses deliver energy so quickly that the epidermis undergoes rapid heating alongside the hair. If the skin contains competing pigment (melanin), short pulses pose a higher risk of surface burns because the skin cannot cool down during the brief energy spike.
Uniformity of Destruction
Short pulses can sometimes damage the hair shaft without fully destroying the regenerative structures of the follicle if the heat does not penetrate evenly.
Long pulse widths allow time for thermal energy to transfer from the melanin-rich hair shaft to the entire follicle structure.
This ensures the required destruction threshold is reached throughout the follicle, minimizing the chance of regrowth while keeping the surrounding collagen and tissue safe.
Understanding the Trade-offs
The Risk of Heat Diffusion
While long pulses are safer for the surface, they technically allow more time for heat to conduct away from the target.
Therefore, the technology must be precise: it must sustain heat long enough to kill the follicle, but not so long that it causes "bulk heating" of the surrounding dermis.
Intensity vs. Comfort
Short pulses are often perceived as sharper or more painful due to the "snap" of high-peak power.
Long pulses are generally perceived as a slow build-up of warmth.
However, if the cooling mechanism fails during a long pulse, the pain can become sustained. Effective contact cooling is essential when using long pulse durations to maintain the safety margin for the epidermis.
Making the Right Choice for Your Goal
To select the appropriate pulse width strategy, consider the priority of the treatment:
- If your primary focus is Epidermal Safety: A long pulse (400ms) is superior, as it allows the skin to dissipate heat during the shot, reducing the risk of burns and side effects.
- If your primary focus is Aggressive Localization: A short pulse (3ms) is effective for locking energy strictly within the target, provided the patient's skin type can tolerate the rapid intensity.
Ultimately, a long pulse width trades the speed of thermal delivery for the safety of thermal control, ensuring the follicle is destroyed without sacrificing the integrity of the skin.
Summary Table:
| Feature | Short Pulse (e.g., 3ms) | Long Pulse (e.g., 400ms) |
|---|---|---|
| Mechanism | Instantaneous energy spike | Gradual heat accumulation |
| Principle | Thermal Relaxation Time (TRT) | Thermal Damage Time (TDT) |
| Epidermal Safety | Higher risk of surface burns | Superior; allows heat dissipation |
| Patient Comfort | Sharp "snap" sensation | Building warmth |
| Best For | Aggressive localization | Maximum safety and darker skin types |
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References
- Leila Ataie‐Fashtami, Hanieh Mohammadreza. Simulation of Heat Distribution and Thermal Damage Patterns of Diode Hair-Removal Lasers: An Applicable Method for Optimizing Treatment Parameters. DOI: 10.1089/pho.2010.2895
This article is also based on technical information from Belislaser Knowledge Base .
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