Long pulse width settings protect the epidermis by leveraging the difference in cooling rates between skin tissue and hair follicles. By utilizing a pulse duration that exceeds the thermal relaxation time of the epidermis (typically around 1 to 10 milliseconds), the equipment allows the skin surface sufficient time to dissipate heat into surrounding tissue before thermal damage can occur.
The core mechanism at work here is Selective Photothermolysis. By ensuring the laser pulse lasts longer than it takes for the skin to cool down, but shorter than the time the hair follicle loses its heat, energy is confined strictly to the target hair structure while the surrounding skin remains safe.
The Principle of Thermal Relaxation Time (TRT)
To understand how long pulse widths provide safety, one must first understand Thermal Relaxation Time (TRT). This is the time required for a specific object to cool down by 50% after being heated.
The Epidermis Cools Rapidly
The epidermis is a relatively thin layer with a large surface area. Consequently, it has a very short TRT, often ranging from 1 to 10 milliseconds. It absorbs heat but releases it almost immediately.
Hair Follicles Retain Heat Longer
In contrast, hair follicles are larger, denser structures with a smaller surface-area-to-volume ratio. This physical bulk means they hold onto heat much longer, with a TRT typically between 7 and 100 milliseconds depending on the hair thickness.
How Pulse Duration Manipulates Heat Flow
The "pulse width" or duration is the specific amount of time the laser beam is actually in contact with the skin.
Allowing the Epidermis to "Breathe"
When the pulse width is set longer than the TRT of the epidermis (e.g., 15ms, 40ms, or higher), the laser is delivering energy slowly enough that the skin can transfer that heat away during the laser shot itself. This prevents the instantaneous accumulation of energy that causes burns.
Trapping Heat in the Follicle
While the skin is successfully cooling off during this long pulse, the hair follicle is not. Because its TRT is much longer, it continues to accumulate thermal energy throughout the duration of the pulse. This ensures the follicle reaches the critical temperature required for destruction, even though the skin remains cool.
Special Considerations for Darker Skin Types
For patients with higher melanin content in their epidermis (darker skin), the risk of surface heat absorption is higher. In these cases, extending the pulse width significantly—sometimes up to 400 milliseconds—is essential. This ultra-long duration gives the melanin-rich epidermis ample time for heat dissipation, preventing hyperpigmentation while still effectively treating the hair.
Understanding the Trade-offs
While long pulse widths are the gold standard for safety, accurate calibration is critical to ensure treatment efficacy.
The Risk of Pulses That Are Too Short
If the pulse width is shorter than the TRT of the epidermis, the skin cannot cool down fast enough. The heat stacks up instantly, leading to potential burns, blistering, or pigment changes.
The Risk of Pulses That Are Too Long
If the pulse width is extended too far—beyond the TRT of the hair follicle itself—the hair will also begin to dissipate heat into the surrounding tissue. If the follicle cools down as fast as it is being heated, the treatment becomes ineffective because the hair never reaches the temperature needed for destruction.
Making the Right Choice for Your Goal
The "perfect" pulse width is a moving target that sits between the cooling time of the skin and the cooling time of the hair.
- If your primary focus is safety on dark skin: Prioritize longer pulse widths (e.g., 30ms to 100ms+), as this maximizes the time allowed for the melanin-rich epidermis to dissipate heat and avoid burns.
- If your primary focus is treating fine or light hair: You may need shorter pulse widths (closer to the lower end of the safety margin), because thin hairs have shorter TRTs and cool down too quickly if the pulse is excessively long.
Ultimately, safety is achieved by timing the laser to match the thermal physics of the tissue, ensuring the skin stays cool while the follicle reaches the point of thermal inactivation.
Summary Table:
| Feature | Epidermis (Skin Layer) | Hair Follicle |
|---|---|---|
| Thermal Relaxation Time (TRT) | Short (1 - 10 ms) | Long (7 - 100 ms) |
| Heat Dissipation Rate | Rapidly releases heat | Retains heat longer |
| Reaction to Long Pulse | Cools down during the pulse | Accumulates heat to destruction |
| Clinical Focus | Safety and burn prevention | Efficacy and follicle removal |
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Precision and safety are the foundations of a successful aesthetic practice. BELIS specializes in professional-grade medical aesthetic equipment exclusively for clinics and premium salons. Our advanced Diode Hair Removal systems feature precise pulse width controls to ensure maximum epidermal protection for all skin types.
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References
- Victor G. Lacombe. Laser Hair Removal. DOI: 10.1055/s-2004-822964
This article is also based on technical information from Belislaser Knowledge Base .
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