Long pulse technology achieves selective photothermolysis by emitting laser energy over a specific millisecond duration that precisely matches the cooling characteristics of hair follicles. This temporal control ensures that the hair follicle absorbs enough energy to be destroyed, while the surrounding skin has time to dissipate the heat safely. It essentially uses time as a filter to distinguish between the target hair and the non-target skin.
The Core Insight Success in laser hair removal is not just about the power of the light, but the timing of its delivery. By extending the pulse width to the millisecond range, the system exploits the difference in "Thermal Relaxation Time" between the hair and the skin, ensuring the follicle cooks while the epidermis cools.
The Mechanics of Selective Photothermolysis
Targeting the Chromophore
The foundational principle of this process is targeting a specific "chromophore," or light-absorbing pigment. In hair removal, the target is melanin, found abundantly in the hair shaft and follicle.
Conversion of Light to Heat
When the laser light hits the melanin, it is instantly absorbed and converted into thermal energy. This creates localized high temperatures intended to destroy the germinative structures of the follicle and the dermal papilla, preventing future growth.
The Role of Long Pulse Technology
Understanding Thermal Relaxation Time (TRT)
The critical variable in long pulse technology is Thermal Relaxation Time (TRT). This is the time it takes for a target tissue to lose 50% of its heat. Large objects (like hair follicles) hold heat longer; small objects (like pigment in the skin) lose heat quickly.
The Millisecond Advantage
Long pulse lasers operate in the millisecond range. This duration is calibrated to align with the slower TRT of the hair follicle.
Selective Accumulation vs. Dissipation
Because the pulse is "long," it allows smaller structures—specifically the melanosomes in the epidermis (skin surface)—to dissipate heat during the pulse. The larger hair follicle, however, cannot cool down fast enough. It continues to accumulate thermal energy until it reaches the point of destruction.
Understanding the Trade-offs
The Risk of Incorrect Pulse Widths
Precision is mandatory. If the pulse is too short (nanoseconds), the energy is delivered too violently, potentially damaging the skin's surface pigment before it can cool.
Efficiency vs. Safety
Conversely, if the pulse is too long (exceeding the follicle's TRT), the hair follicle will dissipate the heat into the surrounding tissue. This not only makes the treatment ineffective but causes unnecessary thermal damage to the adjacent skin.
Applying This to Treatment Goals
If your primary focus is Maximum Safety on Darker Skin:
- Long pulse technology is superior because it allows the epidermal pigment to cool down during energy delivery, reducing the risk of burns.
If your primary focus is Treating Fine Hair:
- You may require shorter pulse widths within the safe range, as thinner hairs have a shorter Thermal Relaxation Time and cool down faster than thick hairs.
If your primary focus is Permanent Reduction:
- Ensure the system delivers sufficient energy density (fluence) alongside the correct pulse width to fully disable the follicle's germinative cells.
True precision in laser hair removal comes from mastering the balance between energy intensity and the specific duration required to target the hair while sparing the skin.
Summary Table:
| Feature | Hair Follicle (Target) | Epidermis (Non-Target) |
|---|---|---|
| Chromophore Content | High Melanin Concentration | Lower Melanin Concentration |
| Thermal Relaxation Time (TRT) | Long (Slow cooling) | Short (Fast cooling) |
| Reaction to Long Pulse | Heat Accumulation & Destruction | Heat Dissipation & Safety |
| Treatment Outcome | Permanent Hair Reduction | Minimal Thermal Damage |
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References
- P. W. Preston, Sean W. Lanigan. Patient satisfaction with laser hair removal. DOI: 10.1111/j.1473-2130.2004.00045.x
This article is also based on technical information from Belislaser Knowledge Base .
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