Super-long pulse duration technology contributes to selective photothermolysis by exploiting the difference in cooling speeds between skin and hair. By extending the laser energy release to a range typically between 5ms and 400ms, the system allows the surrounding skin to dissipate heat safely while ensuring the hair follicle retains enough energy to be destroyed.
Core Takeaway While laser wavelength determines what absorbs the light (melanin), pulse duration determines how that energy affects the tissue. Super-long pulses work by delivering energy slowly enough that the skin can cool down during the process, yet fast enough that the hair follicle—which holds heat longer—reaches the critical temperature for destruction.
The Principle of Thermal Relaxation Time (TRT)
To understand why "super-long" pulses are effective, you must first understand Thermal Relaxation Time (TRT). This is the time it takes for a target tissue to lose 50% of the heat it has absorbed.
Size Dictates Cooling Speed
The physics of thermodynamics dictates that smaller objects cool down faster than larger objects.
- The Epidermis: Contains tiny structures (melanosomes) that have a very short TRT. They gain heat quickly but also release it almost instantly.
- The Hair Follicle: A much larger structure with a significantly longer TRT. It holds onto heat for a longer period before it can dissipate.
The Mechanism of Selectivity
Selective photothermolysis relies on matching the laser's pulse width to these specific timeframes. By using a pulse duration that aligns with the hair follicle's TRT, the laser ensures the energy is confined to the follicle long enough to cause damage.
How Super-Long Pulses (5ms - 400ms) Work
Standard lasers often use shorter pulses, but super-long pulse technology introduces a critical safety margin by managing heat accumulation differently.
Maximizing Epidermal Protection
This is the primary benefit of super-long pulse durations. Because the pulse is delivered over a millisecond timeframe (up to 400ms), it exceeds the TRT of the epidermal tissue.
This means that during the laser shot itself, the tiny pigment particles in the skin are already transferring heat to the surrounding tissue. They do not accumulate enough thermal energy to cause a burn because the energy input is spread out over time.
Ensuring Follicular Destruction
While the skin is successfully dissipating heat, the hair follicle is not. Because the follicle is large and bulky, it cannot shed the heat as fast as the laser delivers it.
Even though the energy delivery is "slow" (super-long), the follicle continues to accumulate thermal energy throughout the pulse. It eventually reaches the coagulation point required to destroy the germinative centers (the parts responsible for regrowth), preventing future hair growth.
Understanding the Trade-offs
While super-long pulse technology enhances safety, it requires precise calibration to remain effective.
The Risk of Pulses Being "Too Long"
If the pulse duration is extended too far beyond the TRT of the hair follicle, the heat will begin to diffuse out of the follicle before it reaches a destructive temperature.
- Result: The treatment becomes ineffective. The hair is heated but not destroyed.
The Risk of Pulses Being "Too Short"
If the pulse is too short (closer to the nanosecond range), the energy is delivered too violently.
- Result: The epidermal melanosomes cannot dissipate the heat in time. This leads to immediate thermal damage to the skin surface (burns) rather than selective heating of the deep follicle.
Making the Right Choice for Your Goal
The effectiveness of super-long pulse technology depends heavily on the specific characteristics of the hair being treated.
- If your primary focus is Coarse/Thick Hair: You need longer pulse durations. Thick hair has a longer TRT and requires extended heating time to fully coagulate the follicle without damaging the skin.
- If your primary focus is Fine/Thin Hair: You generally need shorter pulse durations (within the millisecond range, but on the lower end). Fine hair loses heat faster; a pulse that is too long will allow the follicle to cool down before it is destroyed.
Success in laser hair removal lies in finding the "sweet spot" where the pulse is long enough to spare the skin, but short enough to cook the follicle.
Summary Table:
| Feature | Epidermis (Skin) | Hair Follicle |
|---|---|---|
| Structure Size | Tiny (Melanosomes) | Larger Structure |
| Thermal Relaxation Time (TRT) | Very Short (Rapid Cooling) | Long (Retains Heat) |
| Reaction to Super-Long Pulse | Dissipates heat during delivery | Accumulates heat to coagulation |
| Clinical Outcome | Minimized risk of burns | Permanent follicle destruction |
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References
- Martin Wölz, Jürgen Sebastian. 760nm: a new laser diode wavelength for hair removal modules. DOI: 10.1117/12.2077445
This article is also based on technical information from Belislaser Knowledge Base .
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