A double-pulse mode optimizes hair removal by decoupling epidermal safety from follicular destruction. By utilizing two 60 msec pulses separated by a 40 msec delay, the system exploits the difference in cooling rates between the skin and the hair. This specific timing sequence allows the hair follicle to accumulate lethal heat levels while the surrounding skin is given a "thermal break" to dissipate energy and prevent burns.
Core Takeaway: The 40 msec delay is the critical operational variable; it allows the epidermis to cool down while the hair follicle—which holds heat longer—retains the energy from the first pulse. This enables the second pulse to push the follicle past its thermal destruction threshold without injuring the skin.
The Mechanics of Selective Photothermolysis
Matching Thermal Relaxation Time (TRT)
To destroy a target without damaging surrounding tissue, the energy delivery must align with the target's Thermal Relaxation Time (TRT).
Hair follicles typically have a long TRT, ranging from 40 to 100 msec. This means they retain heat for a significant duration before cooling down.
The Function of the First Pulse
The initial 60 msec pulse is designed to match this follicular TRT.
It penetrates the skin and is absorbed by the melanin in the hair shaft, rapidly raising the temperature of the follicle. At this stage, the follicle is heated but potentially not yet destroyed.
The Strategic Importance of the Delay
Exploiting Differential Cooling
The 40 msec delay is not idle time; it is an active safety measure.
The epidermis (the skin's outer layer) has a much shorter TRT than the bulky hair follicle. It dissipates heat rapidly.
The "Thermal Window"
During this 40 msec interval, the heat generated by the first pulse in the epidermis dissipates, effectively resetting the skin's temperature toward a safe baseline.
However, because the hair follicle's TRT is longer (up to 100 msec), it retains the majority of the heat generated by the first pulse.
Cumulative Heating via the Second Pulse
Achieving the Destruction Threshold
When the second 60 msec pulse fires, it targets a follicle that is already pre-heated.
This creates a cumulative heating effect, spiking the follicular temperature high enough to cause protein denaturation and permanent structural damage.
Maximizing Fluence with Safety
Because the skin was allowed to cool during the delay, this second pulse does not overheat the epidermis.
This allows the operator to use a higher overall energy density (Fluence) than would be safe with a single, continuous 120 msec pulse.
Understanding the Trade-offs
Treatment Speed Implications
While safer, double-pulse modes inevitably extend the total energy delivery time.
The total cycle (60ms + 40ms + 60ms) equals 160 msec per shot, which may slightly reduce the repetition rate (speed) of the treatment compared to single, shorter pulses.
Target Specificity
This specific configuration is highly optimized for standard to coarse hair.
Very fine hair has a shorter TRT (closer to 10-20 msec). In such cases, the 40 msec delay might allow fine hair to cool down too much between pulses, potentially reducing efficacy.
Making the Right Choice for Your Goal
This pulse configuration is engineered for a balance of high efficacy and high safety, particularly for targets that are difficult to treat without surface damage.
- If your primary focus is Patient Safety: This mode is ideal as the 40 msec delay actively cools the epidermis, significantly reducing the risk of burns on darker skin types.
- If your primary focus is Efficacy on Coarse Hair: The cumulative heating profile ensures deep follicular destruction for thick hair structures that resist single-pulse treatments.
Ultimately, this approach turns the hair follicle’s inability to cool down quickly into a fatal vulnerability, ensuring destruction while protecting the faster-cooling skin.
Summary Table:
| Parameter | Duration | Function & Impact |
|---|---|---|
| Pulse 1 | 60 msec | Initial heating of the hair follicle; matches follicular TRT. |
| Delay | 40 msec | Thermal break; allows epidermis to cool while follicle retains heat. |
| Pulse 2 | 60 msec | Cumulative heating; pushes follicle past destruction threshold. |
| Total Cycle | 160 msec | Optimized for coarse hair removal and maximum patient safety. |
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References
- Wolfgang Bäumler, Rolf‐Markus Szeimies. The Effect of Different Spot Sizes on the Efficacy of Hair Removal Using a Long-Pulsed Diode Laser. DOI: 10.1046/j.1524-4725.2002.01117.x
This article is also based on technical information from Belislaser Knowledge Base .
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