A 15 millisecond (ms) pulse width protects the skin by significantly exceeding the thermal relaxation time of the epidermis. This specific duration exploits the difference in cooling rates between the skin surface and the hair follicle. While the epidermis needs only about 3 milliseconds to dissipate heat, the 15ms pulse delivers energy slowly enough to allow the skin to cool down during the shot, preventing burns while still effectively heating the target follicle.
Core Takeaway The safety of a 15ms pulse relies on the principle of Thermal Relaxation Time (TRT). By setting the laser pulse duration (15ms) to be five times longer than the time the epidermis takes to cool (~3ms), the system ensures the skin dissipates heat faster than it accumulates it, while the hair follicle retains the energy necessary for permanent destruction.
The Mechanics of Thermal Relaxation
Understanding Thermal Relaxation Time (TRT)
Thermal Relaxation Time is the time required for a target tissue to lose 50% of its heat. To destroy a target without damaging surrounding tissue, the laser pulse must be shorter than the target's TRT but longer than the surrounding tissue's TRT.
The Epidermal Threshold
According to your primary reference, the epidermis has a TRT of approximately 3 milliseconds. This means the skin surface sheds heat very rapidly. If a laser pulse were shorter than 3ms (e.g., 1ms), the heat would build up in the skin faster than it could escape, leading to burns or hyperpigmentation.
The 15ms Safety Buffer
A 15ms pulse width provides a significant safety margin. Because the pulse duration is roughly five times longer than the epidermal TRT, the skin has ample time to transfer heat to the surrounding air or cooling gel. The energy is delivered gradually enough that the epidermis never reaches a critical burn temperature.
achieving Selective Photothermolysis
Trapping Heat in the Follicle
While the epidermis cools rapidly, the hair follicle is a larger structure with a longer TRT. The 15ms pulse is designed to match this longer retention time.
The laser energy is absorbed by the melanin in the hair shaft. Because the follicle cannot dissipate heat as fast as the epidermis, the thermal energy is confined within the follicle structure.
The Consequence of Differential Cooling
This process creates a temperature differential. The epidermis remains cool and intact because it is shedding heat throughout the 15ms duration. Conversely, the follicle accumulates that same energy until it reaches the coagulation point required for permanent reduction.
Understanding the Trade-offs
The Risk of Pulse Widths That Are Too Short
If the pulse width is reduced below the epidermal TRT (e.g., < 3ms), the "safety valve" of heat dissipation closes. The epidermis absorbs the energy too quickly to cool down, drastically increasing the risk of thermal injury, especially in darker skin types rich in melanin.
Balancing Efficacy and Safety
While 15ms is safe for the epidermis, it is critical that this duration does not exceed the TRT of the hair follicle too greatly.
If the pulse is too long (e.g., significantly longer than the hair's ability to hold heat), the follicle may also dissipate the energy before it is destroyed, resulting in ineffective treatment. The 15ms setting is an optimization intended to balance epidermal protection with follicular destruction.
Making the Right Choice for Your Goal
To maximize clinical outcomes and safety, apply these principles to your parameter selection:
- If your primary focus is Epidermal Safety: Ensure your pulse width is always strictly longer than the epidermal TRT (3ms) to allow for heat dissipation.
- If your primary focus is Treating Darker Skin: Consider that higher melanin content may require even longer pulse widths (potentially exceeding 30ms or more) to further slow energy delivery and prevent surface heat accumulation.
- If your primary focus is Fine Hair Removal: Be aware that finer hair has a shorter TRT; a 15ms pulse is generally effective, but extremely fine hair may cool down too fast if the pulse is excessively long.
By utilizing a 15ms pulse width, you are effectively using time as a shield, allowing the skin to breathe while the follicle burns.
Summary Table:
| Feature | Epidermis (Skin) | Hair Follicle |
|---|---|---|
| Thermal Relaxation Time (TRT) | ~3 ms (Very Fast) | Significantly Longer |
| Reaction to 15ms Pulse | Dissipates heat rapidly; stays cool | Accumulates heat; destroyed |
| Safety Impact | Prevents burns & hyperpigmentation | Efficient permanent reduction |
| Role in Treatment | Acts as a safety buffer | Targeted thermal destruction |
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References
- Daniel Barolet. Low fluence–high repetition rate diode laser hair removal 12‐month evaluation: Reducing pain and risks while keeping clinical efficacy. DOI: 10.1002/lsm.22021
This article is also based on technical information from Belislaser Knowledge Base .
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