The setting of pulse width between 15-30ms is critical because it aligns with the specific physiological window known as the "thermal relaxation time" of the average hair follicle. Maintaining this specific duration ensures that thermal energy accumulates sufficiently to destroy the follicle's reproductive structures without having enough time to diffuse outward and damage the surrounding skin.
The Core Principle: Selective Photothermolysis
Effective laser hair removal relies on matching the laser's pulse duration to the cooling time of the target. The 15-30ms window is the ideal balance where the hair follicle retains enough heat to be permanently disabled, while the surrounding tissue remains cool enough to avoid injury.
The Mechanics of Thermal Relaxation Time (TRT)
To understand why 15-30ms is the standard, you must understand the concept of Thermal Relaxation Time (TRT). TRT is the time it takes for a target object (the hair follicle) to lose 50% of its heat.
Matching the Energy Delivery
For maximum efficiency, the laser pulse duration should be roughly equal to the TRT of the hair follicle.
Most coarse, terminal hair follicles have a TRT that falls precisely within this 15-30ms range.
Heat Conduction Requirements
Heat is absorbed by melanin in the hair shaft, but the goal is to destroy the stem cells in the follicle bulb.
A pulse width of 15-30ms provides enough time for that heat to conduct from the shaft outward to the entire follicle structure, causing irreversible damage.
Why 15-30ms is the "Goldilocks" Zone
Deviating from this specific timing window compromises the delicate balance required for permanent hair removal.
The Risk of Pulse Widths That Are Too Short
If the pulse width is significantly shorter than the follicle's TRT (e.g., <10ms for thick hair), the energy is delivered too aggressively.
This causes the hair shaft to vaporize instantly without allowing time for the heat to travel down to the root.
The result is "singed" hair that grows back, rather than permanent structural damage to the follicle.
The Risk of Pulse Widths That Are Too Long
According to the primary principle of photothermolysis, if the pulse width exceeds the TRT (extending well beyond 30-40ms for standard hair), the follicle cools down as it is being heated.
This allows heat to dissipate into the surrounding tissue rather than building up in the follicle.
This "heat leakage" reduces the efficacy of the treatment and increases the risk of thermal side effects like blistering or hyperpigmentation in the surrounding skin.
Understanding the Trade-offs
While 15-30ms is the ideal baseline for standard treatments, clinical reality requires adjustments based on hair texture and skin tone.
The Fine Hair Exception
Fine hair has a much smaller mass and therefore a shorter TRT.
A 30ms pulse might be too long for fine hair, allowing it to cool before damage occurs. In these cases, shorter pulses (closer to 3-10ms) create a higher instantaneous thermal impact to clear residual thin hair.
The Dark Skin Safety Adjustment
For patients with darker skin (higher epidermal melanin), the priority shifts from maximum aggression to maximum safety.
Operators may intentionally lengthen the pulse width (sometimes up to 400ms) to bypass the TRT rule slightly. This slower delivery allows the skin's surface (epidermis) to cool down effectively while the follicle slowly reaches a damage threshold, preventing surface burns.
Making the Right Choice for Your Goal
The 15-30ms setting is your baseline, but it is not a universal constant. You must adapt based on the patient's specific physiology.
- If your primary focus is standard permanent reduction: Stick to the 15-30ms range to match the thermal relaxation time of the follicle for optimal destruction.
- If your primary focus is safety on dark skin: Increase the pulse width significantly (beyond 30ms) to allow the epidermis time to cool and prevent hyperpigmentation.
- If your primary focus is clearing fine, residual hair: Shorten the pulse width (below 15ms) to shock the smaller follicles before they can dissipate heat.
Success in diode laser treatments is defined by keeping the heat inside the follicle and out of the skin; the 15-30ms setting is the most reliable way to achieve this for the majority of patients.
Summary Table:
| Pulse Width Range | Hair/Skin Type | Physiological Effect | Clinical Outcome |
|---|---|---|---|
| Short (<15ms) | Fine, thin hair | High peak energy for small TRT | Clears residual hair |
| Optimal (15-30ms) | Standard terminal hair | Matches Follicle TRT | Permanent hair reduction |
| Long (>30ms) | Darker skin tones | Slow heat delivery; epidermal cooling | Maximum safety & comfort |
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References
- Navid Bouzari, Yahya Dowlati. Hair removal using an 800‐nm Diode Laser: Comparison at different treatment intervals of 45, 60, and 90 days. DOI: 10.1111/j.1365-4632.2004.02423.x
This article is also based on technical information from Belislaser Knowledge Base .
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