Optimal laser pulse width is primarily determined by the physical diameter of the hair follicle. To achieve permanent hair removal, the pulse width (duration) is typically adjusted between 40 and 100 milliseconds for larger follicles, allowing sufficient time for thermal energy to diffuse from the melanin-rich shaft to the critical stem cells in the bulge area.
The core objective is thermal selectivity: the pulse width must be roughly equal to the hair follicle's thermal relaxation time. This ensures heat accumulates within the follicle to destroy it, while simultaneously being long enough to allow the surrounding epidermis to cool and remain undamaged.
Matching Pulse Width to Physical Dimensions
The Diameter Factor
The physical size of the hair follicle dictates its ability to retain heat. Larger diameter follicles have a smaller surface-area-to-volume ratio, meaning they dissipate heat more slowly than fine hair.
The 40-100ms Window
For these larger follicles, a pulse width set between 40 and 100 milliseconds is ideal. This specific duration aligns with the thermal diffusion models required to heat the entire structure effectively.
Targeting the Stem Cells
The laser energy is absorbed by the melanin, but the goal is to destroy the outer root sheath and the bulge area where stem cells reside. A longer pulse width maintains the temperature long enough for heat to conduct from the hair shaft outward to these vital structures.
The Principle of Thermal Relaxation Time (TRT)
Defining the Metric
Thermal Relaxation Time (TRT) is the time required for an object to lose 50% of its heat. For optimal damage, the laser pulse duration should be approximately equal to the TRT of the specific hair follicle being treated.
Accumulating Energy
If the pulse width is significantly shorter than the follicle's TRT, the peak power might be too high, causing vaporization rather than coagulation. If it matches the TRT, energy accumulates efficiently to cause irreversible thermal inactivation.
Achieving Selectivity: Protecting the Epidermis
The Safety Gap
Your adjustment must account for the difference in cooling speeds between the hair and the skin. The epidermis cools down much faster than a coarse hair follicle.
The "Goldilocks" Duration
To achieve safety and efficacy, the pulse width must satisfy two conditions simultaneously:
- Shorter than (or equal to) the TRT of the hair follicle (to kill the hair).
- Longer than the thermal relaxation time of the epidermis (to save the skin).
Synergistic Cooling
Using longer pulse widths (e.g., closer to 100ms) works in tandem with skin cooling technologies. While the laser fires, the skin has enough time to dissipate the heat into the cooling medium, while the bulky follicle retains the heat and is destroyed.
Understanding the Trade-offs
The Risk of Pulses Being Too Short
Setting a pulse width that is too short (aggressive) relative to the follicle size prevents the epidermis from diffusing heat. This dramatically increases the risk of thermal injury, burns, or pigmentation changes on the skin surface.
The Risk of Pulses Being Too Long
Conversely, if the pulse width excessively exceeds the follicle's TRT, heat will diffuse away from the follicle faster than it is applied. This results in sub-optimal heating of the stem cells, leading to temporary hair shedding rather than permanent destruction.
Making the Right Choice for Your Goal
To maximize results while minimizing risk, base your settings on the thickness of the hair and the sensitivity of the skin.
- If your primary focus is treating coarse, deep follicles: Prioritize a longer pulse width (40–100ms) to match the larger thermal relaxation time and ensure deep heating of the bulge.
- If your primary focus is epidermal safety (darker skin types): Extend the pulse width to ensure it exceeds the cooling time of the epidermis, allowing the skin to dissipate heat effectively.
The ultimate success of the treatment relies on balancing the slow heat loss of the follicle against the rapid cooling of the skin.
Summary Table:
| Follicle Size | Recommended Pulse Width | Thermal Goal | Cooling Focus |
|---|---|---|---|
| Large/Coarse | 40ms - 100ms | Heat diffusion to bulge stem cells | Epidermal protection |
| Fine/Thin | < 40ms (Shorter) | Match lower TRT | Avoid heat dissipation |
| Darker Skin | Extended (Up to 100ms) | Thermal selectivity | Maximum skin cooling |
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References
- Eli Janne Fiskerstrand, J. Stuart Nelson. Hair removal with long pulsed diode lasers: A comparison between two systems with different pulse structures. DOI: 10.1002/lsm.10175
This article is also based on technical information from Belislaser Knowledge Base .
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