Pulse width is the decisive factor that determines whether energy merely strikes the hair shaft or successfully destroys the follicle's growth engine. To achieve permanent hair reduction, the pulse duration must be sufficiently long—typically between 40 and 100 milliseconds—to allow lethal heat to conduct from the melanin in the hair shaft to the surrounding stem cells.
Core Takeaway Permanent hair removal requires a thermal process, not a mechanical one. The laser pulse duration must match the Thermal Relaxation Time (TRT) of the hair follicle; this ensures the heat generated in the hair shaft has enough time to travel to the reproductive structures (the bulge and dermal papilla) to irreversibly destroy them, rather than simply shocking the hair.
The Principle of Thermal Relaxation
Matching the Follicle's Cooling Rate
Every biological structure has a specific Thermal Relaxation Time (TRT)—the time it takes for the object to dissipate 50% of its heat.
For effective destruction, the laser pulse width must be roughly equal to or slightly shorter than the follicle's TRT. Since a hair follicle is a large structure, its TRT generally ranges from 40 to 100 milliseconds.
From Absorption to Conduction
The laser does not hit the stem cells directly; it targets the melanin in the hair shaft.
A longer pulse width maintains the heat within the shaft long enough for that thermal energy to conduct outward into the surrounding tissue. This conduction is necessary to reach the critical biological structures responsible for regrowth: the bulge and the dermal papilla.
The Impact of Pulse Duration
Why Nanosecond Pulses Fail
Pulses that are too short (such as those in the nanosecond range) deliver energy too quickly.
Instead of cooking the tissue, these pulses create a mechanical shockwave. While this may cause the hair to fall out or undergo a temporary growth delay, it rarely results in the permanent thermal inactivation of the follicle because the heat does not have time to spread to the germinative cells.
The Success of Millisecond Pulses
To permanently inhibit growth, the follicle must undergo thermal coagulation and necrosis.
Longer pulses (in the millisecond range) allow the temperature of the entire follicle unit to rise gradually and uniformly. This creates a "slow cook" effect that destroys the regenerative machinery without relying on explosive mechanical force.
Understanding the Trade-offs
The "Sweet Spot" for Safety
There is a critical safety window based on the difference between the skin and the hair.
The TRT of the epidermis (skin surface) is very short, roughly 3 to 10 milliseconds. The ideal pulse width must be longer than the skin's TRT but close to the follicle's TRT (40–100ms). This allows the skin to cool down and disperse heat while the follicle retains it, preventing surface burns.
Handling Large vs. Small Follicles
One size does not fit all.
Larger hair follicles have more mass and retain heat longer, sometimes requiring pulse widths of 120 to 125 milliseconds. Conversely, finer hair heats up—and cools down—faster, requiring settings on the lower end of the millisecond spectrum to be effective.
Making the Right Choice for Your Goal
To maximize clinical efficacy while maintaining safety, you must adjust the pulse width based on the biological target.
- If your primary focus is permanent removal: Ensure your pulse width is set in the millisecond range (40–100ms) to facilitate thermal conduction to the bulge and papilla.
- If you are treating large, deep terminal hairs: Consider extending the pulse width up to 120–125ms to allow sufficient heat accumulation in the larger volume of tissue.
- If you are concerned with epidermal safety: Verify that the pulse width exceeds 10 milliseconds, ensuring the skin has surpassed its thermal relaxation time and can dissipate heat safely.
Ultimately, effective laser hair removal relies on delivering energy slowly enough to cook the root, but quickly enough to spare the skin.
Summary Table:
| Pulse Width Range | Biological Effect | Clinical Outcome | Target Hair Type |
|---|---|---|---|
| < 1 Millisecond | Mechanical shockwave | Temporary hair loss; high risk of regrowth | Not recommended for permanent removal |
| 10 - 30 Milliseconds | Rapid heating | Potential for epidermal damage if too short | Thin/Fine hair |
| 40 - 100 Milliseconds | Thermal conduction | Permanent destruction of bulge & papilla | Standard terminal hair |
| 100 - 125 Milliseconds | Deep thermal accumulation | Safe & effective for large, dense follicles | Thick/Deeply rooted hair |
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References
- Richard J. Ort, Christine Dierickx. Laser hair removal. DOI: 10.1053/sder.2002.33282
This article is also based on technical information from Belislaser Knowledge Base .
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