Adjusting the pulse width is the precise control mechanism for balancing thermal damage to the hair follicle against the safety of the skin. A longer pulse width, such as 25ms, conducts thermal energy more gradually, allowing the epidermis to dissipate heat while the follicle retains it. Conversely, a shorter pulse width, such as 12.5ms, delivers energy more rapidly, creating a higher instantaneous temperature rise that is effective for finer targets but carries higher risk for darker skin tones.
The core objective of adjusting pulse width is to match the Thermal Relaxation Time (TRT) of the target hair follicle. You must select a duration long enough to irreversibly damage the follicular germ center, yet sufficiently short to prevent heat from spreading to the surrounding dermal tissue.
The Mechanics of Pulse Width Selection
The Principle of Selective Photothermolysis
Effective hair removal relies on selective photothermolysis. This principle dictates that the laser energy must be delivered over a specific duration to destroy the target without harming surrounding tissue.
The pulse width must be matched to the target's ability to retain heat. Specifically, the duration should be roughly equal to or slightly shorter than the Thermal Relaxation Time (TRT) of the hair follicle.
The Role of Longer Pulses (20ms - 25ms)
Increasing the pulse width to 25ms significantly alters how heat is managed by the skin. This setting allows thermal energy to be conducted more gradually from the melanin chromophore to the germ centers of the follicle.
The primary benefit of this longer duration is epidermal safety. Because the epidermis contains small melanin particles with a very short TRT, a longer pulse gives the skin time to cool down during the laser pulse.
This makes 20-25ms settings particularly beneficial for darker skin types. It limits the instantaneous temperature spike in the epidermis, preventing burns while still delivering lethal heat to the larger, slower-cooling hair follicle.
The Role of Shorter Pulses (12.5ms)
Shorter pulse widths lock energy into the target more aggressively. A 12.5ms pulse delivers the same amount of fluence (energy) in half the time of a 25ms pulse.
This results in a rapid accumulation of heat. This is necessary for finer hair, which has a shorter TRT and loses heat quickly.
However, shorter pulses provide less time for the epidermis to cool via thermal conduction. This increases the risk of surface damage if the patient has significant epidermal melanin.
Understanding the Trade-offs
The Risk of Excessive Duration
If the pulse width is set too long (exceeding the follicle's TRT), the heat will not remain confined to the follicle. Instead, it will dissipate into the surrounding dermis.
This reduces the treatment's efficacy because the follicle never reaches the temperature required for permanent destruction. Furthermore, it increases the risk of collateral thermal side effects in the surrounding tissue.
The Risk of Insufficient Duration
Conversely, if the pulse width is too short relative to the energy density, it causes an intense spike in temperature. While this ensures the follicle is heated, it may not allow enough time for the heat to propagate fully to the germinative cells.
More critically, ultra-short pulses on high-energy settings can lead to localized overheating. This manifests as burns, pain, or temporary mechanical impressions on the skin surface.
Making the Right Choice for Your Goal
To optimize clinical outcomes, you must adjust the pulse width based on the physical characteristics of the patient's skin and hair.
- If your primary focus is treating darker skin (Type IV-VI): Select a longer pulse width (20-25ms) to allow epidermal heat dissipation and prevent surface burns.
- If your primary focus is treating finer or lighter hair: Select a shorter pulse width (closer to 10-15ms) to trap heat within the follicle before it diffuses.
- If your primary focus is maximizing safety on high-fluence settings: Increase the pulse width to ensure the energy is delivered gradually, avoiding "snapping" or instantaneous overheating.
Ultimately, the ideal pulse width is the longest duration that still effectively destroys the follicle, ensuring maximum protection for the epidermis.
Summary Table:
| Pulse Width | Ideal Hair Type | Skin Type Suitability | Thermal Effect | Clinical Objective |
|---|---|---|---|---|
| Shorter (10-15ms) | Fine, light hair | Lighter (Type I-III) | Rapid heat accumulation | High efficacy for small targets |
| Longer (20-25ms) | Coarse, dark hair | Darker (Type IV-VI) | Gradual heat conduction | Maximize epidermal safety |
| Exceeding TRT | N/A | High risk | Heat dissipation to dermis | Reduced efficacy/Collateral damage |
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References
- Christiane Handrick, Tina S. Alster. Comparison of Long-Pulsed Diode and Long-Pulsed Alexandrite Lasers for Hair Removal. DOI: 10.1097/00042728-200107000-00002
This article is also based on technical information from Belislaser Knowledge Base .
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