Extending the pulse width to 50 milliseconds is a critical safety mechanism that balances energy delivery with skin protection. By stretching the energy release over this specific duration, the laser delivers heat gradually rather than in a sudden burst, allowing the hair follicle to reach a destructive temperature while significantly reducing "instantaneous thermal shock" to the surrounding skin.
Core Takeaway The 50-millisecond pulse width is designed to match the thermal relaxation time of the hair follicle. This ensures that heat accumulates within the follicle long enough to destroy it permanently, yet allows the skin's surface (epidermis) enough time to dissipate excess heat, preventing burns.
The Mechanics of Gradual Energy Delivery
Preventing Instantaneous Thermal Shock
A shorter pulse delivers energy in a violent, rapid burst. This can overload the melanin in the skin's surface, leading to burns.
Extending the pulse to 50 milliseconds slows this delivery down. It creates a gradual heating process that the melanin in the epidermis can tolerate much better than a sudden spike in temperature.
Ensuring Deep Follicular Destruction
Despite the slower delivery, the total energy remains sufficient to destroy the target.
The laser keeps the hair follicle above its critical destruction temperature for the necessary duration. This ensures the protein structure of the follicle is denatured effectively without relying on dangerously high peak powers.
The Role of Thermal Relaxation Time (TRT)
Matching the Target
The effectiveness of the 50ms setting relies on the principle of Thermal Relaxation Time (TRT).
TRT is the time it takes for a target (the hair follicle) to lose 50% of its heat. To destroy the hair, the laser pulse must be roughly equal to or slightly longer than the follicle's TRT.
Locking Heat In
If the pulse is too short, the heat may not penetrate the entire follicle.
By utilizing a 50ms pulse, the system ensures heat is "locked" within the follicle long enough to cause irreversible damage. Simultaneously, this duration is long enough to allow the surrounding skin—which has a shorter TRT—to cool down via thermal conduction.
Understanding the Trade-offs
The Risk of Pulses Being Too Short
Pulse widths significantly shorter than the follicle's TRT (e.g., under 10ms for thick hair) pose a safety risk.
They concentrate energy too quickly. This can cause the epidermis to overheat before the heat has time to dissipate, leading to adverse reactions like blistering, hyperpigmentation, or burns.
The Risk of Pulses Being Too Long
Conversely, if the pulse width is extended too far beyond the necessary TRT without adjusting energy density, the treatment becomes ineffective.
Excessive duration allows heat to diffuse into the surrounding tissue rather than building up in the hair shaft. This results in sub-optimal heating of the follicle and unnecessary bulk heating of the skin, which can cause pain without achieving permanent hair removal.
Making the Right Choice for Your Goal
The 50ms setting is a versatile standard, but adjustments may be required based on specific patient physiology.
- If your primary focus is Patient Safety (Darker Skin): longer pulse widths (like 50ms or even up to 300ms) allow for safer heat dissipation in the epidermis.
- If your primary focus is Efficacy (Thick/Coarse Hair): precise matching of the pulse width to the follicle's size is required to ensure the heat penetrates fully without diffusing.
The 50ms pulse width serves as the critical bridge between aggressive energy requirements for hair destruction and the gentle thermal limits of the human skin.
Summary Table:
| Feature | 50ms Pulse Width | Short Pulse (<10ms) | Long Pulse (>300ms) |
|---|---|---|---|
| Energy Delivery | Gradual and controlled | Rapid burst/shock | Excessive diffusion |
| Skin Safety | High (prevents burns) | Low (risk of blistering) | Moderate (bulk heating) |
| Target Result | Permanent destruction | Superficial damage | Sub-optimal heating |
| Ideal For | Balanced safety/results | Very fine hair only | Darker skin types |
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References
- Neil S. Sadick, Víctor G. Prieto. The Use of a New Diode Laser for Hair Removal. DOI: 10.1097/00042728-200301000-00007
This article is also based on technical information from Belislaser Knowledge Base .
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