A long pulse width, such as 20 milliseconds (ms), acts as a critical safety buffer by delivering laser energy more slowly than the epidermis absorbs and retains heat. Because the skin's surface dissipates heat much faster than the hair follicle does, this extended duration allows the epidermis to cool down via thermal conduction even while the laser is active. Consequently, the peak temperature of the skin remains low enough to prevent burns, while the hair follicle continues to accumulate the heat necessary for destruction.
The Core Mechanism By utilizing a pulse width that is significantly longer than the skin's cooling time—known as the Thermal Relaxation Time (TRT)—but shorter than the hair follicle's cooling time, you achieve "selective photothermolysis." This ensures that thermal damage is confined strictly to the hair root, leaving the surrounding skin unharmed.
The Mechanics of Thermal Relaxation
To understand why a 20ms pulse works, you must understand the concept of Thermal Relaxation Time (TRT). This is the time it takes for a target tissue to lose 50% of its heat.
The Cooling "Race"
The epidermis is a thin structure with a very short TRT, typically between 3 to 10 milliseconds. It sheds heat rapidly.
In contrast, hair follicles are larger, denser structures with a much longer TRT, ranging from 10 to 100 milliseconds depending on hair thickness. They hold onto heat for a longer period.
Exploiting the Difference
A 20ms pulse width exploits this difference in cooling speeds.
Because 20ms is longer than the epidermis's TRT (approx. 3ms), the skin has ample time to transfer heat away to surrounding tissues before it can reach a burn threshold.
However, 20ms is generally shorter than or equal to the hair follicle's TRT. This means the follicle absorbs energy faster than it can release it, leading to a buildup of lethal heat.
Why Long Pulse Widths Protect Darker Skin
The primary risk in laser hair removal is that epidermal melanin (pigment in the skin) will absorb the laser energy meant for the hair.
Slowing Energy Delivery
In patients with darker skin tones, the epidermis has a higher concentration of melanin. A short, aggressive pulse (e.g., 5ms) would dump high energy into that pigment instantly, causing an immediate spike in temperature and potential blistering.
Preventing Pigmentary Changes
By stretching that energy delivery out to 20ms or longer, you lower the "peak power" of the laser.
This gradual heating allows the epidermal melanin to absorb energy slowly enough that the natural cooling mechanisms of the skin can keep up, preventing the thermal damage that leads to hyperpigmentation or hypopigmentation.
Understanding the Trade-offs
While longer pulse widths increase safety, they must be balanced carefully against efficacy. Understanding the limits of this setting is vital for a technical operator.
The Risk of Going "Too Long"
If the pulse width is extended too far (e.g., beyond the TRT of the hair follicle), the hair shaft will begin to cool down during the treatment.
If the laser energy enters the hair slower than the hair can dissipate it, the follicle will never reach the critical temperature required to destroy the stem cells in the bulge.
The Risk of Fine Hair
Fine hair has a shorter TRT than coarse hair because it has less mass to hold heat.
A 20ms pulse is excellent for medium-to-coarse hair. However, for very fine hair, 20ms might be too long, allowing the hair to cool down before it is destroyed. In these cases, the pulse width must be shortened (closer to 10-15ms) to be effective, which reduces the safety margin for the skin.
Making the Right Choice for Your Goal
Selecting the correct pulse width is a balancing act between the safety of the epidermis and the destruction of the follicle.
- If your primary focus is Epidermal Safety (Dark Skin): Prioritize longer pulse widths (20ms to 100ms) to ensure the skin has sufficient time to cool down and avoid pigmentary damage.
- If your primary focus is Coarse Hair Destruction: Use a pulse width that roughly matches the follicle's size (typically 20ms to 40ms) to ensure heat conducts fully to the bulge without leaking into the skin.
- If your primary focus is Fine/Light Hair: You must reduce the pulse width (10ms to 15ms) to "catch" the hair before it cools, acknowledging that this reduces the thermal protection for the skin.
Ultimately, a 20ms pulse width serves as a highly effective baseline for safety because it respects the natural thermal limitations of the epidermis while remaining aggressive enough to destroy the majority of terminal hair follicles.
Summary Table:
| Feature | Epidermis (Skin Surface) | Hair Follicle (Target) |
|---|---|---|
| Thermal Relaxation Time (TRT) | Short (3-10ms) | Long (10-100ms) |
| Effect of 20ms Pulse | Heat dissipates faster than it builds up | Heat accumulates to reach lethal levels |
| Safety Benefit | Prevents burns and pigment damage | Focused destruction of the hair root |
| Best Suited For | Darker skin tones (Fitzpatrick IV-VI) | Medium to coarse hair types |
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References
- Carla Raquel Fontana, Vanderlei Salvador Bagnato. A 12-month follow-up of hypopigmentation after laser hair removal. DOI: 10.3109/14764172.2012.758378
This article is also based on technical information from Belislaser Knowledge Base .
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