A pulse width of approximately 40 milliseconds provides a critical safety buffer by exploiting the difference in cooling rates between the skin surface and the hair follicle. This setting allows the epidermis (skin surface) enough time to dissipate heat and cool down, while simultaneously forcing the hair follicle to retain the thermal energy required for destruction.
Core Takeaway: The 40ms setting relies on the principle of Thermal Relaxation Time (TRT). It creates a specific timing window where the pulse is slow enough to spare the skin from burning, yet fast enough to ensure the hair follicle cannot cool down before it is destroyed.
The Principle of Thermal Relaxation Time (TRT)
To understand why 40 milliseconds is the critical threshold, you must understand Thermal Relaxation Time (TRT). This is the time it takes for a target tissue to release 50% of the heat it has absorbed.
The TRT of the Epidermis
The epidermis is a thin layer with a relatively large surface area. Consequently, it cools down very quickly.
Its TRT is generally estimated to be between 3 to 10 milliseconds. This means the skin can absorb heat and release it rapidly.
The TRT of the Hair Follicle
The hair follicle is a larger, denser structure. It holds onto heat much longer than the skin surface does.
The TRT for a hair follicle is significantly longer, ranging from approximately 40 to 100 milliseconds.
Why 40ms is the Strategic Sweet Spot
A pulse width of 40ms is engineered to fit precisely between the cooling times of these two tissues.
Protecting the Skin (Heat Dissipation)
Because 40ms is significantly longer than the skin's TRT (3–10ms), the laser energy is delivered slower than the skin's cooling rate.
During the pulse, the epidermis is able to transfer heat to surrounding tissues effectively. This prevents the "instantaneous energy accumulation" that leads to burns or hyperpigmentation.
Destroying the Follicle (Heat Accumulation)
While the skin is cooling down, the hair follicle is effectively trapped.
Because the 40ms pulse is equal to or shorter than the follicle's TRT (40–100ms), the follicle cannot release the heat fast enough. The energy accumulates inside the follicle until it reaches the critical temperature for destruction.
The Critical Factor for Darker Skin
This specific timing mechanism is technically essential for patients with higher melanin content in their skin (darker skin tones).
Melanin Competition
In darker skin, the epidermis contains more melanin, which acts as a competing target for the laser energy. This increases the risk of the skin absorbing too much heat.
The Safety Buffer
By extending the pulse width to 40ms (or even up to 400ms for very dark skin), you slow down the rate of heating.
This grants the melanin-rich epidermis sufficient time to relax thermally. It ensures the procedure remains a hair removal treatment rather than a skin resurfacing event.
Understanding the Trade-offs
While 40ms is a robust standard for safety, it is important to understand the boundaries of pulse width adjustments.
The Risk of Short Pulses (<10ms)
If the pulse width is shorter than the skin's TRT (e.g., 1ms or 3ms), the energy is delivered too violently for the skin to dissipate.
This causes rapid overheating of the epidermis, leading to pain, burns, and pigmentary changes, particularly in darker skin types.
The Risk of Excessively Long Pulses
If the pulse width is extended too far beyond the follicle's TRT (e.g., significantly longer than 100ms for fine hair), the follicle may begin to cool down during the pulse.
This reduces the efficacy of the treatment, as the follicle may never reach the peak temperature required for permanent damage.
Making the Right Choice for Your Goal
When configuring laser parameters, the pulse width must be adjusted based on the specific biology of the patient.
- If your primary focus is Epidermal Safety (Darker Skin): Prioritize longer pulse widths (40ms to 400ms) to ensure the pulse duration exceeds the skin's thermal relaxation time, preventing surface burns.
- If your primary focus is Efficacy on Fine Hair: Use a pulse width closer to the lower end of the follicle's TRT (10-40ms) to ensure heat accumulates rapidly enough to destroy the smaller target.
- If your primary focus is Efficacy on Thick Hair: You can safely utilize longer pulse widths (40ms+), as thicker follicles hold heat longer and require more time to absorb energy fully.
Ultimately, a 40ms pulse width is the standard for safety because it respects the skin's need to cool while ignoring the follicle's attempt to do the same.
Summary Table:
| Target Tissue | Thermal Relaxation Time (TRT) | Reaction at 40ms Pulse Width |
|---|---|---|
| Epidermis (Skin) | 3 – 10 milliseconds | Dissipates heat rapidly; prevents burns |
| Hair Follicle | 40 – 100 milliseconds | Traps thermal energy; leads to destruction |
| Clinical Result | Safety Buffer | Effective Hair Removal |
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References
- Carlos Garcı́a, Sameer Zimmo. Alexandrite Laser Hair Removal is Safe for Fitzpatrick Skin Types IV-VI. DOI: 10.1046/j.1524-4725.2000.99185.x
This article is also based on technical information from Belislaser Knowledge Base .
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