A 40ms pulse width is chosen specifically to maximize safety by extending the duration of energy release. By stretching the laser pulse over 40 milliseconds, you allow the melanin in the epidermis sufficient time to release heat into the surrounding tissue, preventing thermal damage to the skin surface while still destroying the hair follicle.
Core Takeaway
In dark Asian skin, the margin for error is slim due to high epidermal melanin content. A 40ms pulse width exploits the principle of Thermal Relaxation Time (TRT): it is long enough for the skin to cool down, yet short enough to ensure the coarser hair follicle retains heat and is destroyed.
The Physiology of Pulse Width
Managing Heat Accumulation
The primary challenge in treating dark Asian skin (Fitzpatrick types IV-V) is that the epidermis contains a high concentration of melanin.
If a laser delivers energy too quickly (a short pulse width), this epidermal melanin absorbs the heat faster than it can release it. A 40ms pulse width slows the delivery of energy, preventing a rapid spike in epidermal temperature.
preventing Adverse Reactions
By controlling the rate of heat deposition, you significantly lower the risk of side effects common in darker skin tones.
Using a shorter pulse on this skin type often triggers epidermal burns, crusting, or blistering. Perhaps more critically, the 40ms setting helps avoid long-term pigmentary shifts, such as hyperpigmentation (darkening) or hypopigmentation (lightening).
The Physics: Thermal Relaxation Time (TRT)
Differential Cooling Rates
The strategy relies on the fact that different structures cool down at different rates.
The epidermis is a thin structure with a short Thermal Relaxation Time (TRT), meaning it cools down very quickly. In contrast, hair follicles are larger and bulkier, possessing a much longer TRT.
The "Window" of Destruction
A 40ms pulse width fits precisely into the window between these two cooling times.
During the 40ms window, the skin absorbs energy but immediately dissipates it to surrounding tissues. However, the hair follicle cannot cool down that fast; it continues to accumulate thermal energy until it reaches the destruction threshold.
Understanding the Trade-offs
Safety vs. Aggression
While 40ms is safer for the epidermis, it is a less aggressive setting for fine hair.
Because the energy is delivered more slowly, fine or light hairs with shorter TRTs might cool down during the pulse, potentially reducing efficacy. This setting is optimal for coarse, terminal hairs typical in body hair removal, which hold heat longer.
The Risk of Going Too Short
It is tempting to shorten the pulse width to increase the "snap" or intensity of the treatment.
However, moving to short pulses (e.g., 3ms to 8ms) on dark skin drastically increases the risk of epidermal injury. While shorter pulses are effective for high-frequency scanning modes or lighter skin, they lack the necessary cooling window required for safe treatment of melanin-rich skin.
Making the Right Choice for Your Goal
Selecting the correct pulse width is a balancing act between protecting the skin and effectively heating the follicle.
- If your primary focus is Clinical Safety on Dark Skin: Prioritize a longer pulse width (30ms–40ms) to allow the epidermis to dissipate heat and prevent pigmentation changes.
- If your primary focus is Targeting Fine Hair: Understand that a 40ms pulse may be less effective; however, on dark skin, you must strictly adhere to longer pulse widths to avoid burns, even if it requires more sessions.
Ultimately, the 40ms setting is the vital control lever that allows clinicians to bypass the skin's natural barrier to treat the follicle beneath without causing collateral damage.
Summary Table:
| Pulse Width Feature | Specification (40ms) | Clinical Impact |
|---|---|---|
| Skin Safety | High | Prevents epidermal burns and hyperpigmentation |
| Physics Principle | Thermal Relaxation Time | Allows skin to cool while follicles accumulate heat |
| Target Hair Type | Coarse/Terminal | Highly effective for thick body hair |
| Skin Type Suitability | Fitzpatrick IV-V | Optimized for melanin-rich and dark Asian skin |
| Risk Mitigation | High | Minimizes blistering, crusting, and pigmentary shifts |
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References
- Mussarat Hussain, David J. Goldberg. Laser-Assisted Hair Removal in Asian Skin: Efficacy, Complications, and the Effect of Single Versus Multiple Treatments. DOI: 10.1046/j.1524-4725.2003.29059.x
This article is also based on technical information from Belislaser Knowledge Base .
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