A long pulse width setting of 60 to 80 milliseconds contributes to efficacy and safety by leveraging the physical size difference between hair follicles and skin pigment. This specific duration allows heat to accumulate destructively within the larger hair follicle while simultaneously granting smaller pigment particles in the skin sufficient time to cool down, effectively decoupling the thermal damage of the target from the safety of the surface skin.
The Core Mechanism
This setting utilizes "kinetic control" based on the principle of thermal equilibrium. It creates a window of time where the laser is active long enough to destroy the hair root, yet gentle enough to allow the epidermis to dissipate heat before it burns.
The Physics of Thermal Equilibrium
Understanding Thermal Relaxation Time (TRT)
The safety of this setting relies on the concept of Thermal Relaxation Time (TRT). TRT is the time required for a target structure to lose 50% of its heat.
The Size Difference Advantage
According to the principle of selective photothermolysis, larger objects (hair follicles) cool down much slower than smaller objects (epidermal melanosomes). A 60 to 80ms pulse exploits this discrepancy.
Enhancing Safety for the Epidermis
Protecting Epidermal Melanosomes
The primary risk in laser hair removal is burning the melanin found in the top layer of the skin (epidermis). Because these pigment particles are small, they release heat rapidly.
Dissipating Surface Heat
A pulse width of 60 to 80ms is "slow" enough to allow these small epidermal structures to dissipate their heat during the laser pulse. This prevents the instantaneous temperature rise that causes burns or pigmentary abnormalities, making it particularly safer for darker skin types.
Maximizing Efficacy for Hair Destruction
Accumulating Heat in the Follicle
While the skin cools down, the hair follicle behaves differently. Because the follicle is bulky, it retains heat for a longer period.
Irreversible Thermal Damage
During the 60 to 80ms window, the follicle cannot dissipate energy fast enough. This causes heat to accumulate effectively within the structure, conducting thermal energy deep into the germ centers to achieve irreversible damage and permanent hair reduction.
Understanding the Trade-offs
The Necessity of Energy Matching
Using a long pulse width requires precise calibration with energy density (fluence). If the energy is not sufficiently high to match the longer delivery time, the follicle may merely warm up rather than be destroyed. Conversely, high-energy devices without proper pulse width matching carry a risk of localized overheating.
Limitations on Fine Hair
This setting is optimized for structures with a longer thermal relaxation time. Very fine, thin hair has a shorter TRT and may cool down too quickly during a 60-80ms pulse, potentially reducing efficacy compared to coarser hair.
Making the Right Choice for Your Goal
Achieving the balance between safety and destruction requires matching the pulse width to the patient's specific physiology.
- If your primary focus is treating darker skin types: A 60 to 80ms pulse width is essential to allow epidermal cooling and prevent pigmentary damage.
- If your primary focus is deep follicular destruction: This setting ensures thermal energy conducts fully to the germ centers without overheating the surface.
Ultimately, a 60 to 80ms pulse width serves as a kinetic filter, ensuring energy destroys the hair root while passing safely through the skin.
Summary Table:
| Feature | 60 - 80ms Pulse Width Impact | Clinical Benefit |
|---|---|---|
| Epidermal Safety | Allows small skin pigment particles to dissipate heat during the pulse. | Reduced risk of burns, especially in darker skin types. |
| Follicular Efficacy | Sustains heat accumulation in large follicles due to slow cooling. | Irreversible thermal damage for permanent hair reduction. |
| Thermal Control | Uses kinetic control based on Selective Photothermolysis. | Decouples target damage from surface skin overheating. |
| Skin Type Suitability | Optimized for higher safety margins on the Fitzpatrick scale. | Versatile treatment options for a broader client base. |
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References
- Mussarrat Hussain, David J. Goldberg. A new long pulsed 940 nm diode laser used for hair removal in Asian skin types. DOI: 10.1080/1476170310000664
This article is also based on technical information from Belislaser Knowledge Base .
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