The adjustment of pulse width is the definitive control mechanism for managing the rate of thermal energy transfer into the skin. By modulating how long the laser emits energy, you directly influence the safety margin for the epidermis. Longer pulse widths allow for a gradual release of energy, granting the skin surface sufficient time to dissipate heat while maintaining the necessary thermal accumulation in the hair follicle.
Core Takeaway Pulse width management relies on the physical principle that skin cools down faster than hair follicles. By extending the pulse duration, you allow the epidermis to "shed" heat during the laser shot, ensuring that thermal destruction is confined strictly to the hair root rather than the skin surface.
The Mechanics of Thermal Relaxation
The critical function of pulse width is grounded in the concept of Thermal Relaxation Time (TRT). This is the time it takes for a target tissue to dissipate 50% of the heat it has absorbed.
Differing Cooling Rates
The epidermis and the hair follicle have vastly different thermal behaviors. The epidermis is a thin structure with a short TRT, generally between 3 to 10 milliseconds.
In contrast, hair follicles are larger and denser, possessing a significantly longer TRT, typically ranging from 40 to 100 milliseconds.
Exploiting the Thermal Gap
To protect the skin, the laser pulse width must be set significantly longer than the TRT of the epidermis.
For example, a pulse width of 15 to 40 milliseconds exceeds the skin's cooling time (3ms). This allows the epidermis to cool down while the laser is still firing, preventing thermal injury.
Concentration of Energy
Because the pulse width is still shorter than or equal to the hair follicle's TRT, the hair cannot cool down efficiently during the pulse.
This ensures the heat remains concentrated within the follicle, leading to effective thermal destruction without collateral damage to the surrounding tissue.
Gradual Energy Release and Skin Safety
Adjusting the pulse width transforms the delivery of laser energy from a sudden shock to a gradual accumulation.
Managing High-Energy Treatments
In 810nm diode laser treatments, high energy is often required to disable the follicle permanently.
By utilizing longer pulse widths—potentially reaching up to 400ms—the system spreads this high energy over a longer period. This prevents the instantaneous temperature spikes that cause surface burns.
Protecting Melanin-Rich Skin
Pulse width adjustment is arguably most critical for patients with darker skin tones (higher epidermal melanin).
Melanin in the epidermis absorbs laser energy just as hair does. A short, aggressive pulse can overheat the epidermis instantly, leading to hyperpigmentation or burns.
Longer pulse durations allow the heat generated in the epidermal melanin to dissipate into surrounding tissue via thermal conduction, keeping the surface temperature below the injury threshold.
Understanding the Trade-offs
While extending pulse width is the primary method for ensuring safety, it must be balanced carefully against efficacy.
The Risk of "Too Long"
If the pulse width is extended too far beyond the TRT of the hair follicle (e.g., significantly longer than 100ms for fine hair), the follicle may begin to dissipate heat too effectively.
This results in the follicle failing to reach the lethal temperature required for permanent removal, rendering the treatment ineffective despite being safe.
The Risk of "Too Short"
Conversly, if the pulse width is too short (e.g., closer to 3-5ms), it approaches the TRT of the epidermis.
In this scenario, the skin accumulates heat faster than it can release it, drastically increasing the risk of erythema (redness), edema (swelling), and burns.
Making the Right Choice for Your Goal
Selecting the correct pulse width is a balancing act between the patient's skin type and the characteristics of the target hair.
- If your primary focus is Patient Safety (especially Dark Skin): Prioritize longer pulse widths (e.g., 100ms to 400ms) to maximize the time the epidermis has to cool down and prevent pigmentary changes.
- If your primary focus is Efficacy (Light Skin/Coarse Hair): Utilize shorter pulse widths (e.g., 15ms to 40ms) that exceed the skin's cooling time but remain strictly within the hair's thermal retention window to maximize heat shock.
Ultimately, correct pulse width adjustment allows you to selectively destroy the hair's regenerative capacity while preserving the integrity of the surrounding collagen and epidermis.
Summary Table:
| Factor | Epidermis (Skin Surface) | Hair Follicle (Target) |
|---|---|---|
| Thermal Relaxation Time (TRT) | Short (3 – 10 ms) | Long (40 – 100 ms) |
| Energy Absorption | Minimal (with proper settings) | High (Concentrated) |
| Cooling Rate | Rapid Heat Dissipation | Slow Heat Retention |
| Recommended Pulse Width | > 15ms (to allow cooling) | Match TRT for maximum destruction |
| Risk of Incorrect Setting | Burns & Hyperpigmentation | Ineffective Hair Removal |
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Our advanced 810nm Diode Laser systems feature intelligent pulse management to ensure the perfect balance between epidermal protection and follicle destruction. Whether you are treating delicate skin with our Pico lasers or performing high-speed hair removal, BELIS provides the technology—including HIFU, Microneedle RF, and Body Sculpting solutions—to give your business a competitive edge.
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References
- Afshan Shirkavand, M R Alinaghizadeh. 94 SIMULATION OF THERMAL DAMAGE PATTERNS DUE TO 810 nm DIODE HAIR REMOVAL LASERS ACCORDING TO THE VARIOUS SKIN TYPES IN ORDER TO OPTIMIZE TREATMENT EFFICACY. DOI: 10.1016/s1572-1000(08)70096-6
This article is also based on technical information from Belislaser Knowledge Base .
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