The pulse width parameter controls the duration of laser energy release. It dictates precisely how long the laser beam remains active on the target area during a single pulse. This setting directly influences the safety and efficacy of the treatment by establishing a critical thermal balance: the pulse must be long enough to heat the hair follicle to the point of destruction, yet short enough to prevent that heat from spreading to the surrounding skin.
Core Takeaway Laser hair removal relies on selective photothermolysis. The pulse width must be calibrated to match the thermal relaxation time of the hair follicle, ensuring the target is destroyed while the surrounding tissue remains cool and undamaged.
The Mechanics of Selective Photothermolysis
The Role of Thermal Relaxation Time
To permanently remove hair, the laser must heat the follicle faster than it can cool down. This concept is known as thermal relaxation time (TRT).
If the pulse width matches the TRT of the hair follicle (often between 15ms and 30ms for specific diode lasers), heat accumulates effectively within the target. This ensures the regenerative capacity of the hair is destroyed without affecting the surrounding dermis.
Balancing Heat Accumulation and Diffusion
The pulse width acts as a "gatekeeper" for heat diffusion.
If the pulse width is too short, the energy delivery may be too intense or fail to penetrate the entire follicle structure. Conversely, if the pulse width is too long, heat dissipates from the follicle into the surrounding skin, increasing the risk of thermal damage to the patient.
Adjusting for Individual Physiology
Calibrating for Hair Thickness
The physical structure of the hair dictates the necessary pulse width.
Thicker hair generally requires a longer pulse width. This allows time for the thermal energy to conduct from the hair shaft throughout the entire follicle structure, ensuring complete destruction.
Protecting Darker Skin Tones
Patients with darker skin tones have more melanin in their epidermis, which attracts laser energy.
For these patients, operators use longer pulse widths to deliver energy more gradually. This "slow and gentle" heating approach prevents rapid heat concentration in the shallow dermis, significantly reducing the risk of blistering or hyperpigmentation.
Understanding the Trade-offs
The Consequence of Miscalculation
Incorrect pulse width settings are a primary cause of adverse reactions.
A pulse width that is too short for a patient's skin type can cause epidermal injury, as the skin absorbs the rapid energy burst before it can dissipate. On the other hand, an excessively long pulse width may result in ineffective treatment, as the follicle cools down before reaching the destruction threshold.
Energy Density vs. Pulse Width
It is vital to distinguish between energy density (intensity) and pulse width (time).
While energy density determines the amount of power delivered, pulse width determines the rate of delivery. High energy delivered over a short pulse width is highly aggressive; the same energy delivered over a longer pulse width is gentler, offering a higher safety margin for sensitive tissues.
Making the Right Choice for Your Goal
To achieve optimal results, the pulse width must be customized to the patient's specific profile.
- If your primary focus is treating thick or coarse hair: Utilize a longer pulse width to allow heat to conduct thoroughly throughout the dense follicle structure.
- If your primary focus is safety on darker skin tones: Select a longer pulse width to slow the rate of energy delivery, protecting the epidermis from overheating.
- If your primary focus is maximizing follicle destruction: Ensure the pulse width is roughly equal to or slightly shorter than the hair's thermal relaxation time to trap heat effectively.
Ultimately, the correct pulse width creates a window of opportunity where the hair follicle is vulnerable, but the skin remains protected.
Summary Table:
| Parameter Influence | Effect on Treatment | Target Application |
|---|---|---|
| Short Pulse Width | Rapid, intense energy delivery | Fine hair and lighter skin tones |
| Long Pulse Width | Gradual heating; protects epidermis | Thick hair and darker skin tones |
| Thermal Relaxation | Heat stays in follicle, spares skin | Achieving permanent hair reduction |
| Energy Density | Determines total power delivered | Must be balanced with pulse duration |
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References
- Hiromichi Yamada. A Study of the Efficacy of Local Anesthetic Lidocaine-Propitocaine Cream for Pain Control in Long Pulse Alexandrite Laser Hair Removal. DOI: 10.2530/jslsm.jslsm-36_0001
This article is also based on technical information from Belislaser Knowledge Base .
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