Adjustable pulse width is the primary safety mechanism that allows laser hair removal equipment to treat a wide range of skin tones effectively. It provides the necessary control to synchronize the laser’s energy delivery with the natural cooling rates of different tissues, ensuring the hair follicle is destroyed while the surrounding skin remains unharmed.
The core function of adjustable pulse width is to match the "Thermal Relaxation Time" of the specific patient. It allows the practitioner to deliver energy slowly enough to protect the epidermis from burns, yet fast enough to thermally destroy the hair follicle.
The Mechanics of Selective Photothermolysis
The Role of Thermal Relaxation Time (TRT)
To understand pulse width, you must understand Thermal Relaxation Time (TRT). This is the time it takes for a target tissue to cool down by 50% after being heated.
For safe treatment, the laser pulse duration (width) must be shorter than the hair follicle's TRT. This ensures heat is trapped within the follicle, destroying it.
However, the pulse duration must also be longer than the TRT of the epidermis (skin surface). This allows the skin to dissipate heat during the laser shot, preventing burns.
Thermokinetic Control
Adjustable pulse width gives the operator "thermokinetic control."
By manipulating the duration of the energy beam, you control how heat accumulates.
You are essentially deciding whether to deliver the energy as a quick, intense "snap" or a slower, sustained "push."
Adapting to Fitzpatrick Skin Types
Treating Darker Skin (Types IV-VI)
Darker skin contains high levels of melanin in the epidermis, which acts as a competing target for the laser energy.
If the pulse width is too short, the epidermal melanin absorbs energy too rapidly, causing temperature spikes that lead to blistering or hyperpigmentation.
Operators must use longer pulse widths (typically 30ms to 34ms, or up to 100ms) for these skin types. This slower delivery allows the epidermis to conduct heat away safely while the follicle still reaches the destruction threshold.
Treating Lighter Skin (Types I-II)
Lighter skin has very little epidermal melanin, so there is less risk of surface absorption.
Operators can safely use shorter pulse widths (typically 6ms to 20ms).
These shorter pulses are highly effective at generating rapid thermal damage within the hair follicle, which is often required for finer hair with shorter thermal relaxation times.
Understanding the Trade-offs
The Risk of Excessive Pulse Duration
While lengthening the pulse width increases safety for the skin, there is an upper limit.
If the pulse width is extended beyond the TRT of the hair follicle (which ranges from 10–100ms depending on thickness), the heat will diffuse out of the follicle before it can be destroyed.
This renders the treatment ineffective, as the follicle never reaches the temperature required for stem cell destruction.
The Consequence of Insufficient Pulse Duration
Shortening the pulse width maximizes heat confinement in the hair, increasing efficacy.
However, on skin with any significant melanin (even a tan), a pulse that is too short prevents the epidermis from cooling down.
This results in immediate thermal injury to the skin surface, regardless of how effective the hair removal might have been.
Making the Right Choice for Your Goal
Correctly adjusting pulse width is a balancing act between the biology of the hair and the safety of the skin.
- If your primary focus is Patient Safety (Darker Skin): Prioritize longer pulse widths (e.g., 30ms+) to allow adequate thermal diffusion from the epidermis, preventing pigmentation changes.
- If your primary focus is Efficacy (Fine/Light Hair): Prioritize shorter pulse widths (e.g., 10-20ms) to ensure heat accumulates rapidly enough to destroy the follicle before it dissipates.
Mastering pulse width adjustment is the defining factor in transforming a standard laser procedure into a safe, personalized medical treatment.
Summary Table:
| Skin Type (Fitzpatrick) | Pulse Width Setting | Primary Goal | Clinical Benefit |
|---|---|---|---|
| Types I-II (Fair) | Short (6ms - 20ms) | Rapid heat accumulation | High efficacy for fine/light hair |
| Types III-IV (Olive) | Medium (20ms - 40ms) | Balanced energy delivery | Optimized safety and results |
| Types V-VI (Dark) | Long (Up to 100ms) | Epidermal cooling | Prevents burns and hyperpigmentation |
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References
- Cylburn E. Soden, Henry G. Skelton. Histologic features seen in changing nevi after therapy with an 810 nm pulsed diode laser for hair removal in patients with dysplastic nevi. DOI: 10.1046/j.1365-4362.2001.01251.x
This article is also based on technical information from Belislaser Knowledge Base .
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