Pulse duration must be adjusted based on the thermal relaxation time of the specific hair type. For thick, coarse hair, you must use a longer pulse duration to allow heat to penetrate the entire follicle structure gradually. Conversely, fine hair requires a shorter pulse duration to deliver energy rapidly before the heat dissipates into the surrounding skin.
Core Takeaway: The effectiveness of laser hair removal relies on synchronizing the pulse width with the target's Thermal Relaxation Time (TRT). Thick hair cools slowly and demands extended energy delivery, while fine hair cools instantly and requires rapid, concentrated bursts to ensure follicular destruction without damaging the skin.
The Principle of Thermal Relaxation
Matching Energy Delivery to Cooling Speed
Every target, including hair follicles, has a specific Thermal Relaxation Time (TRT). This is the time it takes for an object to lose 50% of its heat.
Your goal is to keep the pulse duration shorter than or equal to the TRT of the hair follicle, but longer than the TRT of the epidermis (skin surface). This ensures the hair is destroyed while the skin has enough time to cool down.
Volume Dictates Duration
The volume of the hair shaft determines its TRT. Larger, thicker objects hold heat longer and cool down slowly.
Smaller, finer objects lose heat almost immediately. Therefore, the physical thickness of the hair is the primary variable when selecting your millisecond (ms) setting.
Strategies for Different Hair Textures
Treating Thick, Coarse Hair
Coarse hair has a large volume and a slow cooling rate. To treat it effectively, you should select a longer pulse duration (e.g., 30ms to 100ms+).
A longer pulse ensures that the heat has sufficient time to conduct from the melanin-rich hair shaft to the surrounding germinal cells and the follicle wall. If the pulse is too short, the energy may not fully penetrate the depth of a thick follicle before the pulse ends.
Treating Fine, Soft Hair
Fine hair has a very low volume and an extremely fast cooling rate. It requires a shorter pulse duration (e.g., 6ms to 20ms).
Because fine hair loses heat so quickly, you must deliver the energy in a rapid burst. This allows the heat to accumulate to a destructive threshold before it can diffuse into the surrounding tissue. A long pulse on fine hair is often ineffective because the follicle cools down as fast as it is being heated.
The Critical Intersection with Skin Type
Safety for Darker Skin (Fitzpatrick IV-VI)
While hair thickness is the primary guide for efficacy, skin type is the primary guide for safety. Darker skin contains more epidermal melanin, which competes for the laser energy.
For these patients, you must extend the pulse duration (e.g., 15ms to 34ms or higher), even if the hair is somewhat fine. This slower delivery allows the melanin in the skin (which cools faster than the hair) to dissipate heat, preventing burns and hyperpigmentation.
The Role of Contact Cooling
The pulse duration also dictates how your cooling system functions.
For short pulses (under 30ms), the heat has little time to move; safety relies on pre-cooling the skin. For long pulses (over 100ms), the active cooling of the sapphire tip during the pulse becomes the critical factor in protecting the epidermis.
Understanding the Trade-offs
The Risk of Pulses That Are Too Short
Using a very short pulse on thick hair or dark skin creates a "snap" of high-intensity heat.
While this ensures the hair gets hot, it risks overwhelming the epidermis, leading to surface burns. It allows no time for the skin to cool off through thermal diffusion.
The Risk of Pulses That Are Too Long
Using a long pulse on fine hair results in poor clinical outcomes.
The energy is delivered so slowly that the fine hair sheds the heat into the surrounding dermis before the follicle reaches the temperature required for necrosis. You are essentially warming the skin rather than destroying the root.
Making the Right Choice for Your Goal
When configuring your device, always balance the thickness of the hair against the tolerance of the skin.
- If your primary focus is treating coarse/thick hair: Select a longer pulse duration to match the slow thermal relaxation time and ensure deep follicular heating.
- If your primary focus is treating fine/light hair: Select a shorter pulse duration to rapidly trap heat within the small follicle before it dissipates.
- If your primary focus is patient safety on dark skin: Prioritize a longer pulse duration to allow the epidermis to cool, even if it slightly reduces efficacy on finer hairs.
Successful treatment depends on heating the follicle faster than it can cool, without heating the skin faster than it can recover.
Summary Table:
| Hair Type | Pulse Duration | Thermal Relaxation Time (TRT) | Goal |
|---|---|---|---|
| Thick / Coarse | Longer (30ms - 100ms+) | Slow Cooling | Deep heat conduction to follicle walls |
| Fine / Soft | Shorter (6ms - 20ms) | Fast Cooling | Rapid energy burst to reach necrosis |
| Dark Skin (IV-VI) | Extended (Safety First) | High Melanin Risk | Allow epidermis to dissipate heat |
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References
- Liliana Marza. Laser facial hair removal protocol and key consultation considerations. DOI: 10.12968/joan.2014.3.9.436
This article is also based on technical information from Belislaser Knowledge Base .
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