The relationship between laser pulse duration and Thermal Relaxation Time (TRT) is the critical factor that determines the safety and efficacy of laser hair removal. Ideally, the pulse duration must be set within a precise "safe window": it should be longer than the TRT of the epidermis (3 to 10 milliseconds) to prevent skin burns, but shorter than or roughly equal to the TRT of the hair follicle (40 to 100 milliseconds) to ensure the follicle is destroyed.
The goal is to maximize heat retention in the follicle while minimizing heat retention in the skin. Therefore, the optimal pulse duration operates in the gap between the skin’s ability to cool down (fast) and the hair follicle’s ability to cool down (slow).
The Mechanism: Selective Photothermolysis
Defining Thermal Relaxation Time (TRT)
TRT is the time required for a specific object or tissue to lose 50% of the thermal energy it has absorbed.
Different structures have different TRTs based on their size and volume. Small targets lose heat quickly, while large targets retain heat longer.
The Window of Opportunity
To achieve selective photothermolysis—destroying the target without harming the surroundings—timing is everything.
The epidermis (top layer of skin) has a short TRT, approximately 3 to 10 milliseconds. It dissipates heat rapidly.
The hair follicle is a larger structure with a longer TRT, generally between 10 to 100 milliseconds.
The Ideal Pulse Duration
The laser pulse duration must fall between these two values.
If the pulse is longer than the epidermis's TRT (e.g., >10ms), the skin has enough time to transfer heat away, preventing burns.
Simultaneously, the pulse must be short enough (relative to the follicle) to lock the heat inside the hair shaft before it diffuses.
Adjusting for Hair Texture and Anatomy
Treating Coarse, Terminal Hairs
Coarse hairs have a large volume, meaning they possess a longer TRT. They hold onto heat for a longer period.
For these hairs, a longer pulse duration (e.g., 20ms or 30ms) is effective.
It matches the follicle's natural cooling time, allowing sufficient heat to build up to destroy the stem cells without rushing the process.
Treating Fine, Vellus Hairs
Fine hairs have a small volume and a very short TRT. They lose heat almost as fast as they absorb it.
These require shorter pulse durations (e.g., 5ms).
The energy must be delivered quickly to raise the temperature of the follicle before that heat dissipates into the surrounding tissue.
Understanding the Trade-offs
The Risk of Pulse Durations That Are Too Short
If the pulse duration is shorter than the TRT of the epidermis (<3ms), the skin absorbs energy faster than it can cool down.
This leads to thermal damage in the epidermis, resulting in burns, blistering, or pigment changes.
The Risk of Pulse Durations That Are Too Long
If the pulse duration exceeds the TRT of the hair follicle significantly, the heat dissipates into the surrounding dermis during the pulse.
The follicle never reaches the critical temperature needed for destruction.
Furthermore, this escaping heat can accumulate in the surrounding tissue, causing pain and potential damage to non-target structures.
Making the Right Choice for Your Goal
Correctly matching the pulse duration to the specific hair and skin type is essential for a successful clinical outcome.
- If your primary focus is Coarse or Dark Hair: Use a longer pulse duration (approaching 30–40ms) to match the follicle's slow heat loss and protect the epidermis.
- If your primary focus is Fine or Light Hair: Use a shorter pulse duration (closer to 5–10ms) to "beat" the hair's rapid heat loss and ensure the follicle gets hot enough to be destroyed.
- If your primary focus is Darker Skin Tones: Prioritize longer pulse widths to give the melanin in the epidermis ample time to cool down, preventing surface burns.
By operating within the calculated gap between epidermal and follicular thermal relaxation times, you ensure the energy destroys the root while sparing the surface.
Summary Table:
| Tissue Type | Thermal Relaxation Time (TRT) | Recommended Pulse Duration | Goal of Timing |
|---|---|---|---|
| Epidermis (Skin) | 3 – 10 ms | > 10 ms (usually) | Allow heat to dissipate to prevent burns |
| Coarse Hair | 40 – 100 ms | 20 – 40 ms | Retain heat in follicle for destruction |
| Fine Hair | < 20 ms | 5 – 10 ms | Deliver energy faster than heat escapes |
| Darker Skin | High Melanin Sensitivity | Longer Pulse Width | Maximize epidermal cooling time |
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References
- Whitney Hovenic, John D. DeSpain. Laser Hair Reduction and Removal. DOI: 10.1016/j.fsc.2011.04.002
This article is also based on technical information from Belislaser Knowledge Base .
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