Thermal Relaxation Time (TRT) acts as the definitive "speed limit" for selecting the correct laser pulse duration.
Because the diameter of a hair shaft determines how quickly it loses heat, you must synchronize the laser's pulse width to that specific cooling rate. Coarse hair holds heat longer and requires extended pulse durations (e.g., 20ms) to ensure thorough heating, while fine hair cools almost instantly, demanding very short pulses (e.g., 5ms) to trap the energy before it escapes.
The Core Principle To destroy a hair follicle effectively, the laser pulse duration must be shorter than or equal to the hair's TRT. This ensures the target reaches a lethal temperature before the heat dissipates into the surrounding skin.
The Physics of Heat Dissipation
Defining Thermal Relaxation Time
TRT is defined as the time required for a target structure, such as a hair follicle, to lose 50% of the heat it has absorbed.
This metric is not a fixed number for all patients; it varies significantly based on the physical volume and diameter of the hair being treated.
The Volume-Cooling Relationship
The larger the target object, the longer it takes to cool down.
Consequently, thick terminal hairs retain heat for a relatively long period, while thin vellus hairs shed heat rapidly into the surrounding tissue.
Optimizing for Hair Texture
Strategies for Coarse Terminal Hair
Coarse hair has a large volume and a slow rate of heat dissipation.
Because these follicles cool slowly, they are best treated with longer pulse durations, typically around 20ms (or even higher depending on the device).
This extended duration maintains sufficient heat within the follicle to ensure destruction without delivering the energy so aggressively that it damages the skin.
Strategies for Fine Vellus Hair
Fine hair presents a significant challenge because it has a very short TRT.
If you apply a long pulse to fine hair, the heat will dissipate into the surrounding skin during the laser shot, preventing the follicle from reaching the temperature required for destruction.
Therefore, short pulse durations, such as 5ms, are mandatory to concentrate the energy rapidly before it can spread.
Understanding the Trade-offs
The Risk of Epidermal Damage
While matching the hair's TRT is the primary goal for efficacy, you must also consider the TRT of the epidermis (skin).
If the pulse is too short, the melanin in the skin may not have enough time to release heat, leading to burns or hyperpigmentation, particularly in darker skin types (Fitzpatrick V-VI).
Balancing Efficacy and Safety
There is often a narrow window between the TRT of the hair and the TRT of the skin.
For example, when treating darker skin, it is often necessary to extend the pulse duration (e.g., 15 to 34 ms) to protect the epidermis, even if this slightly reduces the confinement of heat within the hair follicle.
Cooling Requirements
Short pulses (under 30ms) leave little time for heat to transfer to the device's cooling tip during the shot.
This makes pre-cooling the skin essential when treating fine hair, whereas longer pulses rely more on the real-time cooling effect of the sapphire window during the actual irradiation.
Making the Right Choice for Your Goal
Correctly matching pulse duration to TRT is a balancing act between destroying the follicle and sparing the skin.
- If your primary focus is Coarse Hair: Select a longer pulse duration (approx. 20ms to 30ms) to match the hair's slow cooling rate and protect the epidermis.
- If your primary focus is Fine Hair: Select a short pulse duration (approx. 5ms to 10ms) to rapidly heat the follicle before energy dissipates.
- If your primary focus is Darker Skin: Prioritize longer pulse durations to allow epidermal cooling, even if this requires higher fluence to compensate for efficacy.
Success relies on delivering energy fast enough to cook the target, but slow enough to spare the tissue.
Summary Table:
| Hair Type | Diameter/Volume | Thermal Relaxation Time (TRT) | Recommended Pulse Duration | Primary Strategy |
|---|---|---|---|---|
| Fine (Vellus) | Small | Very Short | Short (5ms - 10ms) | Rapid energy concentration before heat dissipates |
| Coarse (Terminal) | Large | Long | Long (20ms - 30ms) | Extended heating to match slow cooling rate |
| Dark Skin (Fitz V-VI) | Variable | N/A (Focus on Skin TRT) | Extended (15ms - 34ms+) | Prioritize epidermal cooling to prevent burns |
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References
- Lucy Millar-Hume. Full-body laser hair removal: assessment and effective treatment of an 18-year-old. DOI: 10.12968/joan.2016.5.8.392
This article is also based on technical information from Belislaser Knowledge Base .
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