To effectively treat hair with different diameters, you must adjust the laser parameters based on the specific thermal properties of the hair shaft. For fine vellus hair, the equipment requires shorter pulse widths and higher energy densities, whereas coarse terminal hair necessitates longer pulse widths to ensure uniform heating.
The Core Principle: The "Thermal Relaxation Time" (TRT)—the time it takes for a target to cool down by 50%—dictates your settings. Fine hair cools rapidly and requires a quick, intense burst of energy; coarse hair retains heat longer and requires a slower, sustained delivery of energy to cook the follicle thoroughly without burning the skin.
Treating Fine Vellus Hair
The Challenge of Rapid Cooling
Fine vellus hair has a very small diameter, meaning it has a short thermal relaxation time.
Because it holds very little volume, it dissipates heat into the surrounding tissue almost immediately.
Adjusting Pulse Width
To counteract this rapid cooling, you must use a shorter pulse width.
The laser energy must be delivered faster than the hair can cool down to ensure the thermal damage is confined to the follicle.
The Need for Higher Energy Density
Vellus hair contains significantly less melanin (the target chromophore) than terminal hair.
Therefore, you typically need higher energy densities (fluence) to compensate for the lower absorption rate and ensure adequate destruction of the follicle.
Treating Coarse Terminal Hair
The Challenge of Large Volume
Terminal hair is thick and has a much larger volume, resulting in a long thermal relaxation time.
These hairs cool down slowly and absorb energy very efficiently due to high melanin content.
Adjusting Pulse Width
For these hairs, you must switch to longer pulse widths.
A longer duration allows the thermal energy to conduct uniformly throughout the entire large follicle structure.
Ensuring Deep Penetration
If the pulse is too short, you risk super-heating only the surface of the hair shaft without destroying the root.
Longer pulses ensure the energy penetrates effectively to the bulb of the follicle, preventing incomplete treatment.
Understanding the Trade-offs
The Risk of Surface Damage
When treating coarse hair, using a pulse width that is too short is a common safety error.
Because coarse hair has so much melanin, delivering energy too quickly can cause the hair to explode or superheat instantly, leading to epidermal burns rather than deep follicular damage.
The Risk of Inefficacy
Conversely, using a long pulse width on fine hair will likely result in treatment failure.
If the energy is delivered too slowly, the fine hair will dissipate the heat into the surrounding skin before the follicle reaches the temperature required for destruction.
Making the Right Choice for Your Goal
To maximize results while maintaining safety, categorize the hair type before selecting your parameters:
- If your primary focus is Fine (Vellus) Hair: Select a short pulse width and increase the fluence (energy) to overcome low melanin absorption and rapid heat loss.
- If your primary focus is Coarse (Terminal) Hair: Select a long pulse width to allow heat to soak evenly through the thick follicle and avoid surface burns.
Match the speed of your laser to the cooling speed of the hair for the safest, most effective results.
Summary Table:
| Hair Type | Diameter & Volume | Thermal Relaxation Time (TRT) | Required Pulse Width | Recommended Fluence (Energy) |
|---|---|---|---|---|
| Vellus Hair | Fine / Small | Short (Cools rapidly) | Short Pulse Duration | Higher Fluence |
| Terminal Hair | Coarse / Large | Long (Cools slowly) | Long Pulse Duration | Moderate to High |
| Goal | Minimal Melanin | Rapid Energy Delivery | Uniform Deep Heating | Prevent Surface Burns |
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المراجع
- NULL AUTHOR_ID. Lazer Epilasyon. DOI: 10.4274/turkderm.46.s1.03
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