A long pulse duration, specifically around 30 ms, is critical because it synchronizes with the thermal relaxation time of the hair follicle. This synchronization ensures that laser energy is delivered slowly enough to heat the entire follicle unit thoroughly, yet quickly enough to destroy the target before significant heat dissipates into the surrounding tissue.
Core Takeaway To achieve long-term hair reduction rather than just temporary delay, the heat must destroy the germinative (stem) cells at the base of the follicle. A 30 ms pulse duration provides the necessary time for thermal energy to conduct from the hair shaft to these deep biological structures, effectively "cooking" the root without vaporizing the skin surface.
The Mechanics of Thermal Destruction
Matching the Thermal Relaxation Time (TRT)
Every object has a thermal relaxation time—the time it takes for an object to cool down by 50% after being heated.
Large objects cool slowly; small objects cool quickly. A hair follicle is a relatively large, pigmented structure with a TRT ranging roughly from 10 to 100 ms.
By using a pulse duration of 30 ms, you are matching the laser's energy delivery to the follicle's natural cooling capability. This ensures the follicle retains the heat long enough to reach the temperature required for decomposition.
Achieving Deep Conduction
Laser light is primarily absorbed by the melanin in the hair shaft. However, the ultimate goal is not to burn the hair shaft, but to destroy the germinative cells (stem cells) located in the bulge and dermal papilla.
These cells are not always directly pigmented. Therefore, the heat must generated in the shaft and then conduct outward to the stem cells.
A 30 ms pulse allows this conduction to occur. It sustains the heat source long enough for thermal energy to travel from the center of the hair to the critical surrounding structures responsible for regrowth.
Precision and Safety
Avoiding Mechanical Shock
Extremely short pulses (such as those in the nanosecond range) deliver energy too violently.
This creates a "mechanical shock" or explosion that snaps the hair shaft but fails to conduct heat to the root. While this might look effective instantly, it often results only in a temporary growth delay because the stem cells survive.
Protecting the Epidermis
The epidermis (the top layer of skin) also contains melanin, but in smaller, more dispersed granules. These granules have a much shorter TRT than the large hair follicle.
A long pulse duration (like 30 ms) delivers energy at a lower peak power. This allows the epidermis—often aided by surface cooling—to dissipate heat and remain safe, while the bulky hair follicle continues to absorb and retain the energy.
Understanding the Trade-offs
The Risk of Pulses Being "Too Long"
While a long pulse is critical for conduction, there is an upper limit.
If the pulse duration excessively exceeds the follicle's thermal relaxation time (e.g., significantly longer than 100 ms for medium hairs), the heat may begin to diffuse into the surrounding dermis.
This reduces the efficiency of the treatment and increases the risk of non-specific thermal damage to adjacent tissues.
The Risk of Pulses Being "Too Short"
If the pulse is too short (e.g., 3 ms) on a thick, terminal hair, the energy is confined strictly to the chromophore (melanin).
While this protects surrounding tissue (selective photothermolysis), it may fail to heat the entire follicle unit. The heat may not have time to conduct to the bulge before the laser stops, potentially leaving the regenerative machinery of the hair intact.
Making the Right Choice for Your Goal
Selecting the correct pulse duration is about balancing the destruction of the target with the preservation of the skin.
- If your primary focus is permanent reduction: Ensure the pulse width matches the estimated TRT of the target hair (typically 20–40 ms for standard hairs, up to 100 ms for very thick hairs) to ensure germinative cell destruction.
- If your primary focus is safety on darker skin: Lean toward longer pulse durations (30 ms to 100 ms), as this lowers peak power and allows the epidermis more time to cool while the follicle slowly reaches destruction temperature.
Ultimately, a 30 ms pulse is the standard for efficacy because it bridges the gap between heating the hair shaft and destroying the living root.
Summary Table:
| Parameter | Pulse Duration (30 ms) | Short Pulses (<10 ms) | Long Pulses (>100 ms) |
|---|---|---|---|
| Mechanism | Thermal conduction to stem cells | Mechanical shock/explosion | Excessive heat diffusion |
| Follicle Effect | Complete root destruction | Temporary growth delay | Inefficient heating |
| Skin Safety | High (Epidermis dissipates heat) | High (Localized heat) | Lower risk of non-specific damage |
| Primary Goal | Permanent Hair Reduction | Instant visual effect | Specialized thick hair care |
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References
- David J. Goldberg. One‐year follow‐up results of hair removal using an 810 nm diode laser. DOI: 10.1111/jocd.12757
This article is also based on technical information from Belislaser Knowledge Base .
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