A long pulse duration is essential because it aligns with the hair follicle’s natural capacity to retain heat. By releasing energy over a period of approximately 10 to 100 milliseconds, the laser matches the follicle's "Thermal Relaxation Time." This ensures the heat has enough time to conduct from the hair shaft to the surrounding stem cells, causing permanent destruction rather than temporary damage.
Effective laser hair removal relies on the principle that the heat must be applied long enough to cook the follicle, but slowly enough to spare the skin. A long pulse duration bridges this gap, enabling the "photothermal" destruction of the hair root while allowing the faster-cooling epidermis to dissipate energy safely.
The Physics of Thermal Relaxation
Matching the Target's Cooling Rate
Every biological structure has a Thermal Relaxation Time (TRT), which is the time it takes for the object to lose 50% of its heat.
The hair follicle is a large, pigmented structure with a relatively slow TRT, ranging from 10 to 100 milliseconds.
To permanently disable the follicle, the laser pulse must be roughly equal to or slightly longer than this TRT to ensure heat accumulates faster than it escapes.
Achieving Photothermal Destruction
The goal of permanent hair reduction is photothermal destruction, not just physical breakage.
A long pulse duration allows the laser to convert light into sustained heat energy.
This sustained heat creates a zone of coagulation necrosis, effectively destroying the follicle's regenerative capacity.
Targeting the Stem Cells
The critical targets for permanent results are the stem cells located in the bulge and the dermal papilla.
These structures are not always directly pigmented; they rely on heat conducting outward from the hair shaft.
A longer pulse provides the necessary time for thermal energy to diffuse from the melanin-rich shaft to these vital germinative centers.
Why Short Pulses Are Ineffective
The Limitation of Nanosecond Pulses
pulses that are too short (such as nanosecond pulses) deliver energy too rapidly.
Instead of generating a deep "burn," these pulses often create a photo-acoustic or mechanical shock.
Temporary vs. Permanent Results
Mechanical shock may snap the hair shaft or cause a temporary growth delay.
However, because the heat does not last long enough to reach the stem cells, the follicle often repairs itself, leading to regrowth.
Safety and Skin Protection
The "Safety Window"
There is a critical difference between the cooling time of the skin (epidermis) and the hair follicle.
The epidermis has a very short TRT, typically between 3 and 10 milliseconds.
By using a pulse duration longer than 10 milliseconds, you allow the skin to cool down while the follicle—which cools much slower—continues to heat up.
Protecting Darker Skin Tones
Patients with darker skin (more epidermal melanin) are at higher risk of surface burns.
A long pulse duration is vital here because it lowers the intensity of the power delivery over time.
This "slow burn" approach gives the pigmented epidermis time to dissipate heat via thermal conduction, while the heat remains locked inside the deeper, larger hair follicle.
Understanding the Trade-offs
The Risk of Overheating
While long pulses are generally safer for the skin, they must be matched precisely to the energy density (fluence).
If the pulse is too long without adequate cooling, or if the energy is too high, heat can diffuse too far into surrounding tissue.
This can lead to localized burns or mechanical impressions on the skin if not monitored correctly.
Impact on Fine Hair
Very fine hair has a shorter TRT than thick, coarse hair.
If the pulse duration is excessively long relative to a fine hair's TRT, the hair may cool down before it reaches a lethal temperature.
Therefore, while long pulses are critical for safety and thick hair, they must be adjusted downward for finer textures.
Making the Right Choice for Your Goal
When configuring laser parameters, the pulse duration must be customized based on the specific physiology of the patient.
- If your primary focus is Thick/Coarse Hair: Use a longer pulse duration (closer to 100ms) to ensure heat fully conducts through the larger diameter of the shaft to the follicle wall.
- If your primary focus is Darker Skin Types: Extend the pulse duration significantly to allow the epidermis to dissipate surface heat, preventing thermal damage to the skin.
- If your primary focus is Fine Hair: Shorten the pulse duration slightly (while staying above the skin's safety threshold) to prevent the thin hair shaft from cooling off too quickly.
True clinical efficacy comes from balancing the time required to destroy the target against the time required to protect the tissue.
Summary Table:
| Treatment Factor | Hair Follicle (Target) | Epidermis (Skin Surface) |
|---|---|---|
| TRT (Relaxation Time) | 10 to 100 milliseconds | 3 to 10 milliseconds |
| Energy Goal | Heat accumulation & destruction | Heat dissipation & protection |
| Pulse Strategy | Long pulse (match TRT) | Short pulse (cooldown gap) |
| Result of Long Pulse | Permanent coagulation necrosis | Reduced risk of thermal burns |
Elevate Your Clinic's Clinical Outcomes with BELIS Technology
To achieve permanent hair removal without compromising patient safety, practitioners require equipment that offers precise control over pulse duration and fluence. BELIS specializes in professional-grade medical aesthetic equipment designed exclusively for premium clinics and high-end salons.
Our advanced laser systems, including Diode Laser Hair Removal, CO2 Fractional, Pico, and Nd:YAG, are engineered to manage Thermal Relaxation Time effectively across all skin types. Beyond lasers, our portfolio includes:
- Body Sculpting: EMSlim, Cryolipolysis, and RF Cavitation.
- Facial & Skin Care: Hydrafacial systems and advanced skin testers.
- Specialized Systems: HIFU, Microneedle RF, and hair growth machines.
Ready to upgrade your practice with industry-leading technology? Contact us today to discover how BELIS can deliver superior value and results for your business.
References
- Henry H. Chan, Lai‐Kun Lam. An In Vivo Study Comparing the Efficacy and Complications of Diode Laser and Long-Pulsed Nd:YAG Laser in Hair Removal in Chinese Patients. DOI: 10.1097/00042728-200111000-00007
This article is also based on technical information from Belislaser Knowledge Base .
Related Products
- Diode Tri Laser Hair Removal Machine for Clinic Use
- Diode Laser SHR Trilaser Hair Removal Machine for Clinic Use
- Trilaser Diode Hair Removal Machine for Beauty Clinic Use
- Clinic Diode Laser Hair Removal Machine with SHR and Trilaser Technology
- Clinic Use IPL and SHR Hair Removal Machine with Nd Yag Laser Tattoo Removal
People Also Ask
- What are the primary safety measures for performing diode laser hair removal on dark skin types? Expert Safety Guide
- Why should clinics conduct detailed literature research before adopting new laser hair removal technologies?
- How does a diode laser facilitate hair removal? Master the Science of Selective Photothermolysis for Smooth Skin
- How does a large spot size, such as 20mm, affect laser hair removal? Master Deep Penetration and Clinical Efficiency
- What are the primary functions of an epidermal cooling system? Enhance Safety and Efficacy in Laser Hair Removal
