A 900-microsecond pulse duration is specifically utilized in Ruby laser hair removal to achieve a critical balance between effective follicle destruction and epidermal safety. This specific timing is calibrated to match the thermal relaxation time of the hair follicle, allowing laser energy to heat the entire structure uniformly without causing the rapid vaporization or mechanical shockwaves associated with shorter pulses.
The 900-microsecond duration serves as a precise "thermal bridge," allowing enough time for heat to destroy the hair's germinative cells while preventing the explosive mechanical damage that occurs with shorter, more aggressive energy spikes.
The Principle of Thermal Relaxation
Matching the Cooling Rate
To destroy a hair follicle without burning the skin, the laser pulse must roughly match the target's Thermal Relaxation Time (TRT).
The TRT is the time it takes for an object to lose 50% of its heat. By using a 900-microsecond pulse, the laser adds heat at a rate that overwhelms the follicle's ability to cool down, ensuring the temperature rises sufficiently to cause damage.
Uniform Energy Accumulation
If a pulse is too short, energy is absorbed only at the surface of the hair shaft.
A 900-microsecond duration allows the energy to soak through the hair shaft and down to the germinative areas (the root responsible for regrowth). This ensures the heat is distributed evenly throughout the follicle rather than concentrated superficially.
Enhancing Safety Mechanisms
Avoiding Mechanical Damage
Extremely short pulses (in the nanosecond range) deliver energy so fast that they cause rapid vaporization or photoacoustic shockwaves.
This mechanical stress can tear the surrounding tissue. The 900-microsecond pulse is essentially a "long pulse" in the context of Ruby lasers, shifting the interaction from a mechanical explosion to a controlled thermal (heating) process.
Controlled Thermal Diffusion
While the follicle heats up, the surrounding skin (epidermis) needs to stay cool.
This pulse duration is short enough that heat remains confined largely to the follicle, but long enough to allow for controlled thermal diffusion. This prevents the heat from spiking too rapidly in the epidermis, significantly reducing the risk of surface burns.
Understanding the Trade-offs
The Risk of Pulse Mismatch
It is critical to understand that 900 microseconds is not a universal setting for all laser types, but rather the optimized window for Ruby lasers specifically.
If the pulse were significantly shorter, the risk of mechanical injury and bleeding increases. If the pulse were too long (beyond the relaxation time of the follicle relative to the skin), the heat would dissipate into the surrounding tissue, potentially causing burns and reducing the treatment's effectiveness.
Skin Type Limitations
While 900 microseconds protects the epidermis from mechanical shock, it is still a relatively short duration compared to the 30ms or 60ms pulses used in diode lasers for darker skin.
Therefore, this setting is most effective and safe on lighter skin tones (Fitzpatrick I-III) where the melanin competition in the epidermis is lower, as the thermal buffer is smaller than with longer-pulse devices.
Making the Right Choice for Your Goal
When evaluating laser parameters, understanding the intent behind the pulse duration helps optimize results.
- If your primary focus is Efficacy: Ensure the pulse duration is long enough (like 900µs) to allow heat to penetrate and destroy the deep germinative center of the follicle.
- If your primary focus is Safety: Rely on this duration to prevent the "snapping" or mechanical damage caused by rapid vaporization, ensuring the tissue is heated rather than shocked.
Ultimately, the 900-microsecond pulse is the technical sweet spot for Ruby lasers, converting raw optical power into a safe, controllable heating mechanism.
Summary Table:
| Feature | 900-Microsecond Pulse (Ruby Laser) | Impact on Treatment |
|---|---|---|
| Energy Delivery | Controlled Thermal Heating | Prevents mechanical shock and skin tearing |
| Targeting | Germinative Cell Destruction | Ensures heat reaches the root for permanent reduction |
| Safety | Thermal Relaxation Balance | Minimizes epidermal burns on Fitzpatrick I-III skin |
| Interaction | Uniform Energy Accumulation | Avoids superficial vaporization for deeper penetration |
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References
- STEPHEN MORLEY, DAVID GAULT. Hair Removal Using the Long-Pulsed Ruby Laser in Children. DOI: 10.1089/clm.2000.18.277
This article is also based on technical information from Belislaser Knowledge Base .
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