The mechanism of action relies on the principle of selective photothermolysis. The long-pulse ruby laser system emits a specific wavelength of light, typically 694nm, which is preferentially absorbed by the melanin located in the hair shaft and hair follicle. By utilizing a "long" pulse width—often designed around 2ms—the system maintains heat within the target long enough to thermally destroy the hair follicle epithelium, effectively inhibiting the mechanism responsible for hair growth.
The success of this technology depends on a precise balance: the wavelength ensures the laser targets the pigment rather than the skin, while the millisecond-scale pulse duration ensures heat destroys the follicle without diffusing extensively into the surrounding tissue.
The Core Principle: Selective Photothermolysis
The long-pulse ruby laser operates by delivering light energy that transforms into thermal energy (heat) upon contact with a specific target.
Targeting Melanin
The system uses a 694nm wavelength.
This specific wavelength is chosen for its high affinity for melanin.
Because the hair shaft and follicle contain high concentrations of melanin compared to the surrounding dermis, the laser energy is selectively absorbed by the hair structure.
Destruction of the Epithelium
Once absorbed, the light energy converts to heat.
This heat radiates outward from the hair shaft to the surrounding follicular structures.
The objective is to raise the temperature sufficiently to destroy the hair follicle epithelium, the tissue responsible for regenerating the hair, thereby preventing future growth.
The Role of Pulse Duration
While wavelength determines what is hit, the pulse duration determines how the heat acts upon the target.
The Significance of the "Long" Pulse
The term "long-pulse" distinguishes this application from lasers used for tattoo removal, which use nanosecond pulses.
For a ruby laser in hair removal, the pulse width is typically designed around 2ms (milliseconds).
This duration is critical because it must be long enough to allow heat to accumulate within the bulky structure of the hair follicle.
Matching Thermal Relaxation Time
To understand why 2ms is effective, one must understand thermal relaxation time (TRT).
TRT is the time it takes for a target tissue to cool down by 50% after being heated.
According to supplementary optical principles, the laser pulse width must be roughly equal to or slightly shorter than the TRT of the hair follicle.
If the pulse is too short, the hair shaft might vaporize without destroying the follicle.
If the pulse is too long, heat dissipates into the surrounding skin, potentially causing burns.
Understanding the Trade-offs
While the 694nm wavelength and 2ms pulse width are effective for many scenarios, there are physical limitations to consider regarding hair texture and skin safety.
Hair Texture and Pulse Width
The optimal pulse duration changes based on the thickness of the hair.
Thick, coarse hair has a larger volume and a longer thermal relaxation time; it cools slowly.
Fine, soft hair cools very rapidly.
While a standard long-pulse ruby laser is effective, operators must understand that very fine hair requires rapid energy accumulation before the heat dissipates, whereas coarse hair requires sustained energy delivery to penetrate the full follicle structure.
Heat Diffusion and Safety
The primary risk in any laser photothermolysis is collateral damage.
The goal is to confine heat to the follicle.
However, if the energy release is not perfectly matched to the follicle's cooling properties, heat can diffuse into the surrounding dermal tissue.
This underscores the importance of the specific 2ms design: it is calibrated to maximize follicular damage while minimizing the risk of pain and injury to the adjacent skin.
Making the Right Choice for Your Goal
When evaluating the parameters of a long-pulse ruby laser, consider the relationship between the target's physical properties and the laser's settings.
- If your primary focus is efficacy: Ensure the 694nm wavelength is being applied to hair with sufficient melanin density to absorb the energy required for epithelial destruction.
- If your primary focus is safety: Verify that the pulse duration (e.g., 2ms) is appropriate for the target hair texture to prevent heat from escaping the follicle and damaging the dermis.
The long-pulse ruby laser achieves hair removal by delivering a precise thermal insult to the follicle epithelium, balancing high melanin absorption with a pulse duration engineered for tissue safety.
Summary Table:
| Parameter | Specification | Function in Hair Removal |
|---|---|---|
| Wavelength | 694nm | High melanin absorption for selective targeting of the hair shaft. |
| Pulse Duration | ~2ms (Long-pulse) | Matches thermal relaxation time to damage follicles without burning skin. |
| Target | Follicle Epithelium | Thermal destruction of the tissue responsible for hair regeneration. |
| Core Principle | Selective Photothermolysis | Converts light to heat specifically within pigmented structures. |
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References
- Philippe Paquet, Gérald Pierard. Long‐pulsed ruby laser‐assisted hair removal in male‐to‐female transsexuals. DOI: 10.1046/j.1473-2130.2001.00015.x
This article is also based on technical information from Belislaser Knowledge Base .
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