The mechanism of a long-pulse ruby laser system relies on the principle of selective photothermolysis, utilizing a specific 694 nm wavelength to treat hypertrichosis. This light energy is aggressively absorbed by the melanin within the hair follicle, converting into heat that structurally destroys the follicle and associated stem cells while leaving the surrounding skin tissue unharmed.
Core Takeaway: The long-pulse ruby laser leverages a 694 nm wavelength that is highly selective for melanin. This specificity allows it to generate intense, localized heat to disable hair regeneration machinery (stem cells) without causing collateral thermal damage to the epidermis.
The Physics of Follicle Destruction
Selective Absorption at 694 nm
The fundamental driver of this technology is the emission of light at a 694 nm wavelength.
This specific section of the spectrum is highly and selectively absorbed by melanin, the dark pigment found in hair shafts. Because the light seeks out melanin, the energy is concentrated almost entirely within the hair structure rather than the surrounding tissue.
Thermal Energy Conversion
Once the laser energy is absorbed by the melanin, it undergoes an immediate conversion from light to thermal energy.
This rapid increase in temperature creates a localized "thermal effect." The heat is not merely for sensation; it is calibrated to physically dismantle the biological structure of the hair follicle.
Targeting Regeneration Mechanics
To effectively treat hypertrichosis (excessive hair growth), the system does more than burn existing hair.
The thermal energy specifically targets and destroys the surrounding critical stem cells within the follicle. By eliminating these cells, the laser inhibits the follicle's ability to regenerate, leading to long-term hair reduction.
Safety and Tissue Preservation
Non-Invasive Selectivity
Despite the destructive heat generated within the follicle, the long-pulse ruby laser is classified as a non-invasive medical aesthetic device.
The safety mechanism is inherent to the wavelength's selectivity. Because normal skin tissue contains less focused melanin compared to the hair follicle, it does not absorb the 694 nm energy to the same degree.
Sparing the Epidermis
The system is engineered to bypass the surface layers of the skin.
While the follicle is destroyed, the surrounding normal skin tissue is spared from thermal damage. This precision allows for aggressive treatment of the hair root without compromising the integrity of the patient's skin.
Understanding the Trade-offs
The Necessity of Pigment
The mechanism of action relies entirely on the presence of melanin to act as a "target" for the laser energy.
This means the effectiveness of the treatment is directly tied to the pigment density of the hair. Without sufficient melanin in the follicle to absorb the 694 nm light, the necessary thermal reaction cannot occur.
Specificity vs. Versatility
While the 694 nm wavelength is highly effective due to its high absorption coefficient for melanin, this specificity acts as a constraint.
Unlike broader spectrum systems or deeper penetrating wavelengths (such as 1064 nm), the ruby laser's intense focus on melanin means it operates within a narrower window of optimal hair-to-skin contrast to maintain its safety profile.
Making the Right Choice for Your Goal
When evaluating the utility of a long-pulse ruby laser system, consider the following parameters:
- If your primary focus is destruction of the growth center: The 694 nm wavelength offers high absorption by melanin, ensuring the thermal effect is strong enough to destroy both the follicle structure and critical stem cells.
- If your primary focus is tissue preservation: The system's selective photothermolysis mechanism ensures that thermal damage is confined to the hair follicle, preserving the surrounding normal skin tissue.
The long-pulse ruby laser represents a precision tool that turns the hair's own pigment into a vehicle for safe, targeted follicular destruction.
Summary Table:
| Feature | Mechanism & Specification |
|---|---|
| Wavelength | 694 nm (High melanin absorption) |
| Primary Principle | Selective Photothermolysis |
| Energy Conversion | Light energy converts to localized thermal energy |
| Target Structure | Hair follicle and regenerative stem cells |
| Safety Focus | Preservation of surrounding epidermal tissue |
| Key Constraint | Effectiveness depends on melanin (pigment) density |
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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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