Solid-state lasers like Ruby and Alexandrite are selected for hair removal because their specific wavelengths (694nm and 755nm) align precisely with the peak absorption range of melanin. This characteristic allows them to exploit the principle of selective photothermolysis, ensuring that laser energy is preferentially absorbed by the hair follicle to generate heat while sparing the surrounding skin.
Core Takeaway
The selection of Ruby and Alexandrite lasers is driven by physics: their wavelengths maximize energy absorption by melanin within the hair shaft. By converting light into thermal energy exactly where the pigment is concentrated, these lasers destroy the follicle's regenerative capacity without inflicting collateral thermal damage on the surrounding dermis.
The Physics of Selection: Selective Photothermolysis
The Targeting Mechanism
The fundamental reason these lasers are effective is their ability to distinguish the target from the background. This relies on selective photothermolysis.
Because the 694nm (Ruby) and 755nm (Alexandrite) wavelengths fall within the peak absorption spectrum of melanin, the pigment acts as a high-affinity "chromophore." This attracts the laser energy directly into the hair shaft.
Conversion to Thermal Energy
Once the laser light hits the melanin, it is instantaneously converted into heat.
This thermal energy does not just singe the hair; it travels down the shaft to the hair bulb. The goal is to thermally destroy the stem cells or replace them with non-regenerative connective tissue, preventing future growth.
Preservation of Surrounding Tissue
Because the wavelength is "tuned" to melanin, the energy passes through the surrounding water and hemoglobin in the skin with minimal absorption.
This ensures the heat is focused narrowly on the follicular target. The surrounding dermal tissue remains cooler and intact, minimizing the risk of scarring or widespread burns.
Technical Specifications and Efficiency
The Advantage of the 755nm Wavelength
The Alexandrite laser (755nm) is particularly noted for its high efficiency in specific demographics.
It is exceptionally effective for treating fine, thin, and dark hair, specifically in individuals with lighter skin tones. Its position in the spectrum allows for deep penetration (aided by large spot sizes) while maintaining the high absorption necessary to treat thinner hair structures.
The Critical Role of Pulse Duration
Wavelength determines where the energy goes, but pulse duration determines how the heat is delivered.
Effective hair removal requires a pulse duration (often 10 to 15 ms) that matches the thermal relaxation time of the hair follicle.
Controlled Heating vs. Explosive Damage
If the pulse is too short, the energy delivery is explosive, potentially damaging the skin surface.
A longer pulse allows thermal energy to act slowly and continuously. This raises the temperature of the germinal cells sufficiently to destroy them, while allowing enough time for the skin surface to cool, ensuring safety.
Understanding the Trade-offs
The Melanin Limitation
The greatest strength of Ruby and Alexandrite lasers—their high affinity for melanin—is also their primary technical limitation.
Because they target pigment so aggressively, they struggle to distinguish between melanin in the hair and melanin in the skin. This makes them risky for patients with darker skin tones (Fitzpatrick skin types IV-VI), as the epidermis may absorb the heat meant for the follicle, causing burns.
Ineffectiveness on Light Hair
These lasers are strictly pigment-dependent.
They are ineffective on white, grey, or very blonde hair because there is insufficient melanin to absorb the 694nm or 755nm wavelengths. Without the "target" pigment to convert light to heat, the photothermal process cannot occur.
Making the Right Choice for Your Goal
When evaluating laser technologies for hair removal, the choice depends heavily on the contrast between the hair and the skin.
- If your primary focus is treating light skin with dark hair: The Alexandrite (755nm) is the superior choice due to its high melanin absorption and ability to target fine hair.
- If your primary focus is patient safety and comfort: Ensure the laser system offers adjustable pulse durations (long pulse) to match the thermal relaxation time of the follicle, preventing surface damage.
The efficacy of these lasers relies entirely on the precise match between the laser wavelength and the melanin concentration in the hair follicle.
Summary Table:
| Laser Type | Wavelength | Primary Target (Chromophore) | Best For | Key Advantage |
|---|---|---|---|---|
| Ruby | 694 nm | Melanin | Light skin, dark hair | High melanin absorption |
| Alexandrite | 755 nm | Melanin | Fine & thin dark hair | Deep penetration & efficiency |
| Technology | Solid-state | Hair Follicle | Fitzpatrick I-III | Selective photothermolysis |
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References
- Detlef Russ, R. Steiner. Simulation of the thermal effect of laser irradiation to optimize laser epilation. DOI: 10.1117/12.409324
This article is also based on technical information from Belislaser Knowledge Base .
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