The long-pulse 755-nm alexandrite laser works by targeting the melanin pigment found within the hair follicle, using the principle of selective photothermolysis. The laser emits a specific wavelength of light that penetrates deep into the dermis, where it is absorbed by the hair shaft and instantly converted into thermal energy. This heat diffuses outward to destroy the hair’s germinative cells, preventing future regrowth while sparing the surrounding skin.
Core Takeaway: The efficacy of this laser relies on the precise balance between wavelength and time. The 755-nm wavelength seeks out dark pigment (melanin), while the "long-pulse" duration ensures the heat is maintained long enough to destroy the follicle but short enough to allow the surrounding skin to cool safely.
The Mechanism: Selective Photothermolysis
Selective photothermolysis is the scientific process of using light to destroy a specific target without damaging the surrounding area. For the 755-nm alexandrite laser, this process occurs in three distinct phases.
Targeting the Chromophore
The laser emits a high-energy beam at a 755-nm wavelength. This specific wavelength is highly attracted to melanin, the dark pigment that gives hair its color.
Because melanin acts as a chromophore (a molecule that absorbs light), the laser energy bypasses the skin and is selectively absorbed by the hair shaft residing in the follicle.
Conversion to Thermal Energy
Once the melanin absorbs the light energy, it is immediately converted into heat. This creates a localized zone of high temperature within the hair follicle.
Destruction of Regenerative Cells
The heat generated in the hair shaft does not stay contained; it diffuses into the surrounding follicular structures.
This thermal transfer destroys the germinative structures and the dermal papilla cells. These are the key components responsible for hair regeneration; destroying them is the only way to achieve permanent reduction.
The Critical Role of "Long-Pulse" Technology
While the wavelength determines what is targeted, the pulse duration determines safety. "Long-pulse" technology releases energy over a millisecond duration, which is essential for protecting the skin.
Matching Thermal Relaxation Time
Every tissue has a specific Thermal Relaxation Time (TRT)—the time it takes for the tissue to cool down by 50% after being heated.
To be effective, the laser pulse must be roughly equal to the TRT of the hair follicle. This ensures the follicle retains enough heat to be destroyed.
Preserving Healthy Tissue
Crucially, the surrounding skin (dermis) has a different TRT than the hair follicle.
By carefully regulating the pulse width, the laser ensures that the heat destroys the follicle but dissipates from the surrounding skin before it causes damage. This precise timing prevents burns and ensures the safety of adjacent tissues.
Understanding the Trade-offs
While the 755-nm alexandrite laser is the gold standard for many patients, its reliance on melanin creates specific limitations that you must understand.
The Contrast Requirement
Because this laser targets melanin so aggressively, it requires a high contrast between the hair and the skin. It works best when the hair is dark and the skin is light.
If the patient has darker skin, the epidermis contains more melanin, which may compete with the hair follicle for the laser's energy. This increases the risk of surface burns or hypopigmentation.
Ineffectiveness on Light Hair
The mechanism of selective photothermolysis completely relies on the presence of pigment.
Consequently, this laser is ineffective on white, grey, or very light blonde hair. Without melanin in the hair shaft to absorb the light, no heat is generated, and the follicle remains unharmed.
Making the Right Choice for Your Goal
The 755-nm alexandrite laser is a powerful tool, but it is not a universal solution. Use the following guide to determine if it aligns with your specific needs.
- If your primary focus is treating light-to-medium skin tones with dark hair: The 755-nm alexandrite laser is likely your most efficient option, offering rapid coverage and high melanin absorption.
- If your primary focus is treating darker skin types (Fitzpatrick IV-VI): You should exercise extreme caution with 755-nm; a longer wavelength laser (such as an Nd:YAG) is generally safer to bypass the epidermal melanin.
- If your primary focus is treating grey or white hair: This technology will not work for you; you should explore electrolysis or alternative non-laser methods.
Success in permanent hair reduction is defined by matching the physics of the laser to the biology of the patient.
Summary Table:
| Feature | Mechanism/Detail |
|---|---|
| Target (Chromophore) | Melanin (Dark Pigment in hair shaft) |
| Wavelength | 755-nm (Optimized for melanin absorption) |
| Core Principle | Selective Photothermolysis (Destruction without skin damage) |
| Pulse Duration | Long-pulse (Millisecond range to match Thermal Relaxation Time) |
| Primary Goal | Destroying germinative cells & dermal papilla |
| Best Candidates | Light-to-medium skin tones with dark hair |
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References
- Steven Paul Nisticò, Keyvan Nouri. Removal of unwanted hair: efficacy, tolerability, and safety of long-pulsed 755-nm alexandrite laser equipped with a sapphire handpiece. DOI: 10.1007/s10103-018-2503-z
This article is also based on technical information from Belislaser Knowledge Base .
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