The 700 to 1000 nanometer spectrum serves as the precise "optical window" for effective hair removal. This specific range is selected because it maximizes light absorption by melanin (the pigment in hair) while minimizing absorption by oxyhemoglobin (blood) and water, allowing energy to destroy the hair follicle without burning the surrounding tissue.
Core Takeaway Achieving selective photothermolysis requires a high "signal-to-noise" ratio in energy absorption. The 700-1000 nm band is the only range where melanin absorption remains high enough to generate destructive heat, while the absorption rates of competing targets—specifically blood vessels and skin moisture—drop significantly, ensuring safety and precision.
The Mechanism of Selective Absorption
Targeting the Correct Chromophore
The fundamental goal of laser hair removal is to heat the hair follicle to a point of destruction. To do this, the laser must target a specific light-absorbing molecule, known as a chromophore.
In hair removal, the primary chromophore is melanin, which is densely concentrated in the hair shaft and follicle. The 700-1000 nm wavelength band is chosen specifically because melanin absorbs light very efficiently within this range.
Minimizing Competitive Absorption
The skin contains other chromophores that "compete" for the laser's energy. The two most significant competitors are oxyhemoglobin (found in blood vessels) and water (found in all skin cells).
If the laser wavelength is too short (below 700 nm), blood may absorb too much energy, causing bruising or vascular damage. If the wavelength is too long (above 1000 nm), water begins to absorb the energy, risking bulk heating of the skin. The 700-1000 nm range sits in a "safe zone" where these competitors have weak absorption, ensuring the energy is preferentially routed to the hair.
Depth of Penetration and Efficacy
Reaching the Follicular Root
To permanently disable hair growth, the laser cannot simply burn the surface hair; it must damage the germinative cells (stem cells) located deep in the dermis, often at the follicle bulge.
Wavelengths in the 700-1000 nm range, such as the common 808 nm diode laser, possess the necessary physical properties to penetrate the epidermis and reach the deep dermis. This deep penetration allows the system to target the entire follicle structure rather than just the surface hair shaft.
Thermal Conduction to Stem Cells
Once the light energy reaches the melanin in the hair shaft, it converts into thermal energy (heat).
This heat does not stay confined to the pigment; it conducts outward to the surrounding structures. By generating intense heat in the hair shaft, the laser effectively "cooks" the adjacent follicular bulb and bulge. This leads to the denaturation and necrosis of the cells responsible for hair regeneration.
The Critical Role of Pulse Duration
Matching Thermal Relaxation Time
While wavelength determines where the energy goes, the pulse width (duration of the laser burst) determines how the heat is managed.
For the 700-1000 nm wavelength to be effective, the laser pulse is typically set to the millisecond range. This duration aligns with the thermal relaxation time (TRT) of the hair follicle—the time it takes for the target to cool down by 50%.
Protecting the Epidermis
This temporal control is a safety mechanism. The epidermis (skin surface) has a much shorter TRT than the hair follicle because it lacks the dense volume of the hair shaft.
By using a longer pulse width, the laser allows the epidermis to dissipate heat and cool down during the pulse, preventing burns. Meanwhile, the larger hair follicle retains the heat, accumulating enough energy to reach the destruction threshold.
Understanding the Trade-offs
The Risk of Epidermal Melanin
While the 700-1000 nm range is safer than visible light, it still poses risks for darker skin types. The epidermis also contains melanin, which acts as a "decoy" target.
In patients with high epidermal melanin, the skin may absorb a significant portion of the laser energy intended for the hair. This reduces efficacy and increases the risk of surface burns.
Limitations on Light Hair
The entire principle of selective photothermolysis in this range relies on the presence of melanin.
If the target hair is grey, white, or very blonde, it lacks the melanin required to absorb 700-1000 nm light. In these cases, the laser light passes through the tissue without converting to heat, rendering the treatment ineffective regardless of the power used.
Making the Right Choice for Your Goal
When evaluating laser systems or treatment protocols, understanding the relationship between wavelength and patient physiology is essential.
- If your primary focus is treatment efficacy on light skin: The 700-1000 nm range (specifically around 755-810 nm) is optimal as it offers the highest absorption peak for melanin while sparing blood vessels.
- If your primary focus is deep penetration for coarse hair: Wavelengths closer to 808-1000 nm are preferred, as they penetrate deeper into the dermis to destroy the follicular bulb.
- If your primary focus is safety: Ensure the system utilizes millisecond-scale pulse widths to allow epidermal cooling, which is critical to preventing collateral heat damage.
By strictly adhering to the 700-1000 nm window, you leverage the physics of light to maximize follicular damage while maintaining the structural integrity of the surrounding skin.
Summary Table:
| Feature | 700-1000 nm Range (Optical Window) | Impact on Hair Removal |
|---|---|---|
| Primary Target | Melanin (Hair Pigment) | High energy absorption for follicle destruction |
| Competitive Targets | Oxyhemoglobin & Water | Minimal absorption, preventing skin & vascular damage |
| Penetration Depth | Deep Dermis | Effectively reaches the follicular bulb and bulge |
| Typical Systems | Alexandrite (755nm), Diode (808-810nm) | Industry standard for safety and clinical efficacy |
| Safety Mechanism | Selective Photothermolysis | Destroys hair while sparing surrounding tissue |
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References
- Gaurang Gupta. Diode laser: Permanent hair "Reduction" Not "Removal". DOI: 10.4103/0974-7753.136762
This article is also based on technical information from Belislaser Knowledge Base .
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