The implementation of an 18 mm spot size in laser hair removal offers two distinct clinical advantages: enhanced optical penetration depth and significantly improved procedural efficiency. By utilizing a larger optical footprint, practitioners can drive energy deeper into the dermis to target stubborn follicles while simultaneously reducing the time required to treat extensive anatomical zones.
Core Takeaway An 18 mm spot size is not just about speed; it is an optical necessity for deep-rooted hair. By minimizing photon scattering, it ensures adequate energy density reaches the deepest follicles while reducing the total pulse count required for large body areas.
The Physics of Clinical Efficacy
overcoming Photon Scattering
The most critical technical benefit of an 18 mm spot size is its ability to overcome tissue scattering. When laser light enters the skin, it naturally scatters, which dilutes the energy density as it travels deeper.
With a small spot size, this scattering occurs quickly, preventing the beam from maintaining effective heat at depth. Conversely, an 18 mm spot size creates a larger volume of light, significantly reducing photon loss at the edges.
Targeting Deep-Seated Follicles
Because of reduced scattering, an 18 mm spot size allows the laser to maintain effective fluence (energy density) at greater depths.
This is essential for treating body areas where hair follicles are rooted deep within the dermis, such as the back or bikini area. Smaller spot sizes often fail to permanently disable these deep roots because the energy dissipates before reaching the target.
Operational Efficiency and Safety
Reducing Treatment Duration
For large surface areas like the back, lower limbs, or chest, an 18 mm spot size drastically reduces the total number of pulses required.
This increases clinical throughput, allowing practitioners to treat more patients per day. It also reduces operator fatigue and improves the patient experience by shortening the time they must remain immobile.
Minimizing Heat Accumulation Risks
Using a larger spot size improves safety by requiring fewer overlapping pulses to cover a specific area.
In smaller spot sizes, practitioners must overlap pulses frequently to ensure coverage, which risks excessive heat accumulation and potential thermal injury (burns). An 18 mm spot size covers more ground per pulse, reducing the likelihood of "hot spots" caused by human error during beam overlapping.
Understanding the Trade-offs
The Challenge of Anatomical Contours
While an 18 mm spot size is superior for flat surfaces, it presents challenges on curved or bony areas.
Attempting to use a large spot on highly contoured regions—such as the ankles, shins, or the upper lip—can lead to poor contact between the crystal and the skin.
Coupling and Efficacy
If the large 18 mm crystal does not make full, flush contact with the skin, the cooling mechanism may not protect the epidermis effectively.
Furthermore, poor contact can lead to energy loss or reflection, reducing the clinical result. Therefore, while 18 mm is a powerhouse for the body, it lacks the agility required for precision work on the face or small extremities.
Making the Right Choice for Your Goal
To maximize clinical outcomes, select your spot size based on the specific anatomical and hair architecture of the patient:
- If your primary focus is Deep/Stubborn Hair (Back/Bikini): Utilize the 18 mm spot size to minimize scattering and ensure lethal thermal damage to deep-rooted follicles.
- If your primary focus is High-Volume Speed (Legs/Torso): Leverage the 18 mm spot size to reduce pulse counts, shorten session times, and minimize the risk of overlap burns.
- If your primary focus is Precision (Face/Ankles): Switch to a smaller spot size (e.g., 10-12 mm) to ensure proper contact and safety on contoured surfaces.
The 18 mm spot size is the standard of care for deep penetration and speed, provided the treatment area is flat enough to maintain full contact.
Summary Table:
| Feature | 18 mm Spot Size | Small Spot Size (10-12 mm) |
|---|---|---|
| Penetration Depth | Deep (Minimal photon scattering) | Shallow (High scattering) |
| Primary Use Case | Large areas (Back, Legs, Chest) | Precision areas (Face, Ankles) |
| Treatment Speed | High (Fewer pulses required) | Low (More pulses required) |
| Safety Profile | Lower risk of overlap burns | Higher risk of heat accumulation |
| Anatomical Fit | Best for flat surfaces | Best for curved/bony contours |
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References
- Pedram Noormohammadpour, Zeinab Aryanian. Effect of Different Pulse Durations on the Efficacy of Long-Pulsed Alexandrite-Assisted Hair Removal; A Split-Face Comparison Study. DOI: 10.34172/jlms.2021.21
This article is also based on technical information from Belislaser Knowledge Base .
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