The fundamental difference lies in the geometry of the light as it travels from the source to the tissue. Articulated arm systems deliver collimated (parallel) beams that maintain a consistent diameter. In contrast, cleaved optical fibers output divergent beams that expand outward at specific angles, such as 53 degrees, causing the energy to spread physically as it enters the skin.
Core Takeaway In scatter-limited phototherapy, the beam shape dictates safety and efficacy profiles. Divergent beams utilize geometric spreading to lower energy density at the surface, which protects the epidermis while still allowing for effective regulation of deep thermal damage.
The Mechanics of Beam Delivery
Collimated Beams: The Parallel Approach
Articulated arms are engineered to deliver light in parallel rays. This means the beam diameter remains largely constant as it travels from the device to the patient. Because the beam does not naturally spread before impact, the energy density remains concentrated upon initial entry into the tissue.
Divergent Beams: The Optical Fiber Approach
Cleaved optical fibers function differently, emitting light that spreads out in a cone shape. The divergence angle is a critical specification; for example, a 53-degree angle significantly widens the spot size as the light travels. This introduces a variable of "geometric spreading" before the light even begins to interact with tissue structures.
Interaction with Tissue Scattering
Geometric Spreading vs. Tissue Scattering
Once light enters the skin, it naturally scatters due to tissue properties (endogenous scattering). Divergent beams add a second layer to this process: physical geometric spreading. The light is expanding due to the fiber's optics at the same time it is scattering due to the tissue.
Modulating Energy Distribution
This combination of physical spreading and tissue scattering alters how energy is distributed at various depths. Unlike a collimated beam, which relies primarily on tissue scattering to diffuse energy, a divergent beam actively dilutes its own surface density through its geometry.
Clinical Implications: Depth and Safety
Protecting the Epidermis
The divergence of the beam reduces the fluence (energy per unit area) at the skin's surface. Because the energy is spread over a wider angle upon entry, the epidermis is spared from the peak intensity that a collimated beam of the same total power might inflict.
Facilitating Deep Thermal Damage
Despite the surface spreading, divergent beams are highly effective for deep treatments. By manipulating the divergence, practitioners can regulate treatment depth more effectively. This geometry allows for sufficient energy to reach deeper layers to cause thermal damage without overwhelming the surface layers.
Understanding the Trade-offs
Precision vs. Volumetric Heating
Collimated beams offer a consistent spot size regardless of minor variations in distance between the handpiece and the skin. Divergent beams, however, change spot size rapidly based on distance. This implies that distance control becomes a variable in maintaining the intended energy density with optical fibers.
Surface Intensity Requirements
While protecting the epidermis is often desired, some treatments require high surface intensity. A divergent beam naturally lowers surface intensity through spreading. Therefore, achieving immediate, high-intensity surface interaction might be less efficient with a highly divergent beam compared to a collimated source, unless power is significantly increased.
Making the Right Choice for Your Goal
To select the appropriate delivery system, you must align the beam physics with your clinical endpoint.
- If your primary focus is Epidermal Protection: Choose a divergent beam (optical fiber) system, as the geometric spreading naturally reduces surface energy density to spare the upper skin layers.
- If your primary focus is Deep Thermal Regulation: Prioritize divergent beams, as they allow for more effective modulation of the treatment zone depth compared to fixed collimated beams.
- If your primary focus is Surface Consistency: Recognize that collimated beams (articulated arms) provide a parallel delivery that is less sensitive to variations in distance from the skin.
Ultimately, the shift from collimated to divergent beams represents a shift from static energy delivery to a dynamic, depth-regulated approach that prioritizes deep tissue impact over surface intensity.
Summary Table:
| Feature | Divergent Beams (Optical Fiber) | Collimated Beams (Articulated Arm) |
|---|---|---|
| Beam Geometry | Cone-shaped/Expanding | Parallel/Constant diameter |
| Energy Density | Spreads out (lower at surface) | Concentrated/Consistent |
| Epidermal Impact | High protection (geometric spreading) | Higher risk of surface intensity |
| Treatment Depth | Dynamic/Regulated deep thermal impact | Static/Dependent on tissue scattering |
| Distance Sensitivity | High (spot size changes with distance) | Low (consistent spot size) |
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References
- Lou Reinisch. Scatter‐limited phototherapy: A model for laser treatment of skin. DOI: 10.1002/lsm.10046
This article is also based on technical information from Belislaser Knowledge Base .
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