A concave lens acts as the primary expansion engine. It transforms a narrow, parallel (collimated) laser beam into a diverging cone of light, effectively increasing the beam's diameter. This expansion is the critical prerequisite that allows the laser to cover a wide surface area before it is mechanically shaped into a rectangle.
The concave lens does not sculpt the rectangular shape itself; rather, it provides the necessary beam width. It ensures the optical field is large enough to completely fill a rectangular aperture, allowing for spot sizes such as 24x32 mm at the treatment target.
The Mechanics of Beam Expansion
Creating Divergence
A standard laser source typically emits a narrow, collimated beam. To treat a large area, this narrow beam must be spread out.
A concave lens functions as a divergent element. When the laser passes through it, the light rays are bent outward, expanding the beam's footprint as it travels forward.
Meeting Dimensional Requirements
For applications like large-area hair removal, the target spot is often significant, such as 24x32 mm.
Without the initial expansion provided by the concave lens, the laser would remain a focused point or a small dot. The lens ensures the beam diameter exceeds the size of the target area at the specific working distance.
The Partnership with Shaping Apertures
Filling the "Cookie Cutter"
While the concave lens expands the beam, it typically expands it into a larger circle or cone. It does not inherently create straight edges or 90-degree corners.
To achieve a rectangular spot, the system employs shaping apertures downstream from the lens. The concave lens ensures the beam is wide enough to "overfill" or fully cover this aperture.
Defining the Edges
The aperture acts as a mask, blocking the outer edges of the expanded circular beam and letting only the central rectangular portion pass through.
This combination allows the system to deliver a geometrically precise spot that matches the treatment window requirements.
Understanding the Trade-offs
Intensity vs. Area
Expanding a beam comes with a cost regarding energy density (fluence).
As the concave lens spreads the photon energy over a larger surface area (divergence), the intensity of the light at any single point decreases. The system must have sufficient initial power to ensure the expanded beam remains effective for treatment.
Precision of Working Distance
Because a concave lens creates a diverging beam, the spot size continues to grow as the distance from the lens increases.
This means the working distance is fixed and critical. If the target is positioned too close or too far from the optimal focal point, the beam dimensions will not match the intended 24x32 mm specification.
Making the Right Choice for Your Goal
To optimize an optical system for large-area coverage, consider how the lens curvature interacts with your target distance.
- If your primary focus is Maximum Coverage: Select a concave lens with a shorter focal length to create a wider angle of divergence, allowing you to fill larger apertures at shorter working distances.
- If your primary focus is Power Density: Minimize the beam expansion to the smallest size necessary to just barely fill the aperture, preventing wasted photon energy on the blocked outer edges.
The concave lens is the foundational component that scales the laser's reach, making high-speed, large-area treatments physically possible.
Summary Table:
| Feature | Role of Concave Lens | Impact on Laser Spot |
|---|---|---|
| Beam Dynamics | Acts as a divergent element | Transforms narrow beams into wide cones |
| Spot Sizing | Increases beam diameter | Ensures the beam fully fills shaping apertures |
| Energy Density | Spreads photon energy | Manages fluence across larger treatment areas |
| Geometry | Foundational expansion | Enables precise 24x32 mm rectangular shaping |
| Distance | Fixed focal properties | Determines the optimal working distance for efficacy |
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References
- Marcel Isper, Matthew Boyd. Design and optimization of dual-spot size optical systems for medical lasers. DOI: 10.1109/honet.2017.8102208
This article is also based on technical information from Belislaser Knowledge Base .
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