Introduction
Selecting the right optical lens is one of the most critical decisions in optical system design. Whether you are building a laser beam expander, a machine vision system, or a fiber coupling assembly, the lens type, material, focal length, and coating directly determine system performance. This guide walks you through the key factors to consider and provides practical recommendations for common applications.
Understanding Lens Types
Optical lenses come in several fundamental geometries, each optimized for different functions. Understanding these types is the first step toward making the right selection.
Plano-Convex Lenses
A plano-convex lens has one flat surface and one spherical convex surface. This is the most commonly used positive lens in optical systems because it minimizes spherical aberration when the curved surface faces the longer conjugate (the side with the greater object or image distance). For collimating a point source or focusing collimated light, orient the curved side toward the collimated beam.
- Best for: Collimation, light focusing, and simple imaging
- Typical focal length range: 5 mm to 1000 mm
- Aberration performance: Good for on-axis applications with proper orientation
PhotonEdge offers a comprehensive range of BK7 Plano-Convex Lenses with focal lengths from 6 mm to 1000 mm, all available with anti-reflection coatings.
Bi-Convex Lenses
A bi-convex (or double-convex) lens has two convex spherical surfaces. It provides a shorter focal length than a plano-convex lens of the same diameter and is symmetric, making it ideal for 1:1 imaging and relay systems. However, for non-symmetric conjugate ratios, a plano-convex lens typically delivers better aberration performance.
- Best for: 1:1 imaging, relay optics, and short-focal-length applications
- Advantage: Symmetric design reduces coma in near-unit-magnification setups
Explore our BK7 Bi-Convex Lenses for symmetric imaging applications.
Plano-Concave Lenses
A plano-concave lens has one flat and one concave spherical surface, producing a negative focal length. It diverges incident collimated light and is commonly used to expand laser beams or increase system focal length in Galilean beam expanders.
- Best for: Beam expansion, diverging light, and Galilean telescope designs
- Key consideration: Orient the curved surface toward the collimated input to minimize aberration
See our BK7 Plano-Concave Lenses for beam expansion applications.
Achromatic Lenses
An achromatic doublet consists of a positive and a negative element cemented together, made from glasses with different dispersions (e.g., crown and flint). This design brings two wavelengths to a common focus, dramatically reducing chromatic aberration compared to singlet lenses. Achromatic lenses are the workhorse of broadband imaging systems.
- Best for: Broadband imaging, microscopy, and photographic objectives
- Chromatic correction: Axial color corrected at two wavelengths; lateral color significantly reduced
Key Selection Parameters
Beyond lens type, several parameters govern lens selection. Here are the most important ones:
| Parameter | What It Affects | Typical Specification |
|---|---|---|
| Focal Length (f) | Image distance, magnification, NA | 5 mm – 1000 mm (±1%) |
| Material | Transmission range, refractive index, cost | BK7, UV Fused Silica, CaF2 |
| Diameter | Light gathering, f-number | 3 mm – 100 mm |
| Surface Quality | Scatter, image contrast | 40-20 scratch-dig (precision) to 60-40 |
| Coating | Reflection loss, transmission efficiency | BBAR, V-coat, or uncoated |
Application-Specific Recommendations
Imaging and Machine Vision
For imaging systems operating over the visible spectrum (400–700 nm), achromatic doublets are strongly preferred over singlets. They correct both spherical and chromatic aberration, delivering sharper images across the field. For monochromatic laser-based vision (e.g., structured light 3D scanning), a plano-convex singlet with a V-coat AR coating can be more cost-effective while delivering excellent on-axis performance.
Laser Collimation and Focusing
Laser applications demand lenses with low absorption and high laser damage threshold. UV fused silica plano-convex lenses are the top choice for UV and high-power lasers due to their low absorption and high LIDT. For visible and near-IR lasers, BK7 plano-convex lenses with V-coat AR coatings provide excellent performance at lower cost. Always orient the curved surface toward the collimated beam to minimize spherical aberration.
Fiber Optic Coupling
Fiber coupling requires aspheric lenses to achieve the tight focus needed for efficient coupling into single-mode fibers. Ball lenses and GRIN lenses are also used for compact coupling assemblies. For multimode fibers, a well-designed plano-convex or achromatic lens can achieve acceptable coupling efficiency. The NA of the lens must match or exceed the NA of the fiber for optimal coupling.
PhotonEdge Lens Products
PhotonEdge manufactures precision optical lenses with tight focal length tolerances and surface quality up to 20-10 scratch-dig. Our standard catalog includes:
- BK7 Plano-Convex Lenses — The most versatile positive lens, available from 6 mm to 1000 mm EFL
- BK7 Bi-Convex Lenses — Symmetric positive lenses ideal for relay optics
- BK7 Plano-Concave Lenses — Negative lenses for beam expansion
- UV Fused Silica variants for UV and high-power laser applications
- All lenses available with MgF2, BBAR, or V-coat anti-reflection coatings
Conclusion
Choosing the right optical lens requires balancing lens type, material, focal length, and coating against your application requirements and budget. For most imaging applications, achromatic doublets provide the best balance of performance and cost. For monochromatic laser applications, a properly oriented plano-convex singlet with the right AR coating is often the optimal choice. PhotonEdge offers both catalog and custom lenses to meet your exact specifications — contact our technical team for personalized guidance.