Definition and Basic Optical Characteristics of Biconvex Lenses
A biconvex lense, also known as a double-convex lense, is a fundamental and crucial optical element. Its structural characteristics include maximum thickness at the center and minimum thickness at the edges, with both optical surfaces exhibiting convex curvature. This symmetrical or asymmetrical biconvex structure gives it positive optical power, enabling it to converge incident light beams.
From a geometric optics perspective, the working principle of a biconvex lens is based on Snell’s law of refraction. When a ray of light parallel to the optical axis is incident on the first convex surface, due to the abrupt change in the refractive index of the medium, the ray is deflected towards the normal. After propagating inside the lens, the ray reaches the second convex surface and is refracted again, eventually converging at the focal point on the optical axis. This cumulative effect of two refractions constitutes the lens’s converging ability.
Key Optical Parameters and Design Considerations of Biconvex Lenses
In practical optical design, the performance of a biconvex lense is determined by several key parameters:
Radius of curvature (R1, R2): The radii of curvature of the two optical surfaces directly affect the optical power of the lens.
Center thickness (CT) and edge thickness (ET): Besides affecting mechanical strength, thickness parameters have a significant impact on spherical aberration correction. While excessively thin lenses reduce weight, they may introduce significant spherical aberration; conversely, excessively thick lenses increase material absorption and weight.
Effective focal length (EFL): The distance from the principal point of the lens to the focal point determines the magnification and field of view of the optical system. Short focal length lenses offer a larger field of view, but aberration correction is more difficult.
Numerical aperture (NA): The NA value directly determines the lens’s light-gathering ability and theoretical resolution limit.
Aberration Characteristics: Biconvex lenses introduce various aberrations in practical applications, primarily including:
Spherical Aberration: Caused by the different convergence points of peripheral and paraxial rays.
Coma: Asymmetric aberration arising from off-axis object point imaging.
Astigmatism: Separation of the meridional and sagittal focal points.
Field Curvature: The image plane appears curved rather than flat.
Distortion: The geometry of the image is dissimilar to that of the object.
Chromatic Aberration: The difference in focal length between different wavelengths of light.
Applications of Materials Science in Lense Manufacturing
The selection of lens materials is a crucial decision in optical design, directly determining the system’s optical performance, environmental adaptability, and cost structure. Modern optical materials science offers a diverse range of choices:
N-BK7 Optical Glass: As the most versatile optical material, N-BK7 belongs to the borosilicate glass family and possesses excellent optical homogeneity and chemical stability. Its main technical characteristics include:
Refractive index nd = 1.5168 (587.6 nm)
Abbe number vd = 64.17, belonging to low dispersion materials
Transmission range: 350 nm – 2.0 μm
Coefficient of thermal expansion: 7.1 × 10⁻⁶/K
Widely used in standard optical systems in the visible light band
UV-grade fused silica: This material exhibits excellent light transmission performance in the ultraviolet and deep ultraviolet regions. Its technical advantages include:
Extremely high optical uniformity (Δn < 5 × 10⁻⁶)
Ultra-wide transmission range: 180 nm – 2.5 μm
Extremely low coefficient of thermal expansion (0.55 × 10⁻⁶) /K)
Often used in combination with low refractive index materials to correct chromatic aberration.
Optical-grade single-crystal silicon: Holds an irreplaceable position in infrared optics:
Transmission range: 1.2-7.0μm (mid-infrared)
Refractive index approximately 3.42 (at 4μm)
High thermal conductivity (149W/m·K)
Applications and System Integration
Biconvex lenses, as fundamental optical components, play a crucial role in numerous high-tech fields:
Imaging and photography systems:
Aspherical lenses in multi-camera modules of smartphones
Telecentric lenses in industrial vision inspection systems
Miniaturized lens assemblies for endoscopes and medical imaging
Lasers and optoelectronics Systems:
Fiber-optic coupled collimating and focusing lenses
F-θ scanning lenses for laser processing heads
Interferometer components for optical coherence tomography
Scientific Instruments:
Entrance slit collimation systems for spectrometers
Excitation and collection optical paths for confocal microscopes
Correcting mirror assemblies for astronomical telescopes
Optoelectronic Communication:
Transmitting and receiving optical antennas for free-space optical communication
Optical coupling systems for fiber-to-the-home optical network units
Selection and Usage Guidelines
When selecting a biconvex lens for a specific application, consider the following factors:
Technical parameter matching:
Correspondence between operating wavelength range and material transmission window
Numerical aperture and system resolution Matching of magnification requirements:
Coordination of focal length and system size limitations
Environmental adaptability assessment:
Matching of operating temperature range and coefficient of thermal expansion
Tolerance to environmental factors such as humidity and salt spray
Stability requirements for mechanical vibration and shock
Economic considerations:
Balancing mass production costs with R&D investment
Lifecycle analysis of maintenance cycles and replacement costs
Domestic substitution and supply chain security assessment
Future development trends of biconvex lenses:
Biconvex lens technology is developing in the following directions:
Metasurface lenses: Utilizing subwavelength structures to achieve wavefront modulation, breaking through the limitations of traditional refraction laws, and realizing ultra-thin and ultra-light optical systems.
Freeform surface optics: Correcting off-axis aberrations through non-rotationally symmetric surfaces, significantly improving system performance while reducing size.
Intelligent optical systems: Integrating tunable materials or microelectromechanical systems (MEMS) to achieve real-time adjustment of focal length and magnification.
Additive manufacturing technology: Using 3D printing technologies such as two-photon polymerization to manufacture micro/nano optical structures, realizing complex surface shapes that cannot be processed by traditional methods.
Computational optics fusion: This approach compensates for optical aberrations through algorithms, reducing the demands on mechanical precision and achieving performance-cost optimization.
Conclusion
As a fundamental component in optics, the design and manufacturing level of the biconvex lens directly reflects the state of modern optical engineering. From materials science to precision manufacturing, from aberration theory to system integration, every step embodies the wisdom and skill of opticians. With the continuous emergence of new technologies, this classic optical component will undoubtedly play a crucial role in more cutting-edge fields, providing humanity with more powerful optical tools for understanding and transforming the world.