Ball Lenses

A ball lens consists of a highly transparent spherical ball, usually made of solid glass (fused silica) or another optical material with a refractive index of less than 2. Most ball lenses have small diameters — typically a few millimeters or even less than a millimeter.  The optical physics can be understood by realizing that these lenses are, equivalently, two plano-convex lenses separated by a parallel plane.

Ball lenses are often used as beam collimators for optical fibers and to improve signal coupling between emitters, detectors, and fibers. They have high coupling efficiency and are easy to align. A single ball lens is used to couple a laser beam to an optical fiber, while two ball lenses are used to couple two optical fibers.

Other applications include objective lenses for endoscopy, optical sensors, and miniature optics used in barcode scanning.

When designing a setup involving a ball lens, you need to consider parameters such as diameter, effective focal length (EFL), back focal length (BFL), and the refractive index of the ball. The effective focal length is equal to the refractive index (denoted by n) multiplied by the diameter of the ball lens (D), divided by 4(n – 1).  We can write this as EFL = [n D]/[4(n-1)]. The back focal length equals the effective focal length minus D/2.

The numerical aperture (NA) of a spherical lens can be calculated from the effective focal length and the diameter of the input source. For collimated incident light, when the refractive index outside the ball lens equals one, you may use the equation:

NA = 1/sqr[1+4(EFL/d)2]

Where sqr[] designates the square root of the expression between the brackets.

Spherical aberration is inherent in all ball lenses, but becomes problematic if light propagation isn’t restricted to a small fraction of the lens’s cross-section. The ratio of the diameter of the ball lens to the diameter of the input source is denoted as d/D. As this ratio increases, the focus spot size at the back focal length of the lens also increases.

Applications of Ball Lenses

1. Fiber Optic Coupling: Ball lenses are used to couple light between fibers or from a fiber to a detector, helping to focus and guide light efficiently, which is essential for telecommunications and data transfer.
2. Laser Collimation: They are used to collimate laser beams, ensuring that the light travels in a parallel direction, which is important in laser scanning, measurement, and alignment systems.
3. Lens Ball Photography: A ball lens creates striking inverted images, offering unique perspectives in photography. It’s used to highlight landscapes, cityscapes, and portraits by focusing and magnifying the scene inside the ball.
4. Camera Lenses: In optical systems, ball lenses improve light coupling and focus in precision applications like fiber optics, endoscopy, and miniaturized imaging, enhancing image quality and light efficiency.
5. Medical Devices: In devices like endoscopes and optical coherence tomography (OCT), ball lenses help focus and transmit light for imaging inside the body with high precision.
6. Barcode Scanning: Ball lenses are used in scanners to focus laser light, improving the accuracy and speed of reading barcodes.
7. LED Light Focusing: In lighting applications, ball lenses focus and direct light from LEDs to achieve specific illumination patterns, often in automotive and display lighting systems.

Factory Standard

• Diameter: 2～50mm
• Scratch & Dig: 20/10
• Sphericity: 0.05mm
• Surface Quality: 40-20
• Diameter Tolerance: +/-0.05mm

Contact us for manufacturing limit or custom specifications.

Selecting a Ball Lens for Fiber Optics Coupling

For laser-to-fiber coupling, the numerical aperture (NA) of the ball lens must be less than or equal to the numerical aperture of the fiber. The spherical lens can then be placed a distance equal to the back focal length in front of the fiber to transmit the coupling light from laser to fiber.

For example, if you’re working with optical fiber that has a numerical aperture of 0.22, you would need a ball lens with an NA of no more than 0.22.

If you wish to couple two fiber optics, you will want to use two identical ball lenses. These can be placed in sequence between the fibers, with each lens positioned a back focal distance from the fiber closest to it.

Materials Used for Ball Lenses

The most common materials used in ball lenses are UV-grade fused silica, sapphire, and BK7. UV-grade fused silica can be used for UV, visible, and near-infrared (NIR) applications, and is a stable substrate with a low coefficient of thermal expansion. BK7, a borosilicate crown glass, has a refractive index of about 1.51 and is often used for visible light applications. Sapphire, while more expensive, offers better optical transmission and  is suitable for both UV and IR applications.