ball lenses (original) (raw)

Definition: lenses which have the geometric form of a sphere

Category: general optics

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Related: lensesmicrolensesfocal lengthaspheric opticsmicro-optics

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Contents

What are Ball Lenses?

A special form of a thick biconvex optical lens is a ball lens, usually having the geometric form of a ball (sphere). They are manufactured from a single material, usually an optical glass with good transparency in the wavelength region of interest. A frequently used material is fused silica.

Another variant is half-ball lenses, which are obtained simply by cutting ball lenses in half.

Ball lenses are usually made with relatively small diameters of a few millimeters or sometimes even less than 1 mm (microlenses). Particularly for such small dimensions, they are easier to fabricate than lenses with traditional designs.

Figure 1: Focusing of light with a ball lens. While the paraxial rays have a focus position as indicated with the gray vertical line, the outer rays are more strongly refracted.

Figure 2: Focusing of divergent light with a ball lens.

Ball lenses exhibit substantial spherical aberrations when light propagation is not restricted to a small fraction of the lens' cross-section. Examples are shown in Figures 1 and 2.

A special kind of micro-ball lens is obtained by heating the end of a tapered fiber such that it melts.

Trade-offs

Ball lenses offer a compact, inexpensive means of focusing or collimating light, particularly in fiber-optic and miniature imaging systems, but their simplicity involves notable trade-offs between cost, ease of alignment, and optical performance.

Because a ball lens is a single spherical surface pair without controlled curvature separation, manufacturing costs are low: They can be mass-produced with high precision using grinding or molding, and they require no complex centration. Their spherical symmetry eliminates orientation constraints, simplifying handling and mechanical alignment compared with aspheric or multi-element optics. This makes them attractive for applications where compactness and robustness outweigh exacting image quality requirements.

However, the same spherical symmetry that simplifies manufacture also introduces significant spherical aberration and off-axis distortion (discussed in more detail below). A ball lens can provide only a limited numerical aperture and small diffraction-limited region; image quality deteriorates rapidly away from the optical axis. Consequently, they are best used for fiber coupling or point-source collimation, where only on-axis performance and small size matters.

Fabrication of Ball Lenses

The manufacture of ball lenses begins with cutting rods of the chosen material into small cylindrical preforms slightly larger than the final diameter. These preforms are rounded into near-spherical shapes by one of several methods:

• flame melting, in which surface tension forms molten glass into spheres
• mechanical grinding, where the pieces are rotated between concave laps
• precision molding, in which molten or softened glass is pressed into spherical molds

Following rough shaping, the spheres undergo fine grinding with progressively finer abrasive slurries — commonly silicon carbide or diamond — to improve the sphere shape and bring the size close to specification. They are then polished on pitch or polyurethane pads using fine polishing compounds such as cerium oxide. This step produces a high-quality optical surface with roughness typically below 10 nm and sphericity within about 1 µm.

After polishing, the lenses are cleaned ultrasonically and inspected for dimensional accuracy, surface finish, and optical performance using interferometric and profilometric methods. Parameters such as focal length and refractive index uniformity are verified, and defective parts are rejected. High-quality lenses may then receive anti-reflection coatings or protective coatings by vapor deposition.

Applications of Ball Lenses

Ball lenses find widespread use in optical systems where compactness and, robustness are required. Their rotational symmetry and short focal length make them particularly effective for coupling light between optical fibers, laser diodes and photodetectors, or for collimating and focusing beams in miniature optical assemblies.

One of their primary roles is as beam collimators in fiber-optic systems. When positioned correctly relative to a fiber endface, a ball lens can convert the divergent output of a single-mode or multimode fiber into a nearly collimated beam, or conversely, focus an incoming beam into a fiber core. This principle underlies fiber collimators and fiber-to-fiber couplers, where precise but simple alignment yields efficient power transfer with minimal insertion loss. Their use is common in optical fiber communications modules, fiber sensing heads, and photonic test setups.

Beyond fiber optics, ball lenses are integral to many miniaturized optical systems. Their small size and high refractive index allow compact designs in barcode scanners, laser rangefinders and optical pickup heads. In endoscopy and other medical imaging devices, ball lenses often serve as objective lenses, providing short working distances and high collection efficiency within extremely limited spaces.

Ball lenses also appear in optical sensors, including position, proximity, and fluorescence detection systems, where they focus or collect light from confined regions. In microscopy, related geometries are found in microscope objectives, especially immersion objectives, where a hyperhemispherical lens — a ball lens segment slightly exceeding a hemisphere — is used to enhance numerical aperture and reduce aberrations by exploiting refractive index matching with immersion media.

Overall, ball lenses are valued for their simplicity, precision, and versatility across both industrial and scientific optical applications, especially where compact, efficient light control is required without the complexity of multi-element lens systems.

Performance Details

Focal Length

There are two different definitions of focal length of a ball lens. The effective focal length, which is the distance between a plane through the center of the lens and the beam waist (focus) of an initially collimated input beam, is given by the equation f = \frac{{n\;D}}{{4(n - 1)}}$$

where ($D$) is the diameter of the lens ball and ($n$) its refractive index.

The back focal length is defined as the distance of the focal point from the lens surface, and is smaller than the effective focal length by half the diameter of the ball.

Optical Aberrations

Just as other spherical lenses, ball lenses exhibit optical aberrations and in particular spherical aberrations (see Figure 1 and 2) when operated with incident beams having a diameter which is not much smaller than that of the ball. Therefore, the minimum possible spot size of the focus is not obtained for the largest possible input beam size, as it would be for a perfect lens.

It is possible to produce aspheric lenses with much weaker aberrations, using spherical ball lenses as a preform which are then appropriately deformed.

Rainbows

Natural ball lenses in the form of small water droplets cause the phenomenon of rainbows. The color effects of the primary (most prominent) bow arise from light paths with a single internal reflection in a droplet. Sometimes one can see a secondary rainbow, arising from beam paths with two internal reflections.

Frequently Asked Questions

What is a ball lens?

A ball lens is a type of optical lens with the shape of a sphere. It is manufactured from a single piece of a transparent optical material, such as fused silica.

What are ball lenses used for?

Ball lenses are primarily used as fiber collimators, for fiber-to-fiber coupling, and in miniature optics for applications like barcode scanning, endoscopy, and optical sensors.

What is a half-ball lens?

A half-ball lens is a lens in the shape of a hemisphere, which is made by cutting a standard spherical ball lens in half.

How is the effective focal length of a ball lens determined?

The effective focal length ($f$) is the distance from the lens center to the focal point and is calculated with the formula ($f = n D / (4 (n - 1))$), where ($D$) is the diameter and ($n$) is the refractive index.

What is the main optical limitation of ball lenses?

Their main limitation is significant spherical aberration, especially when the incident light beam is wide. This aberration prevents achieving a diffraction-limited spot size, unlike with a perfect lens.

Why are ball lenses often preferred for miniature optics?

For small dimensions, typically a few millimeters in diameter or less, ball lenses are often easier and thus cheaper to fabricate than lenses with traditional, more complex designs.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 32 suppliers for ball lenses. Among them:

⚙ hardware

Shanghai Optics offers custom-made ball lenses. Please contact us for manufacturing limit or custom specifications. We can offer balls lenses e.g. with a diameter tolerance of ±50 μm, scratch & dig 20/10, surface quality 40-20, and with <50 μm deviation from a sphere.

⚙ hardware

Ball lenses are commonly used to improve signal quality in fiber coupling applications, or for use in endoscopy or bar code scanning applications. Our ball lenses feature short back focal lengths to minimize the distance needed from the ball lens to the optical fiber. Edmund Optics offers a variety of ball lenses in a range of substrates for performance in the ultraviolet to the NIR. Half-ball lenses are also available to ease mounting or system integration.

⚙ hardware

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

⚙ hardware

Knight Optical provides a wide range of ball and half ball lenses which are ideal for endoscopy and fibre communications. We have a variety of material options available including quartz, sapphire, cubic zirconia, ruby, BK7 and other optical glass, fused silica, borosilicate, and many more. Our stock catalogue consists of 0.4 mm to 10 mm diameter ball lenses, but custom dimensions are available up to 100 mm.

⚙ hardware🧩 accessories and parts🧴 consumables🔧 maintenance, repair📏 metrology, calibration, testing💡 consulting🧰 development

Hangzhou Shalom EO provides ball lenses and half ball lenses made of various substrate materials including sapphire, ruby, BK7, UV fused silica, high index glass, and germanium, ZnSe, ZnS with AR coating. Our ball lenses and half-ball lenses are ideal for applications such as fiber communications, endoscopes, and microscopes.

Available diameters are between 0.3 mm and 300 mm, with surface roughness (Ra) <0.007 µm and a wide AR coating wavelength range. Custom ball and half ball module options are also available on request.

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