A Guide to Ball Lenses and How to Choose the Right Lens for Your Application27th Nov 2018
Ball lenses are the perfect solution where there is a need to improve signal coupling between fibres, emitters, transmitters, receivers and detectors. They are also widely used as objective lenses for barcode scanning and endoscopy applications.
The key feature of the ball lens is its short back focal length which reduces the distance required from the lens to the optical fibre. This allows for precision coupling, particularly useful where space or size is a limiting factor within the application.
Ball lenses are generally used in pairs. One lens acts as a collimator, correcting the output of an optical fibre or diode, whilst the other one focuses the light back into the coupled fibre. For smaller applications, half ball lenses offer a more compact solution.
Why choose ball lenses over GRIN lenses?
Gradient index lenses (GRIN lenses) are often used for the same sort of applications as ball lenses. However, ball lenses offer a number of advantages.
Firstly, size. Ball lenses are physically smaller and less costly than GRIN lenses. In addition, they are easier to mount, position and align thanks to their rotational symmetry. What’s more, the focal length of the ball lens is less sensitive to climatic conditions. Where there is a shift in focal length, reduced coupling efficiency usually results. This makes ball lenses more efficient for applications that are subject to significant variations in temperature.
Ball lens materials – which to choose?
Ball lenses can be manufactured using a range of materials. The key is to choose the material that best suits your application and the conditions in which it is being used.
Fused silica or quartz ball lenses work well in exceptionally demanding applications that make use of the ultraviolet and visible and near infrared spectra. They offer excellent UV transmission from 200nm to 2.2μm. Their low coefficient of thermal expansion is what makes them ideal for use in extreme conditions.
Sapphire ball lenses offer a high refractive index which delivers low spherical aberration. This is a material that is well suited to extreme environments thanks to its high strength, chemical stability and because it is one of the hardest known natural substances. Sapphire also offers superior optical transmission qualities. This material is suitable for use in the 200nm to 5.000nm+ range.
Ruby ball lenses are exceptionally hard and typically used in applications that require wear resistance to metals and chemicals. The colour of the ruby ball lens makes it more visible and therefore easier to handle.
Cubic zirconia ball lenses again offer a high refractive index, making it perfect for optical telecoms products. It is also wear resistant and able to stand up to severe chemical corrosion. Transmitters and receivers require higher coupling efficiency, and cubic zirconia is proving the ideal material to deliver in this respect.
Ceramic ball lenses are able to resist corrosion and abrasion very well and can remain stable at extreme temperatures. Choose this material if you are seeking a ball lens that resists oxidation amongst chemicals, acids and salts, such as in chemical pumps, flowmeters, bearings, gages, valves and down-hole pumps.
BK7 ball lenses make ideal pre-forms for aspheric lenses. Borosilicate is technical glass that is exceptionally chemically resistant with a low thermal expansion. High index optical glasses such as BAF-8 also make excellent ball lenses specifically suited for fibre coupling applications.
There really is an incredible variety of choice when it comes to ball lens materials; precisely why it is so important to obtain tailored advice from technical experts who have the specialist knowledge to guide you as to the most appropriate selection to suit your individual application.
Other considerations aside from ball lens material include focal length, back focal length, diameter and anti-reflective coating. As spherical aberration scales with focal length, something which is directly connected with lens dimeter, it is usually the case that smaller diameter ball lenses deliver better performance. Again, tailored guidance is useful.
Ball Lenses from Knight Optical
Knight Optical offers a comprehensive range of stock and custom ball lenses suitable for a variety of applications. Unique requirements for diameters, quality grades and materials can all be met by our knowledgeable experts, and bespoke technical advice is readily available whenever required. For further information and technical specifications visit