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What are sidelight fibers?

Sidelight fibers are optical waveguiding fibers that are manufactured in such a way that the light guided in their core is not only transmitted to the fiber endface, but also emitted at the side.

Sidelight fibers are a concept for avoiding direct light radiation from the light source to the human eye. Often the direct view into a very bright shining source, such as an LED chip, is uncomfortable and dazzling.

By means of sidelight fibers the luminance of the light is spatially distributed and reduced . For ambient lighting, the illumination with sidelight fibers is an elegantly designed, pleasantly perceived lighting method. As a result of their insensitivity to breakage, sidelight fibers made of polymer materials (SL POF, sidelight POF) are easy to handle and can be incorporated into various types of textiles.

The picture on the right shows a 3mm end light fiber (bottom) and a 3mm side light fiber (top) both coupled with equal light power. The light emitted from the end face of the sidelight fiber is visibly weaker, since it has already been coupled out laterally over the length of the fiber.

Different manufacturing processes

Polymer sidelight fibers, SL POF, are available in a variety of different versions. Three fundamentally different types differ from each other in the manufacturing process, the processability in the illuminated object, the necessary LED emitters for light coupling and, last but not least, the optical effect.

Manufactoring methods:

- extruded or drawn fibers

Drawn or extruded sidelight fibers are made from thermoplastic material in a similar way to the well-known POF endlight fibers. They are coated with a low refracting resin on the lateral surface. The thermoplastic material is often PMMA, thus these fibers behave mechanically and thermally similar to PMMA POF endlight fibers. The lateral radiation is realized by the resin or a roughened jacket surface. The SL-POF fiber attenuation is significantly higher compared to endlight fibers.

Fiber bundles of 0.25.mm (top) and 0.5mm (bottom) in molded and polished ferrules

PMMA sidelight fibers are mainly available in diameters from 0.25mm to 3mm. Even a 3mm thick PMMA POF can be bent only with large radii, even larger diameters are not possible in many applications. Compared to casted fibers, these fibers are inexpensive. For light coupling into thin fibers with diameters of 1mm and less, coupling one LED module per fiber is not very effective; one would need a large number of LED modules to illuminate the entire object. Each of them could couple a small amount of light only into the fiber due to the small fiber diameter. Therefore, several individual fibers are combined into a fiber bundle. All fibers of the fiber bundle are then illuminated from one LED light source. The LED module must illuminate each fiber of the fiber bundle with approximately the equal optical intensity so that all adjacent fibers in the illuminated object shine with the equal brightness. This requirement for uniform illumination places special demands on the LED modules to be used: they must have an internal light mixer like the DieMount LED modules (see light sources for fiber optic illumination) developed for this purpose.

When assembling fiber bundles, an effort should be made to fit a number of circularly symmetrical fibers, i.e. 3, 7, 19, 37, …, in a ferrule with an exactly fitting inner diameter so that the optical losses due to gussets between the fibers are not too great. With large numbers of fibers, this is not easy, as the picture above left shows.
The image above right shows the effect of a fiber bundle consisting of 2 x 19 SL POF (ribbon from F. J. Rammer GmbH).

- casted fibers

Textile surface (0.7 x 0.6m) with integrated 3mm cast sidelight-POF;
Textile surface (1.2m x 0.6m) with integrated 6mm casted SL-POF, embedded in a passe-partout with embroidered logo; in cooperation with Textilforschungsinstitut TITV, Greiz

Alternatively, sidelight fibers can be produced by reaction molding by filling a suitable plastic tube with liquid plastic. The liquid plastic cures and forms the core of the waveguide. Casted SL POF made from flexible materials selected for the core and cladding tube material are very flexible.

Fibers from 1mm core diameter and radii of curvature of a few millimeters are made, for example, from fibers filled with PUR. Thicker, cast fibers consist of acrylate-filled PTFE tubes with core diameters from about 2mm to well over
6mm. The PTFE tubing can be transparent or colored white, so the visual effect is different in both illuminated and non-illuminated conditions. Casted SL POF with white colored cket show uniform white radiation when illuminated with white light, casted SL POF with transparent jacket are not visually perceived, when illumination is not switched on, and have an elegant appearance when illuminated with monochromatic light. Examples

Due to the complex manufacturing process, the price of casted SL POF is higher than that of extruded PMMA SL POF. Casted SL POF usually emit more light over short distances compared to extruded SL POF, and as a result of their high flexibility
they can withstand high mechanical pressure. The robust jacket material PTFE allows cleaning with mechanically and chemically “rough” processes.

A disadvantage of casted fibers is the difficult or impossible integration due to the large diameter that doesn’t allow processing with weaving machines and other standard machines used in textile production. This is the advantage of extruded, thin SL POF.

- half value length

When designing a sidelight fiber object, the question arises as to what length the fiber should be designed to be so that it is perceived as uniformly bright to the viewer’s eye. Reduced brightness at the end of the fiber should not be noticeable. If the fiber is chosen too short, much of the coupled light will be emitted at the opposite end face, so the sidelight fiber system is not efficient. If light is injected into both ends of the fiber, the fiber length of equal brightness will be extended.

The idea of increasing the maximum length of uniform brightness of a SL POF by LED emitters with particularly high light output is unfortunately not correct. The eye perceives differences in brightness as clearly noticeable when the difference in brightness is more than a factor of 2, regardless of the brightness level of the illuminated object as a whole. This observation leads to the following specification, which is useful in practice:

If the half-value length of an SL POF is referred to as the fiber length after which the laterally emitted light has dropped to half of the laterally emitted light at the light coupling point, then the length of an SL POF should be dimensioned so that it is not longer than the half-value length in the case of one-sided coupling, and not significantly longer than twice the half-value length in the case of two-sided coupling.

Adhering to this rule avoids inefficient light loss due to too short fibers, as well as fiber lengths that are perceived as unevenly bright. The half- value length of a SL POF is a characteristic value of the fiber and should be known for system design. The half-value length depends on the manufacturing process, the fiber diameter and the material used. It ranges from a few 10cm to 10m. Half-value lengths of 2 to 3m are common.

Currently available as standard are:

PMMA sidelight fiber:

Casted sidelight fiber:

PUR-SL-POF with silicone jacket cannot be used for white light, because the spectral components of white light are emitted with different intensity over the SL POF length. They should only be illuminated monochromatically.