ROBLIGHT FIBRE OPTIC HARNESSES
The Fibre Harness is the ‘cabling’ of the Fibre Optic lighting system and provides the essential feature of connecting the light output from a remote generator to optical end points.
A Harness is made up of multiple strands of Glass or PMMA (PolyMethylMethAcrylate) fibre collected into ‘Tails’ of 1-10mm diameter each bound into a ‘Termination’ that connect to the end light fitting. Fibres from multiple tails are bound into a ‘Common End’ which is connected to the Light Port of Light Generator.
Historically, glass was the only material used with fibres as fine as a hair (50μm). Standard tail sizes range from Ø1mm to Ø6mm. A Ø6mm tail is built from 11000 raw fibres. Glass use is now limited to high temperature applications or where closer fidelity between source & output light is required.
Most systems now use PMMA fibres which are thicker than glass at 0.75mm or 1mm diameter and are made up into tails from Ø1mm to Ø10mm. A Ø6mm tail is built from 50 x 0.75mm raw fibres. PMMA fibres are easier to work with, are less expensive and are more efficient in capturing light from Generators.
Optical fibres transmit light by the phenomenon of total internal reflection (TIR). When light is incident at the boundary of two dissimilar materials, some light is reflected back and some is refracted and passes through, bent at an angle relative to the refractive index of the two materials. At the boundary between high index material (Glass or PMMA) and low index (Air) there is a critical angle of incidence above which all light is reflected. Total Internal Reflection effectively captures the light in the fibre and bounce it along to the end where it is emitted in a beam with cut off equal to the critical angle.
The critical angle for Glass or PMMA to Air is around 42°.
Fibre tails for End Light application are sleeved in a black flexible tough flame-retardant jacket to protect the surface of the fibres from mechanical damage that would reduce the TIR effect and the efficiency of light transmission.
The Common End gathers the optical fibres together for connection to the Light Port or Light Generator. Fibres are bound tightly to eliminate gaps, the ends are cut and highly polished to maximise the light transfer from the optical system of the Light Generator.
Common Ends are available in two sizes to suit the number of tails to be fitted and the capacity of the Generator. Ø9 for a bundle of fibres up to 9mm diameter or Ø28 for up to 28mm diameter.
The output from the Light Generator tends to be brighter at the centre and diminishes towards the edges, so if most of the fibres within a tail are in the centre, that tail would be brighter than one with fibres on the outside. In order to produce a consistent light level across the whole harness, fibres from each tail can be optionally pseudo-randomised across the Common End so that on average each tail is collecting the same amount of light.
As well as being used for End Light applications, PMMA fibre is also available as ‘Side Emitting’. The fibre surface is modified by a patented twisting construction so that while most of the light is transmitted down the fibre, some light is scattered from the surface to provide glowing illumination along the whole length. Side emitting fibre tails are sleeved with a clear UV stable and algae resistant jacket making them suitable for use in exterior and underwater applications. Tails can be manufactured with a reflector on the open end to improve uniformity. Where longer runs of side emitting fibre are required both ends can be terminated in a Common End to be used with two light generators and lit from both ends, this also helps with the consistency of the output on longer fibre runs.
Side emitting fibre has many uses including step lighting, floating benches, linear architectural detailing, pathways, signage etc….
Fibre tails are available in a number of standard diameters, each with a different number of individual fibres.
For example: Ø1mm PMMA = 1 x Ø1mm fibre, Ø2mm PMMA = 7 x Ø0.75mm fibres and Ø8mm PMMA = 75 x Ø0.75mm fibres.
The size relates to the amount of light the tail can collect from the Light Generator and therefore how much light is transmitted to the end fitting. End fittings can take a range of tail sizes so by varying the tails used, the fitting output can be tuned for the application. Tails of different sizes can be combined within one harness for maximum flexibility.
The free end of the tail is prepared to suit the end fitting it is used with. These range from a simple clean cut end for use in a Starry Sky, to a standard termination with aluminium ferrule cut and polished like a mini common end, to a waterproof acrylic termination for wet applications. Other terminations are specialised for Starpoints, Plasterboard mounting, 90° bends, flexible arms and many others.