HIGH-FIBRE DIET

Fibre optic LED lighting is becoming ubiquitous in decorative, architectural, museum and heritage projects as well as for underwater lighting schemes. But how does it actually work and how, as a lighting professional, can you maximise its use over traditional light sources?

By David Rich

JULY/AUGUST 2021 LIGHTING JOURNAL 13

Lighting technologies

While some architects, designers, lighting consultants and consumers know all about fibre optics, we are frequently asked why they are so popular and what are their advantages over traditional light sources? This article intends to answer both questions.

Fibre optics are versatile, adaptable, energy efficient and low maintenance. They are used for everything from wet area pool and saunas through to starry sky lighting, display cases, chandeliers, ships and shops.

Using fibre optic lighting means the light source and electrical feed are separate from the fittings. Light from the light generator is carried through an optical fibre harness of glass or PMMA (or PolyMethylMethAcrylate) fibre to multiple ‘terminations’ where it is then shaped and controlled as desired. Once installed, the end point (luminaire) need never be accessed again. This makes fibre optic lighting ideal for inaccessible locations, such as:

• Underwater lighting. For example, pools, spas and water features • Architectural lighting. This can include façades, towers and high-level interiors • Multi-point lighting from one source. Examples here include ‘starry’ skies and large decorative schemes

The fact that the remote source removes heat from the end luminaire fitting also makes fibre optic perfect for delicate and sensitive materials, such as:

• Display and showcase lighting • Hermetically sealed conservation cabinets • Ice bars and sculptures

Light generators can be mounted in dry and well-ventilated places, while the end-points will be safe in harsher locations. This, again, makes this sort of lighting viable in:

• Hot or humid areas, saunas, steam rooms, hammams (or steam baths) and ovens • Coastal or saline environments

PUTTING THE PIECES TOGETHER

Unlike traditional lighting, fibre optics combine three separate parts, as shown in figure 1 overleaf. We’ll now look at each part separately.

1). The light generator. The light generator is the ‘engine’ of any fibre optic lighting system. The LED light source housed inside the generator is specifically designed to concentrate the highest possible amount of light into the fibre harness 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.

Before we look at the final core element, the end point or the luminaire, it is worth addressing three other forms of fibre optic application that relate back to the harness and light generator. These are: ‘randomised common end’, ‘side emitting’ and ‘terminations’.

• Randomised common end. 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. • Side emitting. 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, so making them suitable for use in exterior and underwater applications. A good example of side-emitting fibre optic lighting is in the scheme shown here for The 02, with a lighting design by

Cory Berhost. • Terminations. The free end of the tail is prepared to suit the end fitting it is being used with. These range from a simple clean-cut end for use in a starry sky, say, to a standard termination with aluminium ferrule cut and polished like a mini common end. We can also create a waterproof acrylic termination suitable for wet applications. Other terminations are specialised for starpoints, plasterboard mounting, 90° bends, flexible arms and many others.

3). The end point or luminaire. The luminaire is the ‘end point’ of the fibre optic lighting system and directs, shapes and controls the light from the harness tail to illuminate the object or space with precision.

(and we’ll come to this).

The light generator is the only part of any fibre optic lighting system that contains electricity. It is also the only part of the system that will ever require maintenance and can be housed up to 20m away from the light-emitting luminaire for ease of access.

With major technological advances being made with LED fibre optic generators over recent years, generators are now very energy efficient and can use a variety of driver options including 1-10V, DALI, DMX and Push dimming. Most also work with DMX controlled colour or twinkle wheels.

2). The harness. 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 luminaires.

A harness is made up of multiple strands of glass or PMMA fibre collected into ‘tails’ of 1-10mm diameter each bound into a ‘termination’ that connects to the end light fitting.

Fibres from multiple tails are then bound into a ‘common end’, which is connected to the light port of the 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 1,1000 raw fibres. Glass use is now limited to high-temperature applications or where closer fidelity between source and 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.

TIR effectively captures the light in the fibre and bounces 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°, as shown in figure 2 overleaf.

Fibre tails for end-light application are sleeved in a black flexible tough flameretardant jacket to protect the surface of the fibres from mechanical damage that would reduce the TIR effect and the efficiency of

Lighting technologies

Figure 1. This graphic shows the core components of a fibre optic cable

Figure 2. This shows how optical fibres transmit light by the phenomenon of total internal reflection (TIR)

The Frederikshavn Naval Base for the Danish Royal Navy and (right) 200 Aldersgate in London. In both projects a more compact optical system allows for tilt adjustment in even the smallest recessed housings

Luminaires provide beauty and functionality to the only visible element of the fibre optic system. Available in many optical formats, there is a wide variety of styles to help perfect your lighting scheme.

As it is not necessary to accommodate the electrical connections or handle the heat generated by conventional or LED light source, the size of fibre optic luminaires is considerably reduced. A more compact optical system also allows for tilt adjustment in even the smallest recessed housings, as shown here in our projects for the Frederikshavn Naval Base for the Danish Royal Navy, Denmark and 200 Aldersgate in London.

USE OF GOBOS

Another wonderful application can be to use small GOBO and framing spot projectors at the end of the fibre tails.

They produce an adjustable luminous light spot specially designed for the illumination of pictures, paintings and works of art.

Integral shutters enable the light to illuminate precisely the target object without any light spill. The projector, fed by standard fibre optic cable, has three lenses to ensure light quality, sharpness and uniform coverage.

NEXT-GENERATION FIBRE OPTICS

So, what’s next for fibre optic lighting? We are very excited to give you a sneak preview of the next-generation of fibre optic light generators.

First is a fully submersible (up to 10m) IP68 light generator. This has been developed especially for areas where flooding is a risk or in such places where mounting the generator in water is the only solution. The potential is endless also within the global marine and superyacht industry.

The FL 1100 XT B-3 is weatherproof and waterproof. It is available in 3000k and 4000k colour temperatures, with colour change and twinkle effects also available.

It offers various dimming options, has >90 CRI, and offers natural convection cooling Also coming soon to the market is the multi-port light generator. One thing we are frequently asked is can each fibre optic tail from a single light generator be controlled individually? The short answer was ‘they can’t’, until now.

To my mind, the FL 650 MP range is a real game-changer in this context. It is available with 12 or 24 inputs, which means each point of light can be controlled individually via DMX.

Other specifications for it include IP20 for interior use, a DMX control system, 12 x or 24 x Osram Ostar LEDs, available in 4000K or RGB, >90 CRI and natural convection cooling.

FINALLY – A SALES BIT

On a final note – and this is I appreciate a bit of a slight sales pitch but also, I feel, potentially a valuable addition to the lighting professional’s fibre optic lighting toolkit – I ought to point out that RobLight has a downloadable lux calculator.

The RobLight Lux Calculator completes can calculate the light output of any RobLight luminaires with light generator and customised fibre harness.

You just fill in your selection – fitting, light generator, fibre type, fibre length and number of tails – and instantly get the corresponding lux values, beam angle and beam diameter. Sorted!

Registered users can also generate IES files to use in professional light planning software programs and print files (such as shown below). It is available from Light Projects at www.lightprojects.co.uk

David Rich is Light Projects’ product manager for RobLight, formerly Roblon Lighting