Lighting Fabrics - A New Approach for Flexible Light Sources

Fibers and fabrics are an essential part of our technological history. The treatment of fibers into threads, simple skills as braiding or twisting reach back to the Stone Age. Even the more sophisticated technology of weaving dates back 32,000 years - only surpassed by the treatment of wood and stone.

Our relation to textiles and fabrics started with the selection of natural fibers offering numerous possibilities for protection, shelter and comfort. The features of a lightweight, reusable flexible material to be manipulated by a simple technique such as sewing allowed a universal use. Next to clothing, structures like tarpaulins, tents and yurts where home to early mankind. Evolving with them, the progress in textile manufacturing and the use of artificial fibers today allows custom made solutions in all kinds of applications. In this context, it is interesting to note that even early artificial lighting depended on a small piece of fabric to light up: the wick of a candle or lantern as the first controllable sources of light followed by the gas mantle and finally a carbon filament inside the first light bulb.

Next to the generation of light the use of fabrics was always much more obvious in the shaping, modeling and directing of light. Curtains, drapes, shades, louvers, blinds and canopies are tools to separate our very own surroundings from the outside world in a simple, effective but also fashionable way. A curtain not only allows us to control the light and the looks passing through but is also a piece of craftsmanship reflecting style and state of textile technology. Lampshades not only form the light emitted by the light source, they are rather a key factor in the luminaire’s design. A backlit textile works not only as an acoustic element in architecture but also as a homogenous glare free source of illumination.

As light and fabric are literally interwoven in our perception, it seemed just a logical step to actually combine both to create a luminous fabric. As a first attempt to do so, optical fibers were used. These were mechanically treated on their outer surface by abrasion changing the linear light transmission to a 90° angle at the treated areas. Combining these fibers with a fabric created a homogenous lighting piece of textile. Off course light still had to be „fed“ in from a traditional light source. Due to the time and cost intensive treatment this technology never reached an industrial level. Only a small-scale application for photodynamic use in medicine was built. With the evolution of white LED’s came the technical feasibility for bringing a light source directly to a textile surface.

Flexible Solutions

The idea of spreading LED’s on a textile was already patented in 1996 (German Patent DE 19632719 A1) as a way of illuminating banners for commercial use at night. Followed up by an even more futuristic attempt in 2003 (German Patent DE 10320650 A1) combining textile based LED’s and solar cells with batteries for creating a nearly self sustaining lighting device.

Though not all the technologies described in these patents were available at the time of publication, the groundwork was already laid by the use of tinsel wires (the winding of ultra thin wires or foils around a textile carrier thread) long before that. Based on a medieval technique, the so called “Lyonese Wares” used for luxurious decorative purposes in churches and palaces, a technology had been developed to successfully build long lasting voice coils, telephone and headphone cables since the 1940s.

But it was the need for special clothing to help people to withstand the most extreme environments such as arctic regions, underwater and space, that first introduced textile circuitry to larger areas. This lead to the development of wearable circuitry for the use of heating underwear first and later sensor suits, monitoring body functions of divers and astronauts. Today athletes are equipped with sensor suits to enhance their training and performance skills. Of course the military is the ultimate client when it comes to flexibility in terms of “working everywhere.”

In the early stages of these projects tinsel wires were used, being the most flexible conductors at the time. The process of physical vapor deposition took the fabrication of conductive threads to the next level, a reliable way to permanently coat a fiber was found. These fibers, individually or as fabrics, work as excellent conductors when copper or silver is the metal used for the coating. The first use for conductive threads in mass production is heated seats in the automotive industry, taking full advantage of the unbreakable electric conductor capable of adapting to any form and surface. Further applications became electrostatic and electromagnetic shielding, healthcare and smart wearable textiles.

As a result of the European research project PLACE-it the first method for embedding low power LED’s on a textile surface using conductive threads and sequins was developed in 2013 by TITV Greitz, Germany. In 2009, the STELLA research project focused on the other way of flexible circuitry leading Fraunhofer Institute IZM Berlin to the development of the SCB (stretchable circuit board) using meandering copper conductors connecting LED’s sealed in thermoplastic polyurethane.

These two approaches seemed to have the most promising expectations to be picked up by the industry. Yet the difference in technology led to a split between traditional electronic PCB makers such as Würth Germany following the polyurethane based way (TWINflex®Stretch) and on the other hand pushing an even older tradition to new limits: Swiss company Forster Rohner accepted the technological challenge to adapt their decorative embroidery fabrication to e-broidery®.

Figure 1: Polyurethane based circuit boards

Today a whole variety of lighting products claiming to use flexible technologies are on the market such as waterproof sealed led stripes, transparent bendable panels and sheets.

They all have one thing in common: Because of their copper conductors they can either not be bent at a 90° angle or have a limited amount of bending cycles according to the amount of copper used. The thickness of the sealing material on the other hand is a limiting factor for the adaption to given surfaces. This correlation between copper conductors and sealing compound leads to a limitation in either power or flexibility. This limitation also applies to OLEDs being the only other relevant light source in the flexible world.

At this time, there is only one totally uniform way of light to cover its whole surface evenly: Electroluminescent inks can be printed on textiles and connected by conductive threads. Their luminous output and color quality is very limited, though, and therefore cannot be used for real general lighting purposes. This is where the difference between a flexible and a limp circuitry has to be defined: When the only limiting factor can be reduced to the size of the electronic (hard) component connected by a limp conductor, the most versatile use of this set up is given only to be accomplished by textile based technologies. Any other way of connecting components is subject to the limitations mentioned above.

Now this contemplation about flexibility leads to the question where and when it is actually needed in today’s lighting world. Most of our every day lighting requirements work very well with the fixed lamps and lights made from metal, glass and plastic but there is a need when non permanent structures and situations are present. A major role in this area is played by the entertainment industry so it is worth to take a closer look at.

A Special Field of Lighting: The Film Set

The introduction of LED’s in the media and entertainment industry has seen the same reluctance in use as in general lighting when it came to replace “traditional” sources of light with them. Since the producers of audiovisual media products traditionally never paid much attention to their energy balance - due to short production periods - and rather tried to cut on payroll expenses, it took the drastic increase in energy costs and environmental obligations to create an awareness for the need of energy saving production techniques. It was only in the last years that mayor film and TV studios changed their power consuming lighting concepts to led based solutions.


Figure 2: Traditional film set using HMI Fresnel lamps and large diffusion fabrics

And there is even more to be changed: The daily routine on a TV or movie set requires effective, fast working solutions for assembling and disassembling lighting devices, making the equipment’s weight and storage space essential factors. In addition to that efficiency, color rendition and stability, flicker free operation, robustness and compatibility are the other issues to look at. Most of the time the desired lighting effect is obtained by pointing spot or floodlights to large areas of foils, gels or fabrics suspended in frames, using these as reflectors or diffusers for the light source. From the early days of film production the white muslin studio curtains, used to control sunlight have lasted to the present day. Off course high tech fibers and special coatings have enlarged the range of light shaping textiles in the past decades. But because of the lack of alternatives this procedure is the „status quo“ of professional film lighting still today.

This practice always requires a space between light source and diffusing or reflecting media causing bulky, inflexible setups, taking time and logistic resources. The slow technological progress in cinematography lighting is most likely based on the conservative attitude of leading directors of photography who are mainly responsible for he look of a movie. This look was primarily based on the choice of special film stock, development and printing techniques, an all-photochemical process, mostly depending on the right way of exposure. For this purpose, well-established ways of lighting were at hand and therefore no need for innovation.

Tungsten and arc lamps became the worldwide standard beginning at the end of WW1, enabling image capturing with mechanical movie cameras at all frame rates. When HMI bulbs were introduced in 1969, the generation of light became more effective (4 times more compared to a tungsten light source) but the magnetic ballast necessary to run these lamps limited the frame rate of the camera to a single fixed speed (24/25 fps) due to pulsation of the AC arc following the line frequency. Later on, electronic ballasts that operated on higher frequencies solved these problems.

It took another 18 years until the first fluorescent fixture was used to light up a film set. From now on the increase in film stock sensitivity in combination with the growing number of electronic image capturing technologies (CCD/CMOS) prepared the ground for innovative lighting techniques. In 2010 the transformation from analogue to digital cinematography finally marked a new era for the lighting business. It was time for a lightweight, effective light source to be operated everywhere. In this context battery operation became a key feature since more film sets evolved on unusual spots and locations without the need for AC power lines or diesel-powered generators. All led fixtures on the media and entertainment market at that time just replaced the traditional ways of lighting described above by replacing the previous tungsten, HMI or fluorescent light source.

Not all developers of these early LED lights paid enough attention to the color rendition quality of the LED’s used and the driver or dimming setup. The run for luminous output and efficiency led to some poor results that gave early LED lighting a bad name, just as in general lighting. The only progress made in this respect was the use of bicolor or RGB LED’s, creating easy solutions for various color temperatures or even colored lights eliminating the use of expensive heat and UV resistant filter gels. No revolutionary breakthrough had been made solving the problems of heat dissipation; heavy heat sinks or noisy fans are still state of the art. Today’s high end LED fixtures all have excellent color rendition, good thermal management and can be operated with a wide range of power supplies but still rely on traditional shapes and designs as if to mimic the old technology.

LED’s on Fabric

The first attempt to focus on the flexibility advantages of LED technology took place in the area of large displays. But these products rely on traditional PCB, molding, and wiring techniques, resulting in relatively high weight (9 Kg/m.) and are limited to be mounted in scaffolds and trusses when in mobile use.


Figure 3: LED Soft Displays, the first step towards large scale flexibility

After 20 years in the field as lighting technicians, the founders of Carpetlight are aiming at an amount of flexibility that is literally a “shining cloth”, to be produced in various sizes, and which can be handled just like any other fabric. It will be able to replace the traditional setup of fixture and reflective or diffusing media with one single device. In order to give users the most adaptive, form free lighting tool, miniature PCBs are fixed on a textile circuit board, connected by conductive threads embroidered onto a lightweight fabric. By the use of highly efficient LED’s it is possible to lay nearly any desired pattern of light on a piece of cloth, ranging from high luminous output for use as a flood light to low light applications such as ambient lights. The wide emission angle of the LED’s in combination with a dense pitch creates a homogeneous effect over the lighting area.

The choice of materials for that purpose includes rip stop polyamide fabrics as they are used in parachutes, hot-air balloons and sails. They feature extreme tear resistance, low surface weight and a large variety of coating options (water repellent, flame retardant, heat reflecting /absorbing etc.). The textile shell covering the light emitting area from the back is made from this material to ensure maximum mechanical protection just as water repellent properties. It is matte black to make it nearly invisible in a studio or location background. The rip stop weave technique using reinforcement threads in a crosshatch pattern also ensures maximum stability for the textile circuitry inside. The conductive threads embroidered to this surface will not be disrupted when the fabric is punctured or minor tears occur. Extra protection is needed for the junction area between the light’s textile conductors to the “hard” world. In this case a layer of ballistic Kevlar seals the end of the copper wires reaching into the ”soft” edge of the lighting grid. This is the only part that is still limited to the restrictions of the rigid metal connectors mentioned before. Because it is only a small area of the total lighting area (approx.7%) the larger part of it has all the flexibility it was designed for.

Figure 4: The textile solution – Carpetlight

The other materials used on the light emitting side are monofilament fibers which are individually mechanically treated to a helix shape before being woven into a fabric. The optical effect that is achieved here is a lenticular, three dimensional shift of a small circular light source (LED) to a larger rectangular pattern. Underneath this top layer, a spacer fabric is located, enlarging and diffusing the light emitting area as well as protecting the LED’s themselves from direct mechanical impact.

The LED’s heat dissipation is done through a patented multilayer textile compound to be convection cooled. Each LED carrier is connected to a carbon based layer by a thermally conductive adhesive. The carbon fiber structure spreads the heat to a larger area making the whole rear side of the light a “heatsink”. Finally the whole textile is treated to be water and dust repellent.

The standard 2x1 ft. model features a high CRI tunable white reaching from 2,800-5,400 K. Two white LED’s are used to cover the CCT range that’s essential when using movie or TV cameras.

It features low weight (300 g) and high luminous output (8,000 lm). For use as a standard floodlight an aluminum frame is supplied to take up the textile lamp and keep it in a rectangular shape. Velcro fasteners and metal pins are used to fix accessories as diffusers and directing grids.

Driven by a combination of high frequency PWM for ultra fine low light dimming and constant current control the flicker free operation is guaranteed to make sure various camera shutter systems can be operated simultaneously when using the light.

The control unit is powered by an external AC/DC power supply or any DC voltage between 12 and 36 Volts, as is common in the media industry. It can be operated manually to change light intensity and color temperature or by DMX protocol to be remote controlled.

The weight of the control unit is 800 g, making the system the most lightweight floodlight on the market.

Conclusions

The application scenarios for textile-based lighting can be transferred from its current use in the media industry to other fields. All its options can be changed and adapted to the customer’s demands. Different LED combinations, lower LED pitch, variations in fabrics and the use of standardized LED drivers are possible. For example, the outdoor industry is permanently on the hunt for lightweight solutions and things that can easily be packed up and taken along by the modern customer.

Creating built-in lights in tents and canopies, generally in textile structures, was the first step taken. In order to do this the basic protection features of the textile lighting module had to be enhanced. So far coatings of the outer shell and optical cover fabric worked as water and dust repellent but now polyurethane and silicon coatings were necessary to seal the light emitting part itself, to withstand harsh environmental conditions. Other possible use cases are exhibitions, fairs, presentations both indoor and outdoor, basically any non-permanent structure that requires low weight, quick installation and low power consumption lighting.

In the field of interior design, curtains and acoustic fabrics could be “enhanced” with a lighting option adding a new functionality without changing the basic characteristics of the materials used.