Thin, Thinner, Light Guide Film
E-book readers, tablets or smartphones are not only significantly lighter and thinner, but also more energy-efficient than a few years ago. The PLEXIGLAS® light guide films have contributed to this.
Without displays, telephones could have never turned into compact computers. The first screens, however, had nothing in common with the brilliancy of today's models. Today, HD-quality resolutions are no longer a problem; yet at the same time, the energy consumption has decreased significantly. This is mainly due to the fact that the suppliers of the display manufacturers have continuously improved the materials used. An example of this are the PLEXIGLAS® light guide films.
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To produce a visible image on LCD displays, the display must be illuminated from behind. In edge-lit LED displays, light guides made of PMMA (Polymethylmethacrylate), such as Evonik’s brand product PLEXIGLAS®, often serve this purpose. The material conducts the stored light from the LEDs in the entire component surface, and, together with light emitting structures, provides a uniform illumination of the screen. The advantage: Contrary to models where the LEDs are directly behind the screen, displays using this technology can be much thinner.
Within a PLEXIGLAS® light guide, stored light is conducted over long distances by the principle of total internal reflection.
A brilliant picture
In an edge-lit LCD display, LEDs integrate the light via the edges of a light guide plate so that the display is evenly backlit.
Wafer thin: Light guide films
Due to its optical properties, PLEXIGLAS® is an excellent light guide
expert for light guide films at Evonik
During the past years, the light guides themselves have become thinner and thus lighter . In 2009 for example, a light guide sheet in a laptop display was 3 mm thick. Only three years later, the manufacturers halved these dimensions. In tablets, light guides just 0.55 mm thick produced by injection molding or compressing molding were used. “But this was right at the limit of the thickness/surface ratio. The next step towards ever-thinner light guides came in the form of light guide films made from PLEXIGLAS®”, says Markus Parusel, expert for light guide films at Evonik. Today, PLEXIGLAS® light guide films are available with a minimum thickness of only 0.2 mm, while at the same time featuring the same optical properties as the original PLEXIGLAS® light guide sheets. “This way the light guide films contribute to making devices more convenient and, of course, lighter,” Parusel explains. Meanwhile, the restraining factor is no longer the thickness of the material, but the availability of very thin light-emitting LEDs that are positioned at the side and their construction height must be perfectly fitted to the film edge.
Explanatory video: Light guide films
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However, the focus of these display components is not on weight, but on brilliance. “Due to its optical properties, PLEXIGLAS® is an excellent light guide,” Parusel emphasizes. Special microscopic structures that are attached to the film conduct the light from the micro-LEDs to the entire surface and illuminate the display of tablets or smartphones from behind. “This backlighting technology creates a uniformly illuminated impression,” Parusel explains.
E-reader businesses prosper
Films: Easy to process
During production, light guide films offer several benefits: They are delivered on rolls and can be processed efficiently in a roll-to-roll process. For example in this process, the light emitting structures can be attached through different processes such as laser treatment, hot stamping and stamping into a UV-curable varnish.
In other applications, the structures conducting the light must not be visible. For example the e-readers, which become increasingly popular, are usually illuminated through front lighting. This means that the display is illuminated by a light guide that is attached to its front. “It has to be transparent and should conduct the light as far as possible towards the e-reader’s display. Otherwise, the light guide would reduce the contrast,” says Parusel. There are special versions for these applications, in which invisible structures direct the light through the film.
In order to permanently retain the optical brilliance, the light guides must be transparent. “Some polymers turn yellow over time and would spoil the image. As a color-neutral material, PMMA has no negative influence on the colors,” says Parusel.
By the way: Light guide films are not just suitable for mobile end devices. For example they can help present retail furniture in the right light.
Microscopic structures direct the light from the edges over the entire surface. The structures can be attached to only a part of the film as well, as can be seen in the Evonik logo. The unstructured part of the film remains transparent.
© Evonik / Photographer Markus Schmidt
In other applications, such as e-readers, the structures conducting the light must not be visible.
Films for front lighting
E-readers are usually illuminated through front lighting...
© Evonik / Photographer Markus Schmidt
Structure of front lighting
... conducting the light towards the e-reader's display and from there to the user. The light guide must be transparent so that the e-paper display is visible.