Light Guide Film and Optical Fiber

Full-surface backlighting of membrane keyboards

Since membrane keypads are very thin and are partly built into a housing in a curved state, the light guide technology cannot be used for their backlighting, as we use it for long-stroke buttons and capacitive surfaces, for example. Individual LEDs can be applied directly to the conductive film and offer the same function, but do not have the same optical effect. In order to guarantee a large-scale homogeneity of the lighting in membrane keyboards, completely new methods were developed.

The technologies of the Light Guide Film and Optical Fiber have prevailed. Even if all of the buttons on the film are backlit with the same LEDs using these two technologies, which means that separate control and thus use as functional lighting is not possible, they can be a real alternative when it comes to orientation lighting and optical finishing.

With a light guide film, similarly homogeneous backlighting can be achieved with membrane keyboards as with plastic light guides for other input systems. The full backlighting of the film is achieved with just a few LEDs. This saves costs and creates a more visually appealing effect.

In the following, the functionality of a Light Guide film is explained in more detail using the illustration above. In the light guide film technology, the LEDs on the circuit carrier films are not attached directly behind the buttons, but along one side. The LED row is aligned so that it shines into the light-guiding film from the side. The light waves penetrate the film unhindered in all directions and are reflected on its sides in order to illuminate the film homogeneously. A microscopic dot pattern on the underside of the light-guiding film layer reinforces this effect and directs the light waves upwards towards the décor film. Wherever this film is translucent, the effect is visible, from the opaque areas the light is directed back into the Light Guide film.

Optical fiber (in German: glass fiber) is mainly used for fast data transmission using light waves. The best-known example of the use of this technology are the high-speed data cables, which are mainly used for the transmission of Internet signals. The fiber optic core of the cable guides the light waves almost unrestricted, which predestines the technology for data transmission. However, the glass fibers can also be used as light guides for membrane keyboards. Here, too, you can benefit from the loss-free transmission of the light, because with optical fiber only a single LED is required to illuminate the background of an entire film.

Many thin fiber optic cables run parallel and close to each other behind the front film, forming a closed area of fiber optics. These are bundled on one side and illuminated from there with an LED. The light rays penetrate the fibers and are passed through them. As a result, the fibers emit light along their entire length and thus backlight the film.

The fact that optical fiber only requires one LED and still achieve homogeneous illumination is the clear advantage of the technology, but the glass fibers and the small number of suppliers in the production process make optical fiber for light distribution comparatively expensive. Which fiber optic technology is advantageous in which case must be decided on a case-by-case basis during project planning.