LED displays are lit by electricity flowing through each LED. These signs need to be bright – and the brighter the surroundings, the more energy they need simply to stand out. In contrast, liquid crystal displays (LCDs) use natural light. The firm element displays Dr. Wiemer GmbH, part-owned by Steinbeis, has developed a range of new technologies and a new all-in-one solution for low cost, maintenance-free LCD signs which are easy to manufacture and need only very little energy. Some of these innovations have now been patented.
Standard liquid crystal displays remain dark when not energized; the different segments can only switch to transparent when connected to a power source. These displays have a dark background, the exact color of which changes depending on the viewing angle. In contrast, the low twist nematic (LTN) displays developed by element displays have a light-colored background when idle. When energized, the background becomes uniformly black – and although it may appear slightly lighter when viewed at an angle, this background is colorless. The result: an attractive display which remains uniform, even from wide angles. When light shines on the LTN displays, they are noticeably brighter than standard LCD displays, which remain dark unless powered by electricity. Using LTN displays thus helps to reduce energy consumption.
To stay bright, LTN displays need to use natural light from their surroundings. To capture this light, element displays uses UVstable plastic films and panels with integrat- ed diffusers and fluorescent dyes. These “light converters” absorb light with short wavelengths and re-emit it in the emission color of the fluorescent dyes. When exposed to light with longer wavelengths, the light converters are translucent. This means the exact color of the ”lit” display segments is mainly determined by the emission color of the fluorescent dyes.
Twist nematic (TN) or LTN liquid crystal displays must be mounted in special housings, as the polarization filters on the exterior of the LCD cells are not stable enough. This means a transparent screen must be placed in front of the display. Light shining on this screen creates reflections from the screen’s front and rear surface – which make the dark background lighter and reduces display contrast. Not only that, the front of the LCD cell also creates reflections resulting in less contrast. This is normally fitted with an anti-glare layer which scatters incident light over a large area and dissipates sharp contours in the reflected image. Unfortunately, this also lightens the dark background. In the displays produced by element displays (which are optimized for low energy consumption), the LC cells are optically coupled to the front screen. This screen is generally made of float glass coated with a thin anti-reflective film or of Polycarbonate. This solves the reflection problem – with the sole exception of reflection from the front of the display. As a result, the dark background stays considerably darker than on standard displays. The LTN displays therefore maintain excellent contrast, even at low light levels.
LCD displays are usually mounted in opaque metal frames. This prevents light entering from behind, meaning the display must be backlit by energy-consuming light sources mounted beneath the display. element displays has developed a far more practical solution which uses LED backlighting on a transparent board. As the displays run on natural light, this is only needed in the dark – again helping to keep energy consumption significantly lower.
With its optically coupled LC cells, the front screen can be used to build a low-cost LTN display for outdoor use – simply by joining it to the rear of the housing using standard methods for the manufacture of doubleglazing windows.
Peter Wittmann
Steinbeis Beteiligungs-Beratung GmbH (Stuttgart)