Sunlight Readable LCDs
By Frank Jammers (FAE for Europe) and Matthias Pfeiffer (CTO) at New Vision Display
For many years now, TFT LC-Displays have been the dominating technology in visualization. With more and more screens being used outdoors and in portable applications, sunlight readability has become a critical factor in the display market. One way many LCD manufacturers address this issue is to increase the display’s backlight luminance. Although this works well for many applications, it also increases power consumption and the bulkiness of the display. This is not an ideal solution for some portable or small-size applications, so NVD additionally offers “transflective” displays, which have improved readability in any lighting condition without increasing power consumption or bulkiness.
Simple, low resolution liquid crystal displays routinely are offered in reflective, transmissive, and “transflective” (partially transmissive and partially reflective) mode. This is achieved simply by choice of the reflector behind the display , which is usually attached to the rear polarizer. This method is not feasible for TFT displays due to the inherently much lower transmission of these devices. Transflective TFT displays have a unique cell architecture compared to standard TFT display modules. This is necessary to accommodate for the fact that light passes only once through the liquid crystal layer in transmissive mode before reaching an analyser, while in reflective mode it passes the liquid crystal layer twice. In the transflective cell architecture, every single pixel is divided into a transmissive and a reflective section, where the latter is based on a polymer film with micro metal mirrors within the glass cell. The size and partition of these two sections are flexible and depend on the desired focus of the final display. Typically the liquid crystal layer thickness in the reflective part is only half of the call gap in the transmissive part. The front polarizer acts as both, polarizer and analyser for the reflective portion of each pixel. This requires selection of a liquid crystal mode that works with parallel polarizers, unlike transmissive displays where usually polarizer and analyser are crossed.
“Transmissive” and “reflective” modes work together for enhanced performance. In darker environments, the “transmissive” mode is dominant with emissions from the display’s backlight. In bright environments, the “reflective” mode of the display is more dominant through reflectance from the internal mirror structure instead of the module backlight; thereby, saving on battery life in mobile terminals and handsets. In order to achieve satisfactory picture performance in reflective mode, the front surface reflection must be suppressed. The front polarizer needs to be coated with an anti-reflective or an anti-glare layer.
The major difference to other manufacturer’s transflective products is that NVD combines the sunlight readability with it’s wide viewing angle technology, Amplified Intrinsic Fringe-Field (AIFF), which is a variation of Multi-Vertical-Alignment (MVA). The result is stunning viewing angles of typically 80 degrees into all four directions and under all environmental light conditions. The contrast ratio in reflective mode is still 30:1 versus 300:1 in transmissive mode. Color gamut is specified with 60% coverage of the NTSC triangle and the liquid crystal’s response time is 30 ms from black to black.
Transflective TFT display modules can be manufactured in the a wide range of sizes. However, the more complex structure of transflective displays increases the manufacturing costs. NVD focuses its efforts in transflective displays on portable applications, currently up to 4.3 inches with resolutions in the 230ppi range. The operating temperature range is designed at -30 to +80 degrees Celsius. Storage temperature is even larger from -40 to +90 Celsius, so that the product easily complies with outdoor and industrial requirements. 2”, 3.5” and 4.3” are pre-tooled. Other sizes are available as custom designs.
Target applications for these devices – to name a few – include mobile payment terminals, credit card readers, portable measuring devices, navigation and diagnostic systems, outdoor recreational products, medical products and high-end appliances.