LCDs – Monitors for the digital low-energy society

The spectrum ranges from displays for fast moving portable consumer goods to special displays for automotive and industry applications requiring a long lifetime, some of which are also subject to extreme stress. There is one overriding requirement here: reduction of the electricity needed by the displays. In the TV field alone, Sharp estimates that the output of 14 coal fired power stations could be saved if some 1.2billion crt televisions that are in operation around the world were replaced by modern lcd TV sets. Sharp is presenting the fundamental technology approaches to optimise the power consumption of displays. Efficient backlighting intelligently controlled The main consumer of power in each transmissive liquid crystal display is the backlighting. With small diagonals, conventional ccfl backlights require around three quarters of the total power consumption of an lcd module. With large screen diagonals, that figure can be anything up to 90%. Backlighting therefore represents one of the main starting points when it comes to reducing power requirements. High brightness LEDs, such as those manufactured by Sharp, now have a light efficiency of 105 lumens and more per watt and are therefore a real alternative to the cold cathode fluorescent lamps, which have been the preferred illuminant for backlights up to now. From a purely technical perspective, LEDs have the potential to halve the power consumption of backlighting compared to ccfl variants. Television manufacturers are the pioneers in adapting the new technology. For instance, the latest generation of 32in lcd TVs from Sharp with LED backlighting only require 89kWh per year. In comparison, the 32in Aquos lcd television from 2003 used almost three times as much per year. A major part of the energy savings comes from the more efficient backlight technology. LEDs are also increasingly gaining ground in the industrial sector too. At Sharp, new lcd models for industrial applications are now only fitted with LED backlighting. In addition to the efficiency gains due to the technology, LEDs also contribute to energy savings through their rapid response characteristics and their extensive dimmability. They are the basis for intelligent backlight control. In the TV sector, in particular with direct backlit displays, brightness can be controlled per segment with a direct link to the image content. Here, the backlight is simultaneously dimmed in dark image areas, which not only results in a substantial increase in the contrast, but also saves energy. This principle can also be extended to larger parts of the image by special video controllers, which make the semi-dark image elements lighter and compensate for the excess image brightness by dimming the backlight. This procedure, called 'Eco Picture Control' generates a precisely illuminated image with reduced brightness of the backlighting. In addition, the display brightness can be controlled with the aid of sensors, depending on the ambient light. Many TV sets are now fitted with functions like these. Ambient light-dependent control of the display brilliance also makes a significant contribution to the energy efficiency of e-signage solutions. On sunny days, outdoor e-signage displays require a brightness of some 2000cd/m^² to compete with the ambient light; however, the required display brightness is already reduced to half with cloud, and at night it only requires around 400cd/m^². LED backlights can be simply readjusted via a large brightness range in correlation to the ambient light. Accordingly, the power consumption is reduced to around a fifth at night, compared to a full load on sunny days. Secondary effects are also achieved: Many e-signage systems (primarily for the outdoor sector) need active air-conditioning, which is not required as much when the backlighting produces less waste heat with reduced brightness. Increased transmissivity for constant brightness with lower backlight output The lcd panels also offer various approaches for restricting the power consumption of entire lcd modules and consequently of the applications in which they are installed. The key point here is to optimise the light permeability of the panels so that less light output in the backlighting is required in order to achieve the same display brightness. The precise alignment of the liquid crystal molecules plays a pivotal role in the improvement of transmissivity. Only recently, Sharp introduced the new UV^²A alignment technology with which lc molecule can be aligned with precision in the range of picometers. This is carried out by a ribbed microstructure on the panel glass, which is generated by a UV-light induced photochemical process during panel production. The tilt angle of the ribbed microstructure, which subsequently determines the alignment of the lc molecules, corresponds precisely to the incidental angle of the UV light with which the polymer chains of the so-called alignment layer are aligned. Compared to existing alignment technology (ASV), the UV^²A technology improves the transmissivity of the lcd panels by 20%. In addition, the procedure ensures static contrast values of 5000:1 – an increase of 60% in comparison to conventional panels. However, the colour filters also offer potential when it comes to increasing the energy efficiency of displays. The five primary colours technology that Sharp presented in Summer 2009 serves primarily to further optimise the colour rendition of displays. With the new colour filter scheme that also includes cyan and yellow in addition to red, green and blue, 99% of all surface colours can be faithfully reproduced – even difficult to display colours such as the emerald green of the South Pacific, the bronze of trumpets or the crimson of roses. With the standard rgb scheme, the colour space is restricted to between 35 and 60% of the natural surface colours. A positive side-effect of the RGBCY colour filter technology is also the substantially increased light permeability of the panel, which also has a positive impact on the energy efficiency of lcds. However, the technology also requires a new pixel design and more complex control, so it is currently not determined when the first displays with five primary colour filters will be launched on the market. However, so called transflective displays are established technology. Here, part of the inner panel structure - primarily conductor tracks and transistors that do not contribute directly to the image rendition - is covered with reflecting microstructures. Coverage with the reflective microstructures amounting to just a few percent of the display area guarantees an easily visible image rendition in direct sunlight. Depending on how much display brightness is contributed by the exterior light, the backlighting can be dimmed down or switched off, with the process being controlled by a photo sensor. The approach is particularly interesting for all portable devices, from mobile telephones to industry handhelds, in order to facilitate a network-independent operation time that is as long as possible. 'Memory effects' save energy Displays that remember their image content themselves only require electricity when the screen content needs to be rewritten. Although such a technical approach is not suitable for screens for the depiction of moving images (television, media players, etc), this technical approach offers substantial potential for energy saving everywhere where largely static content has to be displayed for a comparatively long time. Two different technologies are available to realise such a concept. Sharp and other companies presented so called bi-stable displays for the first time as early as 2006. They are based on a special type of liquid crystal, cholesteric lcs. In contrast to nematic lc material, these cholesteric lcs have a helical structure and two stable states – planar and focal conical. Both states can be maintained without a permanent power supply. A voltage impulse is only required to 'switch over'. In the planar state, part of the incident wave spectrum is reflected depending on the step height of the helix, while all other wavelengths are absorbed by the display backside, resulting in a bright, coloured dot that corresponds to the reflected wavelength. In the focal conical state, the lc material lets the entire incident light through and all the wavelengths of the display backside are absorbed so that a dark dot appears. In this way, you get a monochrome display with light-dark contrast. This technology can be refined through the combination of lc material with various helix step heights and corresponding colour reflectors on the backside so that colour displays are also possible. Bi-stable displays can potentially be used everywhere where image information has to be exchanged regularly, but also needs to be displayed over longer periods unchanged and a permanent power supply is not available, or only in restricted form. Examples of application areas include price labels on shelves, door signs in conference hotels, offices and public buildings, as well as e-books. Large-format e-signage displays, such as those offered by Distec, can also be realised with this technology. In addition, Sharp presented a new kind of memory lcd in 2009 that fundamentally differs from bi-stable displays. Based on Sharp's proprietary Continuous Grain Silicon technology, it has been possible to develop a new type of display in which each pixel is equipped with a 1bit memory that stores the pixel status. Image information therefore only needs to be rewritten in the pixels in which the content has changed compared to the previous screen frame. In contrast, with conventional lc displays, microcontrollers have to rewrite the entire screen content from frame to frame at a speed of 50 to 60Hz, even if the majority of the image content remains the same. The power consumption of standard lcds is therefore around 130 times higher than the power consumption of the new memory lcds with 1bit memory in the pixels. At a size of 1.35in, these require only 15µW in operation and can therefore also be fed directly via the smallest of solar panels, as presented recently by Sharp. This combination is the ideal basis for innovative, network-independent devices such as so-called "smart meters" – thermometers and sensors for home automation systems – and for GPS-assisted bike computers, pulse meters and intelligent car keys that report whether all windows and doors are actually closed, etc. Conclusion Liquid crystal displays offer various options for reducing the power consumption of screens. The main consumer of power in most devices with a screen is the backlight unit that, depending on the size and technical design, can use up to 90% of the power needed of a lcd module. They are therefore the focal area in efforts to reduce the power consumption of liquid crystal displays. LEDs represent an important starting point as an alternative to the ccfls as illuminant for backlights. Firstly, they offer substantially higher energy efficiency than ccfls. Secondly, due to their rapid response characteristics, they also facilitate intelligent backlight operation that reduces the light output in correlation to the image content and exterior light. Innovative lcd panel technologies such as Sharp's proprietary UV^²A alignment technology and the five primary colours filter technology also contribute to substantial improvements in the transmissivity of the panels. This means that the same display brightness can be achieved with reduced light output from the backlight. Last but not least, alternative technologies are available in the form of bi-stable lcds and memory lcds with 1bit memory per pixel for special application niches. Compared to classic TFT-lcd technology, these have extremely low power consumption as they can 'remember' image contents, meaning that electrical power is basically only required for new image content.