Low-Power Large-Area Cholesteric Displays
Compared to an emissive or transmissive display, a reflective display can offer low power consumption, especially if it is also capable of utilizing low-power supporting systems. Such often forgotten systems can consume more power than the display itself.
by David Coates
LARGE-AREA DISPLAYS can be found in applications that range from showing moving images at sports stadiums to presenting a cycle of static images on outdoor billboards. They are also used as transportation and information signs. Wall displays conveying mood or ambience rather than images are also being developed. These displays can be employed outdoors or indoors and used for short- or long-range viewing; it is unlikely that any one technology or display type will suit all these diverse applications.
Light-emitting-diode (LED) displays have enjoyed success for over 10 years in some of the above applications, e.g., sports stadiums, but their growth into other applications such as the outdoor-advertising market has been quite slow; this may suggest that LEDs are not universally suitable for all applications. LED displays may, for example, consume too much power for use in long-term applications or, due to the large bright area of the screen, they may be deemed overbearing and intrusive in confined indoor spaces. They may also be judged to produce too much light pollution (even though they can, in fact, be dimmed). They may also suffer image break-up if the viewing distance is quite varied, not have enough contrast and brightness in full-sunlight locations, or not meet various local regulations that dominate the static-billboard market. In such circumstances, another option is required that ideally should provide a different set of properties that complement rather than mimic LED technology and that would be more suitable for some applications. One such option comes in the form of low-power-consumption large-area full-color reflective displays.
Cholesteric liquid-crystal displays (ChLCDs) have been used for many years and have been suggested1 for applications that require very low power, usually in hand-held devices but also in monochromatic signs (green/black and blue/white). In these applications, to achieve a very-low-power specification and also to attain fast scan rates, the optical properties of the ChLCD are often compromised. For large-area displays, this need not be the same paradigm; the image quality may be more important, and if the display itself is not the major power user (other electronics besides the LCD are using power), then the power used by the display becomes less critical.
The key features of ChLCDs are that they are reflective and use a bistable mode of operation; thus, there is no backlight, and once an image is generated no further power is required by the display; some power may be needed to keep the system alive, but the LCD itself requires no power. Here, a distinction is made between the display, i.e., the part that shows the image, and the system that drives the display.
A ChLCD operates between two stable states, a colored reflecting state (planar state) and a weakly light-scattering state (focal-conic state); between these two states many stable gray levels exist. A black absorber behind the display absorbs light that is not reflected (i.e., transmitted) by the display and provides a black color. The stable states are accessed by applying a high voltage (>30 V) which produces the transparent metastable homeotropic state which, when allowed to relax, forms the planar state, and with further lower voltage pulses the focal-conic state, or forms mixed focal-conic/planar gray levels. A single cholesteric film will typically reflect a waveband (~ 60–90 nm) of light; to reflect white light and achieve full-color operation, three layers (red, green, and blue) are used.
Power Requirements
By using this mode of operation, ChLCDs have been studied for use in very-low-power applications such as mobile phones. Despite the bistable nature of the display, the relatively high voltage required can counter this potential advantage of very low power, especially if the display has to be updated regularly. This has reduced the applicability of ChLCDs in many mobile applications, especially those that require frequent image updates.
However, large-area displays require the integration of other components to form a system whose power requirements as a whole must be considered; the overall power used can then be divided into three categories:
• Power to change the image (display power): In three-layered devices, this is in the range of 5–15 W/m2; it occurs only while the image change is taking place. The average power therefore depends on the time taken (defined, for example, by the number of displayed lines and the drive scheme) and the voltage required. Clearly, the contribution to the overall average "image change" power budget depends on how often an image change is made.
• Power to operate the display as a whole (i.e., background or system power): This includes any PCs and/or controllers to capture incoming data (usually the display is connected to some outside source to download information and images), power supplies, and other electrical items involved in running the display. This is typically in the region of 350–700 W per display. In applications where the image requires very infrequent changes, most of this equipment can be shut down between image changes to reduce the power to a very low level (<1 W/m2).
• Power to peripheral equipment such as lighting, heating, and/or cooling as required: This is dependent on the location of the display (indoors or outdoors), the climate (hot or cold), and specification requirements such as image refresh time, etc. The mechanical design of the system is a key feature in defining these parameters.
° Power for lighting depends on how reflective the display is, how bright the surroundings for the display are, and the design and type of lamps; typically, this is in the region of 30–100 W/m2.
° Because the transmitted light is taken up by an absorber behind the display, there is a conversion of light into heat at this point; thus, some cooling is often required and can consume up to 100 W/m2, depending on the design of the mechanics in the display and the climate in which the display is used. Indoor displays should not require cooling.
° Heating may also be required to pro-vide a reasonable switching time and can consume 50–250 W/m2, again depending on the climate, display design, specification requirements, and drive scheme used. Indoor displays should not require heating.
Thus, when comparing the power used by large-area displays, it is important to under-stand which power-consuming components are included or not included in the power specification. Displays used indoors will require much less power than those used outdoors.
When comparing reflective displays with typical values for other large-area display types, the system in which they work also has to be considered. For example, a large-area backlit LCD designed for outdoor use has to emit in the region of 2000 cd/m2 to compete with direct sunlight2 – in this mode, it uses about 2 kW/m2, but at night this can be much less, at about 0.5 kW/m2. "Background" power (PC, etc.) to run the system should be added to achieve an overall power budget that is in the region of 1.0–2.5 kW/m2. The move to more efficient backlights will help reduce this power budget.
Large-area (20 m2) outdoor LED displays (typically having a pitch of 20 mm) use in the range of 0.5–0.7 kW/m2 over a 24-hour period (depending on brightness and pixel pitch). Smaller-pitch LEDs will consume significantly more power, and displays used indoors will use less power.
A bistable ChLCD cannot show video images, so like-for-like comparisons are not possible. Therefore, the power required to provide repeated changing of static images has to be considered. The reflective nature of ChLCDs replaces the power required to handle direct sunlight by what should be a lower power requirement to illuminate the display at night.
Large-Area ChLCDs
In 2002, Magink and Mitsubishi Electric Corp. launched the first large-area outdoor ChLCDs, showing static images on displays ranging between 2 and 13 m2. Figure 1 shows such a display, which consists of ChLCD panels in a glass-fronted temperature-controlled box. The contrast ratio was 6–8:1 with an image refresh taking place every 7 sec and lasting for about 2 sec at 20°C. The display and background system used about 25 W/m2 for each image change and 16 W/m2 between image changes; overall, about 17 W/m2 was used. The overall power consumption was dominated by heating (100–250 W/m2), cooling or air circulation (<6 W/m2), and illumination (80–100 W/m2). Overall, the display used an estimated 150 W/m2, but this was the first device of its kind.
Magink Display Technologies
Fig. 1: Shown is a 2-m2 bistable ChLCD introduced in 2002 in Mumbai, India.
Other bistable ChLCDs have been used as train timetables, indoor information displays,3 and traffic guidance signs.4 Displays of this type tend to be smaller and of higher resolution; thus, there are more lines to address per m2, which uses more power. For example, AEG multicolor displays4 of this type consume 150 W (~70 W/m2) in daylight and 750 W (~340 W/m2) at night.
High-Contrast ChLCDs
Image quality is a critical issue for applications such as advertising billboards. The typical contrast ratio of ~6:1 for the bistable operation mode, while adequate for alphanumeric displays, was not good enough for full-color images. Improvements to the liquid crystal and the drive schemes resulted in a contrast of 10:1, but at the expense of a longer image refresh time.
To provide the quantum leap in image quality required for many high-end applications and also to address some other issues found in the use of ChLCDs for large areas, the contrast ratio was improved by making use of the homeotropic phase as a display state rather than as an intermediate state. Magink developed a method of doing this that has led to a high-contrast ChLCD featuring a contrast ratio of 50:1 on a pixel and, as a result, also has major impact on the color gamut, which increased in area by about 90%.
The homeotropic state is metastable and only exists while an electric field is applied across the thin film of a cholesteric liquid crystal. Thus, the display requires constant power to maintain the areas of an image that are required to be in the homeotropic state. Some areas of an image are used in the bistable mode; thus, the display is a hybrid bistable-driven device. Consequently, more power is used by the ChLCD itself in the high-contrast mode, compared to the fully bistable mode. However, when using the high-contrast mode of operation, other features are improved that can allow power savings elsewhere. For example, the total image refresh time is very fast (currently <170 msec at 20°C and <800 msec at –10°C), which reduces the requirement for heating in very cold climates, and for indoor use (over the range of 15–50°C), video images at 60 fps can be shown. The high-contrast display can also be used in the lower-power conventional bistable mode if required; in this case, the contrast is about 15:1.
The high-contrast mode was initially used in a box similar to that used by the bistable display mode (Fig 2). It consumed about 54 W/m2 when showing an "average" image. The power budget per image change is not relevant in this case, as the power is provided all the time and image changes cause negligible increases in power use. When video mode is used, the power consumption increases to about 107 W/m2. Additionally, there is the background power (approximately 500 W per display) and cooling during the sunshine hours, which, in this design, can be up to 150 W/m2. In this first design, some significant heating was also provided (150–350 W/m2) but was not really required. At night, when little or no cooling is required, the lights consumed about 60 W/m2. Thus, on average over 24 hours, the display, depending on conditions, required over 300 W/m2 for a 13-m2 display (this was the largest display size available).
This system design did not take full advantage of the properties of the high-contrast mode; it used the same basic design as that used in the bistable mode of operation. By making more use of the properties of the high-contrast mode, lower overall power can be realized. For example, taking the display out of the glass-fronted box can, depending on the design, reduce the need for forced cooling (although some cooling is still currently used) and the lack of front glass reduces parasitic reflections, which leads to a further improvement in image quality, which in turn allows less lighting to be used. The fast image changes are less susceptible to low temperatures (at –10°C the image refresh time is <1 sec), thus eliminating the need for heating of the display.
(a) (b) Magink Display Technologies
Fig. 2: Shown is a 6-m2 high-contrast ChLCD – (a) day and (b) night views – erected in Cannes, France, in 2006.
The first outdoor displays of this type (Fig. 3) that show high-contrast static images have no heating and require less lighting and involve an average power consumption of 110–120 W/m2 (over 24 hours). Lower-power drive electronics, background, and lighting requirements suggest that this can be reduced to below 80 W/m2 for a 20-m2 outdoor static display.
Making use of these savings for indoor use (often with video images), where there is no cooling (or heating) requirement but with constant and more powerful lighting, about 120–130 W/m2 is possible. Compared to equivalent LED or backlit LCDs that also show video images, the virtues of a reflective display are quite clear in terms of power savings.
Summary
Large-area outdoor reflective displays, bistable or not, can use less overall power than equivalent emissive and transmissive displays which have to deal with direct sunlight. ChLCDs can be operated in a conventional "low-power" bistable mode. However, in large-area displays, it is the system and peripherals requirements that dominate the power budget. In a high-contrast display mode that inherently uses more power than the bistable mode, the positive attributes lead to less peripheral power being required. This leads to a reduction in power consumption. Figure 4 illustrates some typical average values mentioned here – the low-power nature of the reflective mode is readily appreciated, and when used optimally the unexpected advantage of employing a higher-power display (i.e., a high-contrast ChLCD) to provide a lower-power display overall is also clear.
Acknowledgments
The author wishes to thank colleagues at Magink Display Technologies and Dr. Z. Hara at the Mitsubishi Electric Corp. for discussions and data used in this review.
References
1See displays from Kent Displays Inc., which pioneered this type of device.
2Screen Technology Ltd. data sheets.
3LCTEC M1 SVGA ChLCD module.
4AEG MIS Geameleon bistable ChLCDs. •
Fig. 3: Shown is a 20-m2 high-contrast ChLCD from Pensecola, Florida, 2008.
Fig. 4: Comparison of power usage in different outdoor large-area displays. Source: Coates, Magink.