Increasing LCD Energy Efficiency with Specialty Light-Management Films

With increasing focus on the demand and costs of energy, LCDs must become more efficient interms of utilizing light and energy. Low-loss components in LCDs can greatly improve system efficiency and help address the need to reduce power consumption. This article describes tests that show that LCD efficiency can be improved 50% or more by using a reflective polarizer and high-efficiency reflectors.

by Tao Liu and Mark O'Neill

UNLIKE other display devices, liquid-crystal-display (LCD) devices are predominantly transmissive devices. A backlight placed behind the LCD is used to illuminate the image formed by the display pixels. An LCD is inefficient, as typically less than 10% of the available light passing through the display and less than 1% of the total input electrical power to the display is emitted as usable optical power.¹

With increasing global demand for energy resulting in rising energy costs, consumer awareness of energy use is increasing, and energy consumption has become a key purchasing factor for items ranging from vehiclesto LCD TVs. In addition, global energy regula-tions are focused on addressing the increasing power consumption in consumer electronics.

Improving the efficiency of LCDs has been an industry trend for years. Improved manufacturing of the LC cell, light sources, and image-processing schemes have all enhanced the performance of LCD panels. 3M is continuously developing low-loss reflective polarizers and high-efficiency reflector components that work in concert with these improvements to further increase LCD energy efficiency.

Efficiency Improvements

To date, LCDs have mainly relied on cold-cathode fluorescent lamps (CCFLs) as the backlight source. Because newer technology light sources such as light-emitting diodes (LEDs) offer improved efficiency, the electrical-to-optical power-conversion efficiency is improving. More light is generated at lower power.

Geometric-enhancement films, such as prismatic films and gain diffusers, can be used to further improve efficiency by concentrating more of the backlight angular distribution toward a centralized viewing region, resulting in more light toward the viewer. However, the luminance outside of the centralized region is compromised, which reduces viewing-angle performance. Moving the light from one region to another results in more light energy for viewers in the central region, but the net overall efficiency is often lower due to losses in the added films. In addition, the concentration of light toward the central region may result in a viewing region too narrow for the intended application.

LCD-panel transmission has improved through manufacturing improvements. LC-cell architecture and smaller transistors (such as the use of poly-Si TFTs) have increased the aperture of each pixel, allowing more light to be transmitted through the panel. Some wide-view technologies counteract this improvement. As cell structures get more complicated to provide wider viewing angles, the amount of light transmitted through the panel may suffer.

Active dimming of the backlight has become increasingly prevalent. 0-D dimming, where the entire backlight is modulated as the local-scene luminance changes, reduces the power consumption on an overall scale. Local dimming, where localized areas are modulated as its average luminance requirements change, can further reduce the power consumption of LCDs. But while these techniques reduce power consumption, neither improves optical efficiency.

Low-loss components can be applied with all these techniques to enhance display efficiency. Reflective polarizers, such as Vikuiti^™Dual Brightness Enhancement Film (DBEF), are low-loss components that pass one polarization of light and reflect the orthogonal polarization of light.² When aligned with the LCD panel's rear polarizer, the reflected polarization is recycled in the backlight cavity for further use instead of being absorbed in the rear display polarizer, resulting in efficiency improvements of 30–50%. High-efficiency reflectors, such as the Vikuiti^™ Enhanced Specular Reflector (ESR), have high reflectivity over all angles of incidence, resulting in low absorption losses.² Because recycling elements such as geometric-enhancement films, reflective polarizers, and diffuser plates are used, the benefit of high-efficiency reflectors increases.

Power that is saved by using a more-efficient LCD can be applied to different areas of a device, benefiting other consumer needs. Mobile devices including cell phones, music players, and notebook computers run on batteries that can limit their operation. 3M's measurements indicate the LCD module consumes 25% or more of the total system power in portable electronic devices. Efficiency gains can enable such devices to run longer between charges, increase display luminance without added power consumption, reduce the battery size for display-performance requirements, or allow the power budget to be re-distributed to other device needs.

Plug-in devices such as computer monitors, TVs, and digital signs also benefit from more-efficient LCDs. The backlight consumes 70% of the power needed to run these devices. Improvements in display efficiency can be used to increase the luminance without additional power consumption, or reduce total power consumption and energy costs with the same luminance, or reduce the device's heat load.

System Design and Supply-Chain Considerations

All displays have some basic requirements, including proper luminance level, luminance angular profile, and uniformity. In addition, the form factor, battery life (for mobile devices), power consumption, and general viewing perception must be considered. Meeting all these requirements is challenging because some specifications counteract others. One tenet in system design is using low-loss components to improve system efficiency. Managing improved efficiency is left to the display designer. By altering the stack of optical films used in the backlight, designers can improve the efficiency by achieving the following:

• Higher luminance at the same power consumption.

• Reduced power consumption for the same luminance.

• Lamp-count reduction for the same luminance, reducing the bill of materials of the backlight maker.

• Lower display temperatures with different thermal-management requirements, which may allow for less-expensive options or even different design considerations.

The above can accomplished while maintaining visual quality standards, product reliability, and lifetime. However, changing backlight components has multiple impacts on the supply chain. The addition or substitution of films may increase the cost at one level, such as the backlight supplier, while simultaneously lowering the total bill of materials and operating cost.

As the film configuration changes, luminance and optical efficiency improvements can be implemented in various ways that can affect the entire supply chain.

• Fewer CCFLs or LEDs reduce the bill of materials of the backlight maker.

• Fewer light sources may require fewer inverters.

• Fewer light sources may also allow replacement of power supplies or other electrical components to less-expensive options at the system integrator.

• Lower system temperatures may allow for changes in the thermal management to less-expensive options or even different design considerations

• Lower power draw will reduce energy consumption for the end user.

The benefits of reflective polarizers and high-efficiency reflectors can easily be characterized in terms of luminance and power consumption. The total cost of a display device is realized throughout the value chain with the final decision at the brand.

To investigate how much the film stack can affect the power budget of a mobile device, we conducted experiments comparing displays using reflectors and reflective polarizers against those that did not. The results are presented here.

Results: Mobile Devices

The run time of mobile devices can be enhanced by using high-efficiency reflectors and reflective polarizers. The film stack of a 3.5-in. mobile entertainment system was modified to demonstrate the benefits of low-loss components. The luminance of the device was measured using various film stacks as listed in Table 1.

The device backlight was further modified to control the power to the LED sources and monitor luminance. Luminance as a function of power was mapped for each device in Table 1 with results shown in Fig. 1. Each device can now be set to the same luminance level by controlling the power settings for the sources.

Run time was evaluated each time a single modification was made. After each modification, the backlight power settings were adjusted to produce the same luminance as was present before the modification. The savings in backlight power required, caused by an increase in light efficiency after each modification, thus enabled a net increase in run time.

The display power settings were modified to simulate each film stack. To evaluate device #1, the backlight was set to full power. To evaluate device #2, the backlight settings were changed to 84% power (the setting if film stack #2 was in the device under test). Similar adjustments were made to evaluate devices #3–#5. A video was played on continuous loop with the backlight on. Run time was measured from fully charged battery to when the device shut down from lack of power.

Modifying device #1 to device #2 eliminated one component, modified the diffuser level, and added the reflective polarizing capability of Vikuiti^™ Multifunctional Film BEF RP2-RC (BEF RP2-RC), increasing run time by 7%.² The luminance profile for device #2 is broader than that for device #1. Typically, this decreases the peak luminance. With the addition of the reflective polarizer, polarized light is emitted from the backlight, resulting in higher transmission through the LCD panel. In device #3, the reflector is changed from silver (device #2) to ESR. The improvement in reflectivity results in lower absorption losses in the reflector. The higher peak luminance translates to an additional 13% improvement in run time. In device #5, the addition of Vikuiti^™ Thin Brightness Enhancement Film II (TBEF2-T-i) compresses the luminance profile of device #3 to be comparable to device #1.² The geometric enhancement further improves the peak luminance resulting in a 48% increase in run time of device #5 as compared to device #1. Table 1 lists peak luminance and run-time improvements for various reflector/film combinations in this experiment.

By changing to low-loss components and maintaining the same display luminance, a mobile entertainment system's run time increased 48%, from 232 to 343 minutes.

Results: Notebook Computer

The power allocation of notebook computers can benefit from using high-efficiency reflectors and reflective polarizers. The film stack of a 15.4-in. notebook computer was modified to demonstrate power savings. The backlight power was regulated by external control. Power and luminance levels were recorded for the film stacks in Fig. 2.

Fig. 1: Luminance as a function of backlight power for the each modification listed in Table 1.

Fig. 2: Luminance as a function of backlight power for the modifications listed in Table 2. Energy savings is shown for a luminance of 195 cd/m² in each device.

The power to the backlight was modulated to provide an axial luminance of 195 cd/m² for each film-stack configuration. Power draw for the backlight at this luminance and the associated power savings are listed in Table 2. Modifying the enhancement films from crossed Vikuiti^™ Brightness Enhancement Film II (BEF2) to Vikuiti^™ Brightness Enhancement Film II (BEF2 G2) and Vikuiti^™ Dual Brightness Enhancement Film II (DBEF II) allows removal of the coversheet while adding the reflective polarizer capability.² The subsequent luminance profile of device #2 is broader than device #1 because only a single prism film is used. The reflective polarizer more than compensates for the expansion of luminance profile from the change in geometrical enhancement. The power required for a luminance of 195 cd/m² decreases by more than 300 mW (>5%). Further modification to include a second sheet of BEF2 G2 in device #3 (to obtain a crossed BEF2 G2) compressed the luminance profile to be slightly less than that for device #1. This geometrical enhancement permits an additional power reduction of more than 600 mW while maintaining a luminance of 195 cd/m². Finally, replacing the diffuse white reflector with the Vikuiti^™ Durable Back Reflector (DBR-220), a modification of ESR for notebook computers, results in less light absorbed by the reflector component.² Device #4 further reduces power consumption of the backlight by more than 250 mW as compared to device #3 and more than 1.2 W (20%) compared to device #1.

Changing the film stack of a notebook computer to include low-loss components and maintaining the same luminance decreases the power consumption of the backlight by 1.2 W or 20%. This power can be reallocated to add functionality such as increasing processor performance, cameras, or fingerprint readers. Alternately, the portability of the computer can be improved by reducing the battery size and computer weight.

Results: LCD Monitors

The energy consumption of LCD monitors can benefit from using high-efficiency reflectors and reflective polarizers. Benefits from improving the efficiency of an LCD can result in reduced power use by reducing light source and power-supply components.³

Two mainstream Lenovo monitors (models L174 and L197w) clearly demonstrate power savings using a reflective polarizer in their backlights and half the number of CCFLs as their original counterparts (models L171 and L194w). Vikuiti^™ Diffuse Reflective Polarizer Film (DRPF2) was added to the L174 model.²

Vikuiti^™ DBEF-D2-280 (DBEF-D2-280) was added to the L197w model.² With the addition of the reflective polarizer, the backlight emits polarized light to the LCD, resulting in less absorption by the rear polarizer. Optical-efficiency improvements allowed the removal of two CCFL lamps and their associated electronics from each monitor. The modified displays meet luminance and uniformity specifications while enabling power savings of 6 W or more than 30%.³

The addition of a reflective polarizer results in energy savings and fewer components, including some that contain mercury. These new Lenovo monitors are Electronic Product Environmental Assessment Tool (EPEAT) Gold qualified. EPEAT is among the tech industry's most coveted environmental designations.⁴

Results: LCD TV

The energy consumption of LCD TVs can be reduced by using reflective polarizers. Benefits from improving LCD efficiency can reduce power consumption, the number of light sources, and TV temperature.

The film stack of a standard 40-in. LCD TV having a diffuser plate, gain diffuser, Vikuiti^™ Brightness Enhancement Film III (BEF3-10T), and gain diffuser was modified to a diffuser plate, crossed BEF2 G2 and Vikuiti^™ LED Efficiency Film (LEF-D), and a reflective polarizer.^2,5 The combination of the geometrical-enhancement films and reflective polarizer recycle much more light than the original film stack. This results in higher peak luminance from the prism films as well as light polarized for the LCD from the reflective polarizer. Luminance levels were recorded and total power monitored with a plug-in watt meter. Internal temperatures of the TV backlight cavity were also measured.

The TV settings were adjusted to best match the luminance levels of the TV in each configuration. The increase in efficiency and luminance due to the prismatic enhancement films and reflective polarizer allowed removal of eight CCFLs from the backlight without affecting system uniformity. The results are listed in Table 3. Thermal images are shown in Fig. 3 showing a temperature drop of more than 7°C.

The use of a reflective polarizer in a TV application improves the optical efficiency of the TV. Less light is absorbed by the LCD. The efficiency improvement can be used to reduce the number of CCFLs from 20 to 12. In addition, a power savings of 102 W (52%) and a 10°C lower internal temperature can be realized.

Conclusions

LCDs are continuing to proliferate throughout electronic applications. This is occurring as concerns for energy demand and costs are increasing. Regulatory guidelines for energy consumption are beginning to appear. Low-loss components in LCDs can greatly improve system efficiency and help address the need to reduce power consumption. Tests show that LCD efficiency can be improved 50% or more by using a reflective polarizer and high-efficiency reflectors.

Benefits of the efficiency boost can be used in a variety of ways across the applications of LCDs. Mobile devices may have extended the battery life to provide up to 48% longer run time in mobile entertainment systems. The power budget of notebook computers can be reallocated to improve functionality as the power consumption is decreased by up to 20%. Cameras, fingerprint readers, and processor speed can use the power redirected from a more-efficient display. Monitors can reduce power consumption by eliminating light-source components, saving more than 30% of the energy consumption. Finally, power consumption of TVs can be reduced by more than 50% while simultaneously reducing heat load and removing components from the bill of materials.

Acknowledgments

The authors are grateful for useful discussions with Kris Tyson, Paul Kelly, Dave Lamb, Tracey Peacock, and Fei Lu from 3M Optical Systems Division.

References

¹White Paper: "The Power to Change," Meko Ltd., 2008, from data presented by Toshiba at DisplaySearch U.S. FPD Conference 2005.

²Additional information on Vikuiti^™ display films can be found at http://www.vikuiti.com and http://solutions.3m.com/wps/portal/3M/ en_US/Vikuiti1/BrandProducts/main/productliterature/.

³Data presented by Lenovo at DisplaySearch US FPD Conference 2008.

⁴Lenovo press release, http://www. lenovo. com/news/us/en/2008/03/epeat.html.

⁵J. Anderson, C. Schardt, J. Yang, B. Koehler, B. Ostlie, P. Watson, K. Ingham, S. Kienitz, and A. Ouderkirk, "New Back Reflector and Front Film for Improved Efficiency of Direct-Lit LED Backlights for LCD TV," SID Symposium Digest 38, 1236 (2007). •

Fig. 3: Thermal images of a 40-in. LCD TV with modifications as listed in Table 3. With the use of low-loss components, the front-panel temperature decreased from 37.8 to 30.1°C.