Display Applications

Overcoming the Drawbacks of Fluorescent Lamps

Liquid crystal display (LCD), thanks to continued improvements in resolution, response rates and scalability, has become the pervasive display technology for mobile phones, monitors, notebooks, HDTVs and other consumer electronics. Since LCD panels are transmissive and emit no light of their own, they require a backlight to provide illumination. Commonly, LCD backlighting units (BLUs) employed cold cathode fluorescent lamps (CCFLs), similar to those used for commercial overhead lights, as their light source. However, CCFLs have a number of drawbacks. They require a high voltage power supply and generally are the highest power consuming component in large format displays and HDTVs. CCFLs contain mercury which has special disposal requirements and faces increasing limits on its use in many countries. Also, the space needed by CCFLs constrains how thin an LCD panel can be made. And as CCFLs are a tube-based technology, they are usually the first component to fail in an LCD display.

Light emitting diodes (LEDs) offer a semiconductor-based lighting solution which overcomes the limitations of CCFLs. With continued advancements in brightness and efficiency, LEDs are displacing CCFLs in backlighting applications, and as their price continues to drop, will take their place as a general lighting solution as well. LEDs deliver higher brightness than CCFLs and better power efficiency (more lumens per watt), use a lower-voltage power supply and generate less heat. LEDs can produce a much wider color gamut making movies and images appear more vibrant and lifelike. Because of their compact nature, LED backlights can enable ultra-slim displays and HDTVs less than half an inch thick. As a solid state component, like the other semiconductor devices in mobile phones, computers and HDTVs, LEDs have much longer lifetimes than CCFLs.

Harnessing the Benefits of LEDs

However, harnessing all the benefits of LEDs for backlighting still entails challenges. As point sources of light, LEDs can be used in an array topology in the backlight to directly illuminate the LCD panel. An array requires a high number of LEDs and therefore can be very expensive. In addition, in order to properly diffuse the light, arrays require a greater distance between the LEDs and the LCD panel, resulting in a thicker display. A thinner and more cost-effective solution is to use LEDs in an edge-lit configuration with a light guide panel (LGP) to turn the light into the viewing plane and distribute it across the display. This requires fewer LEDs but introduces the problem of maintaining uniformity of brightness over the entire backlight area. Maintaining uniformity and achieving the full benefits of edge-lit technology necessitates a high-efficiency LGP that can be economically manufactured.