Backlight-Unit Basics
by Adi Abileah
Flat-panel displays have come a long way in recent years. LCDs are the dominant technology today, and behind every LCD panel is a backlight. Even the newly developed transparent displays rely on illumination from behind. In this issue, we will present two papers on backlight technology, one describing the general direction of backlight units (BLUs) and the second showing state-of-the-art design considerations for a compact optical structure with local dimming.
There are two basic BLU concepts: (a) a cavity backlight with many light sources (e.g., CCFLs or LEDs) distributed in the area, a reflector behind and diffusers above, and (b) an edge-illuminated light guide with light sources at the edge and optical elements on the light-guide surface to extract uniformly the light propagating along the cross section. In both cases, it is very important to have efficient light coupling and proper diffusers to obtain good uniformity and enhanced illumination.
Each BLU consists of (a) light sources (CCFL, LEDs), (b) light-distribution means (cavity reflectors or a light guide), (c) diffusers and light-shaping elements (BEF, holographic diffusers, DBEF), and (d) driving electronics for the light sources. When optimizing the BLU, all aspects have to be included. In the articles in this issue, we will touch on some of these aspects.
Until recently, the BLU light source of choice was the well-established cold-cathode fluorescent lamp (CCFL). Over the years, CCFLs have improved in efficiency (20–60 lm/W) and at the same time became thinner (e.g., 2.8-mm outer diameter). This allowed thinner designs both in cavity mode and with edge illumination. The thin CCFLs match thinner light guides, with decent coupling efficiency. The CCFL also improved over time in terms of better gas mixtures and more robust hollow cathodes. And its total luminance, efficiency, and lifetime (e.g., 30,000 hours) improved. Last, in mass production, they were also quite inexpensive.
Recently, LEDs have been taking over the backlight arena with higher efficacies (80–130 lm/W). As LEDs move into mass production, their prices are becomingcompetitive with CCFLs. LEDs are also penetrating into the area of general lighting (automotive, industrial, household).
The major breakthroughs in LED development have been (a) invention of blue LEDs. (b) light-extraction techniques from the emitting layer, and (c) development of white LEDs. The blue LED was the missing link needed to make multi-color arrays of LEDs. It was developed through the pioneering work of Dr. Shuji Nakamura form Nichia Japan.
The main leap-frog of luminance efficacy was performed when people realized that the LED theoretical maximum was very high (250 lm/W), but that the emitting layer had a very high index of refraction and the light was trapped internally. Surface interface techniques to extract the trapped light provided a significant improvement from the 20-lm/W range to the recently reported 150 lm/W. This represents amazing progress in just a few years. With commercial LEDs in the 80–130-lm/W range, we expect a fast penetration not only to backlights but also to general illumination, as presented in this issue's article, "Light Guides Evolve from Display Backlighting to General Illumination," by Brett Shriver of Global Lighting Technologies (GLT). Recent reports about LCD-TV trends show that more than 50% are using LED-based BLUs.
The white LEDs are actually a blue LED with yellow phosphors coated on the upper layer. The blue peak is one component of the spectra. Part of the blue light is absorbed by the yellow phosphor and emits wide-band spectra with green and red peaks. This generates a well-controlled white spectrum. Controlling the phosphor concentration and mix affects the white balance. Individual red, green, and blue (RGB) LEDs have wider color gamut (e.g., 110% NTSC) combined with the LCD color filters. However, their relative stability and long-term aging have to be controlled. White LEDs have a smaller color gamut (e.g., 72% NTSC), but much more stability in terms of white balance over time. They are preferred for BLUs due to this stability, but also because of lower cost and fewer driving electronics.
A key topic related to LEDs is heat management. LEDs have very fast responses and operate in a wide range of temperatures. However, in hot temperatures they shift the dominant emission wavelength and have reduced luminance. This is followed by reduced life span (< 100,000 hours). The junction temperature should be below the vendor specs (e.g., 110°C), and preferably much lower (e.g., 80°C). It requires very careful design to extract the heat from the inner layers of the LED structure. Recent designs are handling this challenge efficiently.
A major link in the development of BLUs has been the development of diffusers, which make the light emitted from either a cavity BLU or light-guide structure become uniform and controlled over angles. Uniformity is needed to hide individual light sources, or surface-extracting elements. The idea of angular control is to direct the light to the useful viewing range and gain luminance. For instance, LCDs are viewed within wide angles left and right from normal, while span of the the vertical angles' span is relatively narrow. This control can be achieved with brightness enhancement films (for example, 3M's BEF™) or holographic diffuser (angular controlling films). The BEF was the first concept to gain wide acceptance in many LCD devices. In this case, the small prismatic structures are set horizontally to concentrate light in the vertical direction. This topic is close to my heart (my patent introducing this technology was sold to 3M). However, now there are more improvements and developments, some of which are covered in the GLT article, as well as in the second article, " Creating a More Efficient Light Guide for a 2-D-Type Local-Dimming LCD Backlight" by Dr. Kälil Käläntär.
As mentioned previously, the two major BLU types include cavity and edge-illuminated light guides. For a while, the cavity BLU was preferred because it is more efficient. However, a cavity BLU is significantly thicker. There has therefore been a trend to switch to edge-lit light guides. This coincides with improvements in edge-light coupling. In earlier designs, only a small portion of the light of the source was captured in the light guide (e.g.. 30%). With better optical designs that include reflectors surrounding the light sources, and the matching of light-source emission patterns to the light-guide acceptance angles (numerical aperture), it is now possible to more than double the efficiency over earlier designs. This makes the edge-lit light-guide option even better than the cavity BLU. The topic of light coupling is covered in Käläntär's article and also explained in the Shriver piece.
As we have seen, the BLU plays a major role in LCD structure and performance, and it is a complex subsystem. One additional development to improve the overall performance is local dimming. The concept is to dim the backlight in areas of the image that are dark, while keeping full luminance in areas with full brightness. This process has advantages: (a) improved contrast ratio of the image and (b) reduced power. In parallel, there are several challenges involved: (c) complex backlight design with sectional illuminants, (b) sectional driving electronics matching with the backlight design, and (c) image processing that generates the backlight sectional driving matching to the LCD image (frame by frame). The solution proposed by Käläntär handles the optical sectional design in a very efficient and elegant manner.
Future work on BLUs will focus on (a) further improvements of light coupling at the edge of light guides, (b) integrated optical design of the light guide with optical elements, (c) polarization enhancement through optical elements with better light recycling that matches the polarization orientation to the display rear polarizer, (d) local-dimming integral elements that allow easier control of local dimming, (e) optical elements for control of angular distribution and enhanced luminance, (f) LED optimization within their structure and as part of the BLU system, (g) transparent BLUs for new designs, and (h) color-saturation improvements.
I hope that this introduction provides a helpful, general overview of backlight units and that you will enjoy reading these two articles. •