Once used primarily for backlighting small–to–medium-sized LCDs in portable and handheld devices, LED-based edge-illuminated light-guide technology has transitioned from cell-phone backlights to PC monitors and large TV backlights and is now even being used to illuminate entire rooms.
by Brett Shriver
WHAT IS THE BEST backlighting technology? That depends on what your application is and who you talk to. One thing is certain: LCD backlighting has come a long way since the days of passive-matrix mono-chrome character displays. Historically, many different light sources have been used for LCD backlighting, including incandescent lamps, electroluminescent (EL) lamps, vacuum fluorescent (VF) lamps, cold-cathode fluorescent lamps (CCFLs), and, more recently, light-emitting diodes (LEDs).
CCFLs are still used in legacy applications such as medical equipment (e.g., defibrillators and patient monitors) and point-of-sale (POS) equipment (e.g., ATMs and slot machines). In medical applications, for example, components tend to be specified for use over a long period of time. Extensive and expensive testing and verification are required to replace them, so a switch to new components is not undertaken lightly. But, as is well-known, CCFLs are being displaced by LED-based backlight units (BLUs) in virtually every LCD, from smart phones to laptop and tablet PCs to large-screen TVs. According to a survey conducted by LED Inside, the LED research division of TrendForce, the LCD-TV market is expected to increase to 70% in 2012 and, according to WitsView, the panel research division of TrendForce, LED technology's penetration rate is expected to increase to 65–70% of TVs sold.1
LEDs solve many design challenges, providing increased luminance with less heat and greater uniformity, longer lifetime of the BLU (including better white-point consistency over the operating lifetime), improved color gamut (when discrete RGB LEDs are used), wide-range dimming to suit changing ambient light conditions, wider operating temperature range, lower voltage (no inverter), no mercury, and a thinner BLU that is more efficient and easier to integrate into slimmer designs than is CCFL-based technology. Also, since luminous efficacy and color consistency among LEDs in batches can vary, LED manufacturers can bin the components for flux, color, and forward voltage. Fine binning for brightness and color can also be used to obtain the proper consistency for color-critical applications. While there are still some applications for EL and CCFL, LED backlighting is being used by the vast majority of LCD applications, while LED technology continues to improve and the cost premium over CCFL moves ever closer to zero.
BLU Efficiency Backgrounder
Where space is at a premium and the thickness of the BLU must be kept to a minimum – and where isn't it? – a light guide using white or tri-color (RGB) LEDs is the implementation of choice for most designers. Years ago, backlight units utilizing edge-illuminated LED lighting with high-efficiency molded light guides containing pixel-based micro-optical light-extraction technology were just emerging as the technology of choice for a variety of backlighting applications. These included displays and keypads in cell phones (including flip and clam-shell versions), as well as the LCDs in digital cameras, MP3 players, and PDAs with cellular phones and Internet capability.
The introduction of these light guides using molded-in LEDs goes back to the 1990s, when they were also referred to as "light pipes" and were starting to look like viable alternatives to LED arrays (and CCFL and EL backlights). A light guide is a device designed to transport light from a light source to a point at some distance with minimal loss. They are typically made of thermoplastics such as acrylic or polymethyl methacrylate, also known as PMMA. Light is transmitted through a light guide by means of total internal reflection (TIR). The author's company, Global Lighting Technologies (GLT), pioneered molded light-guide technology in 1998 using pixel-based light-extraction technology combined with high-brightness white LEDs encapsulated within the polymer structure of the light guide (Fig. 1). These light-guide assemblies utilized hundreds of thousands of micro-optical elements – miniature reflective and refractive surfaces – per square inch to extract light precisely and uniformly across the LCD panel to deliver more collimated light, which increased the efficiency of the optical system, requiring up to 30% less power or up to 30% fewer LEDs than other light-extraction technologies such as printed, stamped, etched, or V-groove. By 2007, these molded light-guide assemblies had moved beyond their established turf in the portable/ handheld arena and were delivering luminances from 3,500 to 12,000 cd/m2 (depending upon the number and type of LEDs), with excellent uniformity for mid-sized LCDs (3.5–7.0 in. on the diagonal) while lowering overall manufacturing costs.
Fig. 1: An early example of a light-guide BLU shows molded-in LEDs (inset).
Enter Edge Lighting
As opposed to direct lighting, which places LEDs directly behind the LCD in a cavity, edge-lighting positions the LEDs on the edge of the light guide using side-firing LEDs that focus the light into a high-performance optical light guide, which extracts, directs, and distributes the light as required by the application, employing one of several different light-extraction technologies available. The light-extraction technology can be printed dots, chemical or laser-etched dots, V-grooves, or pixel-based, depending on the requirements of the application (Fig. 2). Up to this point, the molded-in LEDs had worked well in use with the focused LEDs with the built-in reflector cup, etc., but when the LED industry began developing LEDs specifically designed to couple with light guides (surface mountable, side emitting, thinner, and with wide-output-profiles) there was no longer a need for the insert molding, which was actually rendered impractical. GLT continued to refine LED-based edge-lighting technology for use in a wide range of LCDs, from < 1 in. to > 50 in. on the diagonal, using high-brightness white LEDs (HBLEDs) as the light source.
Fig. 2: LEDs positioned on the edge of the light guide employ pixel-based light-extraction features. As shown in the background, mechanical holding features can be designed into the backlight.
With regard to white vs. RGB LEDs, only white LEDs are currently used in GLT's LCD backlights. Though color mixing can be addressed during initial design by using larger transition areas and lens arrays in front of the devices, the RGB-LED-based BLUs create issues with the successful control of color mixing not only initially during production, but also because of drift in color over the life of the product due to the LEDs degrading at different rates.
A wide range of display applications would utilize the benefits of edge lighting – LCD and keypad backlights for mobile phones, GPS devices, portable DVD players, scanners, automotive interior displays, programmable touch-screen thermostats, home-security system controllers, industrial instrumentation and control displays, laptop and desktop computers, LCD TVs, and more.
Laptop computers widely adopted edge-illuminated backlighting with white phosphor LEDs and have been a driving force behind the development of ever thinner BLUs. In these applications, a blue LED is imbued with an amber phosphor to convert the visible wavelength to white. The white LEDs still deliver a percentage of the color spectrum (per CIE 1931 chromaticity diagram XYZ color space) comparable to CCFLs (approximately 75% compared to 70–80% for CCFLs) while consuming less power. Traditionally, these backlights have been 2–3 mm thick; however, with advances in LED and light-guide technology, backlights as thin as 0.4–0.6 mm are being created, enabling reductions in the devices' overall weight and thickness.
TV Backlighting
Techniques to utilize LEDs for LCD-TV backlighting were being explored as early as 2005. One solution included RGB-LED backlight modules or "tiles," each incorporating a color-mixing waveguide with light-emitting optical elements arranged in a pattern behind the LCD. This provided wider LED spacing that minimized thermal issues and offered higher efficiency with fewer LEDs.
In 2006, GLT teamed with Luminus Devices, Inc., Woburn, MA, to produce modular LED-based edge-lighting assemblies for large-screen LCD TVs. Luminus's PhlatLight LED technology, combined with GLT's light guides with pixel-based light extraction and high-brightness LEDs, enabled large-sized LCD panels to be edge-lit, as opposed to directly backlit. For more about the PhlatLight technology, see "Rapid Progress in High-Brightness LEDs for Projection" in the September 2007 issue of ID.
The rapid evolution of LEDs in the last few years has brought edge lighting to a new level of performance and efficiency. In 2008, HBLED light sources for LCD backlighting were offering efficiencies of ~ 43 lm/W 2 compared to ~ 40 lm/W for CCFLs. Today, HBLED components are routinely delivering efficiencies > 80–100 lm/W and above (in cool white @ 5500K).
Light guides using white HBLEDs are now being used to backlight larger-sized LCDs with a thinner form factor. The light guides themselves are much thinner – as thin as 3.0 mm at up to a 60-in.-diagonal area for large LCDs.
Light-Extraction Technologies
Edge lighting utilizes several light-extraction technologies, depending on the application.
Printed, chemical, and laser dots are diffuse light-extraction technologies in which light is scattered in all directions (360°). They offer the advantage of not requiring additional films to prevent headlight effects, etc., due to the diffuse nature of the light extracted (Lambertian spread of light), and iterations can be completed very quickly. (A headlight effect is a bright area near the LEDs at the edge of the light guide where the illumination looks like the headlight of a car.) However, brightness is from low to medium for most applications. Output angles cannot be precisely controlled, and any non-transparent dots can block a portion of light that is scattered downward from reflecting back through the panel and exiting the top surface. Figure 3 compares the acceptance angle of light between pixel-based and other methods – printed, chemical, or laser-etched.
Fig. 3: Pixel-based light extraction (top) is compared to other light-extraction technologies.
V-grooves or pixel-based technologies can achieve higher efficiencies in some designs and can control the target direction of the light being extracted, but they require more optical design and development time. And V-grooves, though they offer higher brightness with very good efficiency, are limited in that 2-D uniformity correction (side-to-side correction) is not possible.
A specular, pixel-based light-extraction technology provides high brightness and reflects and transmits light from the optical features in a specular manner. Here, 2-D uniformity corrections are possible, optical features are transparent, and the angles at which light is emitted from the light guide can be controlled. The benefits of this approach include better optical control – especially of color and uniformity – fewer LEDs, better repeatability at all levels, reduced power consumption, and the thinnest possible light-guide package. Table 1 compares some of these light-extraction technologies.
The Push to Thinner and Touch
Every manufacturer wants a more-efficient BLU, and a big part of the efficiency equation is thinner. Light guides are being manufactured as thin as 0.3 mm or less for applications such as keyboards, touch sensors, and small smart-phone-type backlights. GLT, for example, is addressing the push to thinner BLUs (and the challenges this creates) by sourcing thinner and thinner LEDs as they are being released by the LED manufacturers, as well as pushing manufacturing processes in order to manufacture light guides 0.25 mm thick or less. These thickness reductions, however, can lead to challenges with uniformity and brightness, which can be addressed through the use of custom-designed optical-extraction features.
Edge-lit LED light guides employing pixel-based light extraction technology have become so thin; in fact, that they are now being used to backlight touch screens and illuminate touch-enabled display graphics (on/off and function buttons, menus, keypads, directional symbols, etc.) used in applications such as appliances, printers, home-entertainment devices, and multi-device desktop speakerphone systems for office environments with LCD and multiple graphic touch inputs.
In cases where the light guide is employed between a capacitive touch sensor and a top graphic overlay to illuminate the overlay, reducing thickness is a critical parameter in improving the sensitivity of the combined touch-sensitive control. Edge-lit light guides with thicknesses of 0.2–0.6 mm can be used in this application, allowing acceptable touch sensitivity.
The use of low-profile side-firing white LEDs placed along the edge of an ultra-thin light guide in concert with a pixel-based light-extraction technology that spreads the light uniformly across the area to be illuminated has proven to be particularly effective in applications utilizing capacitive touch technology, which transmits approximately 90% of the light from the backlight and has become the touch technology of choice for appliances and a variety of other touch-enabled applications where a slim, touch-friendly backlight is desirable. Now, in many applications, one LED can be used to illuminate multiple graphic icons, eliminating the need to use individual LEDs for each graphic interface or multiple LEDs for larger areas such as company logos (Fig. 4).
Fig. 4: In many touch-enabled display graphics applications, one LED can be used to illuminate multiple graphic icons.
Keeping It Uniform
The use of binned HBLEDs combined with advanced light-extraction technologies and optics go a long way toward providing a truly uniform backlight. An issue that remains, however, is headlight effects. Headlighting can also be seen when using a lens-based light-extraction technology because it will create a direct reflection of the LED in the user's eye. This can be addressed through the use of a diffuser and BEF films on the light-guide panel (LGP) as well as the addition of optical features on the top surface of the LGP, which will break up the user's view of the lens-based features.
One Side? Two Sides? More?
Edge-lit LCDs have utilized side-firing LEDs on one, two, or more sides of the BLU. However, as white HBLEDs become ever brighter and more efficient, the number of LEDs required has decreased. This makes it practical – and more effective – to move all the LEDs onto a single edge of the LGP and keep the LED spacing as close together as possible. This allows the LGP manufacturer to still maintain the smallest possible distance between the LEDs and the visible edge of the output area, and also lowers the bill of materials for the manufacturer. It does, however, make the design of the LGP more difficult, as the light must travel farther down the light guide. This is compensated for by advanced optical light-extraction technology as well as careful material selection, enabling maintenance of a high level of uniformity and efficiency, even with the increased lengths. And in some applications, both in backlighting and general illumination, it is still most efficient to space the LEDs along two sides of the light guide, such as in the 2 x 2-ft. troffer downlight shown in Fig. 6.
Fig. 6: The excellent uniformity of color and luminance provided by the edge-lighting approach is seen in the OL2 Series 2 x 2-ft. downlight, which can be provided in four color temperatures ranging from warm white (left) to cool white (right).
General Illumination
Edge lighting has also made it possible to take existing light-guide technology and expand it into a variety of general illumination applications.
LEDs are becoming more and more commonplace in general lighting, and their benefits over incandescent bulbs and regular hotcathode fluorescent tubes are well known (longer life, less energy usage, greater control and directionality of light, more design flexibility, wider color gamut, and smaller form factor). The proven benefits of edge-lit light guides as BLUs for LCD backlighting are being leveraged to make the LED light source an integral part of light fixtures in general illumination applications rather than a replaceable part to be designed around reducing the time-to-market and increasing the appeal of LED-based light fixtures in general.
Common examples of edge-lit LED-based light guides at work in the general illumination arena include room lighting (ceiling downlights, wall sconces); under-cabinet, splash, and desk task lighting; illumination of industrial and commercial enclosures (refrigerators, for example.); and architectural lighting, as well as in outdoor applications such as pathway illumination and pedestrian traffic signs.
These applications were a natural design evolution for edge-lit light guides, as they were developed to take light from a point source (i.e., LEDs) and provide uniform distribution of it over large areas (Fig. 5). The "large area" has expanded from a cell-phone backlight to an LCD TV to illumination of an entire room or office. (You could, in fact, think of the floor of your office as the back of a big LCD panel.)
The downlight shown in Fig. 5 was designed to provide a brighter, more efficient replacement for 2 x 2-ft. fluorescent lay-in troffers used in recessed ceiling lights, such as those shown in Fig. 6. Switching from fluorescent tubes to LEDs was made easier from a design perspective and more cost effective from a manufacturing perspective because of the design and form-factor flexi-bility and excellent repeatability of edge-lit light guides with embedded optics.
Fig. 5: The cone shows the illumination distribution from a light source – in this case, a 2 x 2-ft. LED flat-panel downlight employing edge-lighting technology, with 100 LEDs spaced along two sides of the light guide for optimal light dispersion – at a distance of 1, 2, and 3 m (roughly 3, 6, and 9 ft.) over four different color temperatures ranging from warm white (3000K) to cool white (6000K).
Many companies are also starting to realize that in order to take advantage of the true benefits of an LED package, they have to design the luminaire to work with the LED rather than trying to take a luminaire that was designed to use a florescent bulb or an incandescent lamp and trying to retrofit it to LEDs.
Measuring Efficiency
The edge-lighting approach achieves maximum efficiency in light dispersion. But how do you measure efficiency? Application efficiency – i.e., the actual amount of light delivered to the targeted area in relation to the total light output of the fixture – is the true measure of performance. It includes
• Luminous efficacy (lm/W)
• The optimal efficiency of the total fixture
• Ray-angle control
Luminous efficacy, sometimes referred to as brightness, is measured in lumens per watt (lm/W) and indicates how well a light source produces visible light. Lumens per watt are calculated based on the total output of light from the final product as measured within an integrating sphere versus the power that is input into the product. Currently, the target for most BLU suppliers is 70 lm/w because this is the requirement to achieve Energy Star performance on a luminaire. Luminous efficacy is one component of the luminaire's total application efficiency. In order to achieve high application efficiency, a lighting fixture must direct as much of the light to the target area as possible instead of scattering it in all directions, focusing on the target area to avoid wasted light emissions. That is where ray-angle control comes in. Specular optics embedded in the light guide can direct the light in the specific direction desired. As shown in Fig. 5, pixel-based light-extraction technology can be optimized to deliver light within the acceptance angle of the optical system. In order to reach maximum application efficiency, the luminaire manufacturer must work with each supplier in the system to achieve the highest possible efficiency. This includes those responsible for the power supply, LEDs, drivers, LGPs, and films.
Luminaires Leading the Way
The luminaire is where LED-based solid-state lighting is going to really succeed because that is where LEDs can be most elegantly used, as opposed to being retrofitted into an old light-bulb form factor. Over the last year, products have emerged that represent a redesign of the entire luminaire rather than just the light source. For example, GLT, in cooperation with luminaire manufacturers, has developed recessed ceiling lights that involve a top-down design of the entire luminaire around the end light source – the LED. One of the fixtures that has seen growth in the last year are the industrial 2 x 2-ft. troffer-type replacements. When designed as an entire fixture to take advantage of the LEDs, efficiency increases significantly. Some of these luminaires have efficacies greater than 60 lm/W with a power consumption of 45 W, CRI of ≥75%, and lifetimes of 30,000 hours.
As in everything, there is what people say they would like to have and what they are willing to pay for. When the author's company shows customers the price differential between a luminaire with 50-lm/W luminance and a CRI of 80 at a unit cost of approximately $125, and a luminaire with 60 lm/W and a CRI of 85 for approximately $175 per unit, many choose the lower cost module without hesitation. For high-volume products, the most important specification is often price, although that involves several factors. For example, pricing for GLT's UL-certified OL2 Series troffer 2 x 2-ft. downlight assemblies begins at about $122 each in volumes of 100 pieces, compared to $45-80 per unit for similar conventional fluorescent troffers. Given the much greater lifetime of LEDs and the effective elimination of costs for re-lamping, customers have found the LED troffers to be very competitive.
In general, edge lighting using LED-based light guides lends itself well to cost-efficient manufacturing in high volumes. This is made possible due to the use of fewer LEDs, advanced light-extraction technologies that extract light precisely where needed to provide bright, uniform light in a thinner form factor without hot spots or dark areas, the excellent repeatability of light guides, and the approach of designing around the LEDs rather than trying to retrofit them to designs meant for bulbs or tubes, as well as production in multiple dedicated facilities in Asia. All these factors have combined to make the production of not only the most efficient light guides for LCD backlighting a practical reality, but entire assemblies for general illumination, and at competitive prices. Moving forward, the author hopes to see lighting companies working with LED, driver, and light-guide manufacturers to form more partnerships rather than continuing to operate as completely separate suppliers. Such cooperation will enable a truly efficient system for presenting LED-based general illumination products to the market.
References
1Source: electronicsfeed.com, Feb. 2, 2012, Evertiq New Media AB.
2Source: "Advances in LED Technology for LCD Backlighting", LED Journal (May/June 2008). •