IOndputsimtryi zdienmga nbda fcokr ltihginhnteinr,g m woriet-he fLficEieDnt ebdacgkelig-lhigtsh isti nbrg inging to the fore edge-lit illumination solutions that use fewer LEDs
By Brett Shriver, Vice President, Sales & Marketing Global Lighting Technologies, Inc., Brecksville, OH, USA
Superior light-extraction (EL) backlighting, but with all used in a variety of consumer, efficiency, combined with an the advantages of today’s industrial, automotive, and optimum backlight-unit (BLU) advanced LEDs combined with medical displays, as well as in system design, maximizes the superior light extraction. the larger LCDs used in advantages provided by the notebook computers, desktop latest advances in LED LEDs have moved well beyond monitors, and flat-panel TVs. technology. Those advances their established turf in the Today more than ever, have been considerable, with portable/handheld arena and designers and manufacturers of LEDs rapidly become thinner are the standard in midsize these devices are leveraging the and more efficient. LCDs (3.5 to 7 in. diagonal) advantages of LED backlighting: wider color gamut, longer life, dc The result of these advances is power with no inverter, lower smaller, thinner, and more power consumption, greater efficient LCDs, keypads, and design flexibility, a smaller form touchscreens for handheld factor, and lower cost. And, of devices, as well as thinner course, LEDs’ mercury-free portable and desktop computer construction and longevity offer LCDs, with ultrathin keyboard a “green” environmentally backlighting. The latest designs friendly backlight. provide high luminance and uniformity in virtually any color New solutions beyond display all with fewer LEDs. Thinner, backlighting applications more lightweight BLUs with include edge-lit light-guide units lower manufacturing costs for exit signs and all manner of including backlight assemblies egress lighting as well as edge- less than 0.6 mm thick utilizing lit illumination for downlights light guides as thin as 0.25 mm such as task lights and troffers. are now a reality (see Fig. 1). New design approaches such BLUs have reached the Fig. 1. BLUs have achieved amazing as edge-lighting are enhancing thinness while providing lighting thinness of electroluminescent uni formity as well as brightness. these benefits.
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Edge-lit LED backlighting To maintain optimum efficiency, the light guide thickness should Edge lighting employs side-firing be matched to the output profile high-efficiency LEDs that focus of the LED. As LEDs are the light into a high-performance, designed with thinner profiles, very thin light guide. There are the thickness of edge-lit light several light-extraction guides will continue to decrease. technologies utilized: printed, However, manufacturing etched (using chemical, laser, efficient light guides in very thin or other means), V-groove, and profiles requires the latest in pixel-based, each of which can Fig 2. Using a lens array manufacturing and optical light- configuration like the one shown be used to provide the optimum extraction technology. permits a reduction in the number solution for a particular of LEDs required while still application (see Table 1). achieving a uniform backlight. angular output distribution of the Table 1. Comparison of edge-lighting methodologies light within the light guide to Technology Efficiency 2D pattern possible provide for a uniform visual appearance using a reduced Printed dots Low Yes number of light sources while Etched dots Good Yes minimizing the hot spots that can occur when an LED is too close V-grooves Very good No to the viewing area. Using lens arrays permits a reduction in the pixel-based Very good Yes number of LEDs required while still achieving a uniform backlight. Modeling with lens arrays This enables savings in cost, Since the LEDs are located on space, components, and power. the edge of the light guide, this A key technique used to approach offers benefits that enhance uniformity is lens arrays For the examples shown in Fig. include better optical control (see Fig. 2). Lens arrays are light 3, two side-emitting high- especially of color and distribution features that are brightness LEDs with optimal uniformity fewer LEDs, better located on the input edge of the light coupling to the edge-lit repeatability at all levels, light guide (in front of the LED). light guide were chosen. Then, reduced power consumption, These arrays increase the baseline optical modeling was and the thinnest possible LED lighting solution.
Edge-lit backlighting allows for color mixing within the light guide. This not only eliminates the LED-to-LED deviations that can be evident in the direct lighting (light array) approach, but also makes edge-lit light guides very adaptable to today’s multi-function displays, allowing designers to use fewer Fig. 3. By using customized optical lenses, the natural spread of light in the LEDs and reduce costs. original design (top) is enhanced to provide a broader spread of light (bottom).
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performed on the system to brightness and uniformity determine the natural spread of along with the ability to utilize light. An array of customized fewer LEDs, thereby reducing optical lenses was then added the cost and power to improve the usable consumption of the backlight. distribution of the light from the The backlight design utilizes LEDs and to spread the light pixel-based light extraction more widely than it was in the technology and optical manufacturer’s original design. engineering to increase the Fig. 4. Light blades can be efficiency of light transmittal constructed using a 0.2-in.-thick edge- Fig. 3. By using customized through the keyboard’s key lit light guide (above). The drawing optical lenses, the natural caps, increasing brightness (below) shows the construction of a 3 x 42 x 0.3-in. light blade that uses a spread of light in the original while creating a more uniform light guide edge-lit by 8 LEDs; this design (top) is enhanced to illumination. design approach can be used in many provide a broader spread of different sizes and LED configurations light (bottom). Beyond displays for various applications. Backlighting applications The benefits of edge-lighting are by no means confined to Portable computers have widely backlighting LCDs or keyboards. adopted edge-illuminated Edge-lit molded light guides backlighting with white phosphor offer an extremely flexible design LEDs. For such applications, a that allows them to be used with blue LED is imbued with an various light extraction amber phosphor to convert the technologies and molded to a visible wavelength to white. wide variety of sizes for use in The white LEDs still deliver a general illumination applications percentage of the color such as egress lighting; spectrum comparable to CCFL downlighting; under-cabinet, to illuminate areas over 60 in. (approximately 75% compared splash, and desk task lighting; diagonally while maintaining a to 70% to 80% for the CCFL) refrigeration lighting; and very thin profile. while providing lower power cabinet illumination (such as consumption. medical cabinets). New advances in light extraction efficiency and manufacturing Traditionally, these backlights The edge-lit panel in Fig. 4 capabilities, coupled with lens have been 2 to 3 mm thick; shows a single light blade that is arrays and optical modeling however, with advancements in thin and very efficient at techniques, are using the LED and light guide technology, providing diffuse lighting with an increased efficiency of today’s backlights as thin as 0.4 to 0.6 indirect light source. It can be LEDs to create edge-lit light mm are currently being created custom designed for a variety of guide units from small (0.24-in.- to allow for reductions in the applications, such as troffer diagonal) to large (60-in.- total weight and thickness of downlights, under-cabinet and diagonal) sizes. They are these devices. desk task lighting, and growing steadily thinner and refrigeration illumination. The require fewer LEDs to suit a New, thinner LED-based blades can be “tiled” to facilitate wide range of applications, from backlights are also being large-area illumination or display backlighting to general designed into portable device manufactured in vary large sizes illumination. keyboards, providing optimal so as to use a single light guide
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