Though often drowned out by the excitement generated by general illumination LEDs, LED backlighting technology has evolved dramatically over the past 30 years. Continued advances in design and performance have allowed each new generation of LED backlighting technology to significantly grow the number of relevant applications. This trend continues in 2012, with the introduction of cutting-edge flexible backlight technology which allows LED backlights to be cut in custom shapes and used on curved surfaces, opening the door to a number of new applications.
A look back at progress achieved in each of the early stages of technology development makes it possible to appreciate the exciting promise of the latest generation of LED backlights.
Evolution of LED Backlighting Technology
Stage 1: Early 1980s – Edge-Lit LED Backlights
In the early 1980s, the first generation of LED backlight technology was used mainly for backlighting of LCDs. Since LCDs do not produce light themselves, they need illumination to produce a visible image. The first generation of LED backlight was a format known as “edge-lit” where the LEDs illuminated the LCD from the side of the display panel to light up the whole viewing area.
The technology provided several key benefits over existing alternative technologies such as cold-cathode fluorescent lamps (CCFL). Compared to the commonly used CCFLs, the LED backlighting technology provided up to 10-times longer life span, up to 70% lower power consumption, and the ability to eliminate the traces of mercury found in CCFLs.
However, early generations of LED backlighting also suffered from a number of performance drawbacks. The die used in the LED technology was not efficient or high-output and as a result the brightness achieved was only in the ballpark of 25cd/m2. Also the edge-lit format created hot-spots on the display, resulting in non-uniform light distribution. Finally, color options for backlighting were limited to only three choices – red, green and yellow.
Stage 2: Early 1990s – Chips-On-Board Backlights
About a decade later, a second generation of LED backlight technology was introduced. Applications still focused largely on LCD backlighting, however, a new format of backlight called “chips-on-board” or “COB” was introduced. With the COB format, the LED die is wire-bonded to the printed circuit board (PCB) in a matrix array. A diffuser tape is added to the reflective frame to evenly distribute light over the viewing area. This approach provided significant engineering and performance benefits.
From an engineering perspective, the COB format doesn’t require reflow soldering like previous backlights that used packaged surface-mounted (SMD) LEDs directly behind the required viewing area, thus removing potential reliability issues associated with high-temperature reflow soldering. The COB format further simplifies production by incorporating a backlight, reflective boarder, chips and diffuser as a single unit (compared to SMD backlights where reflective borders need to be added in a separate secondary process after soldering). Each of these processing advancements reduces manufacturing costs by as much as 10 to 30%.
The COB format provided significant performance benefits over early edge-lit technologies. Light could be more uniformly distributed as hotspots were eliminated. Also, as there is a larger surface behind (rather than on the edge) the LCD, more LEDs could be applied providing brighter displays. Early COB technology provided brightness of up to 200 cd/m2. Finally, color consistency was improved as it was now possible to select die directly off the wafer with LED binning, eliminating mismatched LEDs.
Stage 3: Mid to Late 1990s – More Efficient LEDs
Throughout the mid to late 1990s, LEDs grew increasingly more efficient, creating even brighter backlight technologies. As performance improved, particularly with edge-lit format LED backlights, the technology was integrated into a growing number of applications. Still used widely for LCD backlighting, LED backlights also began to be used for accent or indication lighting applications such as backlighting “exit” or “on-air” signs, or providing accent lighting for artwork.
During this time period, edge-lit format backlights became more compact in size allowing for greater design flexibility. With the introduction of high brightness LEDs, the overall height of the edge-lit backlight was reduced by up to 70%. The introduction of the more efficient LED die made it possible to achieve greater brightness with fewer die and reduce the size of the backlight technology.
Starting in the late 1990s, full RGB color options became available which made it possible for backlights to be designed in any color. Brightness enhancement films (BEF), in combination with the new high-brightness LEDs, significantly enhanced the visual performance of edge-lit backlights and made them more competitive with COB formats.h
Enhancements made possible by texturing reflectors and/or film, combined with higher brightness and smaller size, allowed this new generation of edge-lit technology to compete with COB backlights in terms of light uniformity and light output. Edge-lit backlight technology provided the additional benefits of lower power consumption and lower costs.
Stage 4: Early 2000s – Introduction of White LEDs
In the early 2000s the development of high-quality warm, cool, and neutral white LED technologies further expanded the number of potential applications for LED backlights. LED backlights could now begin to replace CCFLs in many applications including cell phones, computers and TVs.
At the same time, the technology behind RGB LEDs was further improved to allow for thinner profiles, enhanced color mixing, and plug-and-play installation while becoming increasingly cost-competitive with CCFL technology.
In addition to backlighting, indication and accent lighting applications, this generation of LED backlights created a new purpose for backlighting technologies — brand differentiation. For example, logos could be backlit in a brand-specific color, heightening brand awareness. The superior performance and wide color options combined to allow backlighting to play a more central role in product differentiation and branding.
Between 2000 and 2010, new LED backlighting applications continued to emerge. Due to their high brightness performance, backlights were also increasingly adopted for small space and task lighting applications including cabinet illumination, refrigerator down-lights, and safety/exit lighting.
Latest Generation LED Backlighting Technology
Stage 5: 2010-2012 – Introduction of Flexible Backlight Technology
In late 2010, an entirely new generation of LED backlighting technology was introduced worldwide. This latest generation of cutting-edge backlight technology features a bendable/flexible format and represents an exciting and completely new era of LED backlighting technology.
Flexible LED backlighting technology features an ultra-thin profile and bendable format that allow LED backlights to be used for the first time on curved surfaces, dramatically expanding the scope of potential applications.
Lumex QuantumBrite™ Flexible Backlight LEDs, for example, offer a low profile in an ultra-slim 0.125mm thickness compared to 5mm for COB and 2- 3mm for edge-lit technologies. In addition to its thin profile, the new flexible backlight technology is exceedingly durable and shadow resistant, even when a hole is poked through it. While the brightness of the flexible backlight is not comparable to the COB backlight, it still has the same brightness performance as the old edge-lit version at 90% less thickness.
The latest update to this innovative technology (to be introduced worldwide during Q2 2012) can be cut to shape in any custom size for unparalleled design flexibility. This allows the flexible backlights to be used to illuminate an untold number of curved surfaces that were previously not compatible with LED backlight technology. Potential applications are nearly endless but can include previously unimaginable applications such as backlights to illuminate knobs, switches, or clocks, inexpensively. The ability to cut to shape has also enhanced product durability. If, for example, a hole is pierced into the new custom-shape, it will not create any kind of shadowing in the light performance.
It will be exciting to see how brands worldwide incorporate this latest generation of LED backlight technology.
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