This article was published in the November/December 2011 issue of LEDs Magazine.
View the Table of Contents and download the PDF file of the complete November/December 2011 issue.
Planar light fixtures offer aesthetic advantages in many applications and the inherently-diffused concept can efficiently deliver the required light to a task plane. The potential market for such fixtures has driven the solid-state lighting (SSL) industry to pursue several approaches to planar lighting including OLEDs and panels that are edge- or back-lit by LEDs. Technology with an LCD-panel backlight unit (BLU) heritage can enable planar fixtures and may prove the best option in such luminaires in terms of light control and application efficiency.
In general, the introduction of LEDs to the application of general lighting presents lighting architects and fixture designers with unprecedented flexibility that can't be achieved with the constraints of bulbs and tubes. LEDs can slash energy use and offer greater control and directionality of light. Moreover, the light source can now become an integrated part of the fixture itself rather than a replaceable object to be designed around.
In the BLU segment, meanwhile, the growing popularity of LEDs has driven innovations in the LEDs themselves, and has yielded new LED subsystems in the BLU that control and distribute the light. These subsystems include advanced optics that enable precise control of the uniformity and emission angle of light out of the BLU, while helping maximize the amount of light delivered through the LCD panel.
With shared needs for lower power and cost, longer lifetimes, improved color quality and the desire to eliminate light sources that contain hazardous materials, the lighting industry can benefit from the advances of LED backlighting in the display industry. The proven technologies originally implemented in display backlights can be leveraged to accelerate the time-to-market and adoption of LED-based light fixtures.
Thin is in
Indirect fixtures offer a softer aesthetic by bouncing the light off a surface, such as a back reflector or a ceiling, before illuminating the desired area. While indirect fixtures avoid the glare drawbacks of direct-lit architectures, they do so at the cost of efficiency. A portion of the light is lost through absorption by the surfaces used to reflect it, as well as that lost when propagated in undesired directions.
A third alternative is the edge-lit architecture that places LEDs along one or more edges of a light guide whose principal function is to direct and distribute light as desired. Light guides are typically made of thermoplastics such as acrylic or polymethylmethacrylate (PMMA). Light emitted from the LEDs is directed into the edge of the light guide and distributed throughout using the properties of total internal reflection (TIR). Different types of optical elements that are printed, etched or embedded into the light guide are then used to extract the light via the properties of refraction (Fig. 2).
Forming optical elements
All of these various patterning techniques for edge-lit fixtures can produce good uniformity, but they have a wide range of optical efficiencies, tooling costs, manufacturing costs, and optical characteristics. Printed and chemically-etched dots offer low-cost manufacturability, but produce diffused light outputs that offer virtually no control of the beam pattern. Laser-etched optical features provide improved optical efficiency, but require long manufacturing cycle times. Light guides with embedded optics combine specular reflection with fast and highly-repeatable manufacturing to deliver the highest level of ray-angle control and lowest overall cost.
In many ways, edge-lit architectures combine the benefits of direct and indirect fixtures while also providing unique benefits. They offer high optical efficiency, control of the light distribution, reduced number of LEDs, and superior aesthetics. The adoption of edge-lit BLUs has enabled sleek, ultra-thin notebook computers, tablets, monitors and HDTVs that are less than 0.3 inch thick at a lower cost than direct-lit counter parts. Those same benefits can be leveraged for lighting. Light guides with embedded optics can be implemented in countless shapes and sizes with a wide variation of light distribution patterns. Edge-lit fixtures will usher in a new era of efficient, flexible and beautiful products. But with these new capabilities and flexibility of form factor, what is the best method for comparing relative performance and efficiency?
Introducing a new metric
With the introduction of energy-efficient light sources, such as fluorescents and LEDs, the reference to the wattage of a bulb is no longer an acceptable means of expressing the light output. In an effort to ease the transition, the term "watt equivalent" has been used to provide a reference. For example, a typical 60W incandescent bulb generates about 800 lm. But a 60W-equivalent LED bulb uses less than 12W to produce the same amount of light. The use of lumens has also been incorporated to describe the luminous flux or light output of a light source. Luminous efficacy, measured in lumens-per-watt (lm/W), is an important metric when describing the efficiency of a light fixture, but it doesn’t tell the whole story.
Lumen output provides an accurate representation of traditional light sources as it is an averaged value used to measure total light output from an omnidirectional light source. However, LEDs are directional, so these metrics fail to accurately depict the true benefits of LEDs as general light sources. By using lumen output as a measurement for LED-based fixtures, the actual amount of light the luminaire is capable of delivering to a specific surface can be misrepresented.
In order to truly maximize the amount of light delivered to a desired area with the least amount of energy required, designers must focus on the application efficiency of the fixture. Application efficiency is the percentage of light delivered to the targeted area as it relates to the total light output of the fixture. In other words, the amount of light emitted from a fixture and directed to a specific surface. In order to achieve high application efficiency, a fixture must combine high luminous efficacy with the ability to direct as much of the emitted light to the desired surface, or area, as possible (Fig. 3).
Application efficiency depends on three elements: the efficacy of the LEDs, the optical efficiency of the total fixture, and the degree of ray-angle control provided by the fixture’s optics. Today’s high-brightness LEDs deliver upwards of 140 lm/W, but not all of the light emitted from the LEDs is effectively delivered to the fixture. Special design techniques are required in order to maximize the efficiency of the interface between the LED light source and the light guide.
There are additional losses introduced by the fixture inherent in the LED driver used to condition the power supplied to the LEDs, the thermal-management system used to cool the fixture, and the optics used to extract light out of the fixture. A system-design approach which takes all of these elements into account is required to maximize the overall efficiency of the light fixture.
Finally, light emitted by the fixture in undesired directions is effectively lost. The ability to control the light delivered from the fixture to the desired area depends on the nature of the optics used in the light guide. While diffuse optics cannot direct the light in a specific direction, specular optics embedded in the light guide, such as MicroLens optics, can. As a result, more light can be directed to the desired surface, with less light straying to an unneeded area.
A system approach
By optimizing the LED efficacy, optical efficiency, and driver design, and then providing ray-angle control, fixture designers can achieve maximum application efficiency to reach their end goal of creating a productive, functional space with the least amount energy.
In the display market, edge-lit architectures are the leading way to employ LEDs for BLUs. They deliver efficiency, thin displays, and low cost. Ultimately, these same benefits will translate to edge-lit architectures becoming the preferred solutions for lighting. The added benefit of form-factor flexibility means edge-lit solutions offer tremendous freedom of design. Lighting designers no longer need be constrained by the limitations of legacy bulb and tube-based fixtures. Nor should they evaluate lighting solutions only in terms of legacy measures. By designing with maximum application efficiency in mind, they can create beautiful, functional spaces with fewer fixtures and lower energy consumption.