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Dr. Nisa Khan Tells of LED’s Next Step

High-quality LED signage, display and lighting improvements remain distant.

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As president of IEM LED Lighting Technologies, Dr. M. Nisa Khan consults in the solid-state lighting industry and educates consumers about LED lighting. She has a bachelor’s degree in physics and mathematics, and master’s and Ph.D. degrees in
electrical engineering. Email her at [email protected]

LED display buyers should exercise caution when acquiring low-price devices, because they typically lack high-quality LEDs and sophisticated electronic-control systems.

Unquestionably, modern LED lamps offer unique benefits in many signage applications, but they still face quality, cost and dependability challenges, despite the increasingly impressive LED-based display, signage and lighting technology developments and any accompanying marketing efforts. Today, interestingly, many early nay-sayers have switched to LED lamps for both replacement and new-installation applications. Further, the technology has provided new business opportunities for distributors and retailers and, in turn, many of us have become LED-product consumers. Also, in addition to signage, lighting and displays, the LED industry is making remarkable inroads in numerous other application areas. Health, agriculture, and security, are such examples.
Writer Heather Clancy, in her December, 2013 Forbes magazine article, “Why LED Retrofits Are The Trojan Horse of The Internet of Things,” predicted LED lighting integrating into Internet-of-Things (IoT) systems as an outlier revenue source. Her primary themes comprised:
-Reduced LED prices and increased LED use;
-Expansion of lighting-control systems;
-LEDs’ inherently controllable nature and easy integration into light-management systems; and
-LED luminaires’ use as carriers for such “smart” sensor systems as environmental-, surveillance- and promotional-lighting sensors.

Clancy referenced a parking-lot owner who decreased power bills and simultaneously increased security by installing a surveillance system integrated with the LED lighting.
Although integrating such technologies may succeed, we still face LED lamps’ general illumination quality,which hasn’t been addressed properly. Nor have desirable solutions been realized.

LED lighting shortcomings
Because lighting and illumination are both subjective, the practice of wrapping “bells and whistles” around off-the-shelf LED lamp applications has been an effective sales method. For example, novelty light systems that provide different color temperatures from the same LED lamp appear more intriguing to shoppers than enjoying glare-free, omnidirectionally uniform light distribution, such as that provided by standard, Edison-type, incandescent bulbs.
Light directionality is advantageous for LED-lamped, signage-type displays, both for backlighting screen systems and in the facings of electronic message centers. Unfortunately, both display types can become glary if the lamp intensity is brighter than a human-comfort level; they can also appear uncomfortably bright against a dark background.
Simply applying color tuning or dimming adjustments to LED displays can optimize any ambient-light conditions. However, the preferred systems feature high-quality LEDs, those which feature high-luminous efficacy, brightness and a long-life span. Such systems can be dimmed to span broad luminance ranges, adjusted for color gamut, and balanced for viewing comfort while maintaining color fidelity. Contrastingly, low-quality LED modules could cause dim lighting with undesirable color variations. Similar adverse effects may arise due to inadequate electronic-control systems.
LED shortcomings, compared to other applications, are more easily solved in display and signage products, because both makers and buyers better understand the brightness, glare and color parameters. The signage and display industry is also more mature and accustomed to LED light measurements for required parameters; it can also implement electronic controls that handle various, ambient-lighting conditions.

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Defining LED light
The general lighting industry still struggles to define and understand LED lamps’ glare. Essentially, glare is related to luminance, which translates to brightness in layman’s terms. Further, the lighting industry hasn’t quantified brightness properly because its leaders aren’t acquainted with the complications. The problem is magnified for solid-state lighting because many lighting designers and engineers aren’t willing to measure LED luminance, because of inherent difficulties in defining and measuring it.
LED lamps’ luminance lacks uniformity because the lamps are constructed with discrete LED chips or modules with gaps between them. Typically, light-emitting elements with periodic gaps create a non-uniform luminance distribution at the lamp surface that’s difficult to measure. The more recent, chip-on-board (COB) LED technology permits reduced gaps between LED chips, but the luminance from these COB constructs is extremely high and directional. In addition, COB thermal management is significantly more difficult to accomplish.
In the lighting field, glare is only empirically defined to some degree; however, there is no consensus and, globally, the industry hasn’t set glare standards. Therefore, many LED lamps, including car and truck lights, expectedly emit excessive glare. As a correction, some lamp units use thick, translucent covers to reduce glare, but such practices decrease luminous efficacy and, you’ve surely noticed, the light doesn’t have a uniform distribution over any broad angular range.

Light quality
In contrast to LEDs, incandescent, fluorescent and neon lamps are uniformly omnidirectional. Such uniformity in the lamps’ illumination provides a soothing appearance for the human eye. In addition, such lamps and luminaire systems uniformly span over broad angles to produce natural object illumination.
Although rarely discussed by consumers, acceptable light quality requires uniform-light distribution over broad angular spans, minimum or no glare, dependability and acceptable color properties for the intended use.
Today’s LED lamps lack such quality characteristics. Dependability remains an issue because many LED lamps are distributed by manufacturers that lack the proper, solid-state-lighting development platform required for producing high-quality LED chips.
Today, achieving basic light qualities from solid-state lighting devices requires the manufacturer to have a good understanding of light and lighting science. Despite remarkable LED technology improvements and product developments, achieving quality, LED lamps suitable for general, human-centric illumination is conspicuously difficult. To get past any barriers, profit-seeking manufacturers unashamedly offer energy-efficiency (“green”) marketing pitches, and add various functionalities to overshadow the basic, lighting-quality issue.
The most human-centric lighting characteristics are lack of glare, a good color-rendering index (CRI), correlated-color temperature (CCT) ratings and broad, light distribution in a homogeneous fashion, so that surrounding objects look natural. Can such functionalities as variable CCT, dimming and remote controls outweigh these basic light qualities?
Not really.

Orthogonal rays
The reason inorganic LEDs are glary compares to why the sun naturally emits orthogonal rays from its circular boundary (Fig. 1). Some diffusion around the sun comes from the aperture affect. But the long and strong rays rising orthogonally to the sun’s surface are due to the Divergence Theorem, which says energy contained in an object with bounded volume is released from its surface in the direction orthogonal to the object’s surface.
How does this relate? Inorganic LEDs are very flat and thin, and the light energy is released orthogonally to the LED chip’s surface, which produces concentrated and directional light with high-glare characteristics. Light emitted from such an LED source only has one dominant direction — orthogonal to the chip’s flat surface.
To produce more human-centric LED lamps for use in signage and space-illumination applications, the light-emitting surface must be curved so the light emission is less direct and glary. It needs to be more broadly distributed. Manufacturers could accomplish this by appropriately designing secondary, optical elements that follow the LED chips into a luminaire ensemble. An entire LED luminaire that incorporates such a design, and is also optimized for thermal management and other aspects, could provide a complete, high-quality and dependable light source. Next-generation LED lamps are expected to bear these features.

Figure 1: The diverging rays, orthogonal to the sun’s surface, illustrate the Divergence Theorem in nature.

 

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