Dr. M. Nisa Khan Discusses LED Sign-Lighting Challenges

Lighting is a subjective consumption item
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LEDs’ energy efficiency and electronic prowess has misguided consumers, business owners and marketers as much as LED lamps’ blinding glare will off-track them. As LED lamp efficacy, cost and electronic controllability have improved, the signage and general illumination industries have unhesitatingly flocked toward adopting LED lamps as replacements for their conventional lighting products. However, users and sellers have realized the challenges LED lamps are bringing and the repercussions that can arise from certain types of LED lamp usage. It isn’t popular to talk about LEDs’ challenges; dampening a growing business practice is not a well-liked idea. Add that lighting is a subjective consumption item, much like food and clothing, and that certain detrimental effects from lighting may only become obvious after the passing of time. In truth, lighting could parallel what we have seen in with the fast- and inorganic-food industry, i.e., affecting people’s health decades after the introduction of such conveniences.
Different challenges for signage and lighting
So what are the LED illumination challenges? They are, of course, different for sign illumination and general lighting. Even within the LED signage and display industries, the challenges are distinct and require various mitigation techniques. For example, LED lamps used for display backlighting and channel letters are not exposed to the eye; therefore glare is not an issue. However, certain LED EMCs can have annoying glare, but this problem can be fixed with proper design and operational-management schemes.
However, serious problems abound with LEDs used for general lighting because we’re exposed to general-light appliances for much longer periods of time than we spend looking at illuminated signs. In addition, the 3-D aspects of general lighting play a more significant role for our vision, which is not so when viewing signs.
This unique dynamic is rather difficult to demonstrate to a general audience, but experienced artists, filmmakers, photographers and architects easily recognize the appearance differentiations created through 3-D-lighting and color-balance variations. Most challenging is that current LED lamps, from a
3-D perspective, create undesirable illumination and often render color imbalances.
LED lamps face challenges because the basic light sources are 2D, i.e., flat, which produces enormously high luminance and directional light that, when viewed unexposed, leads to blinding glare. Although the glare diminishes when the LED modules are covered with translucent material, the directionality remains, and such light distribution produces unnatural and undesirable illumination, especially for lighting 3-D objects. Unless you are artistically inclined, the immediate negative affect is quite subtle.
Although some LED lamps now have impressive color-rendering indexes that exceed 90, their light distribution is still not ideal for 3-D object illumination or for comfortably illuminating such large volumetric spaces as lecture halls, museums, theatres and auditoriums. It’s because the total-lumen output and luminous efficacy properties, along with certain beam-angle specifications currently used by the LED industry, are still not sufficient for judging light quality. They fail to encompass the comprehensive set of lighting metrics needed to ensure quality illumination.
Is this a new phenomenon? Yes. Traditional lighting products did not have to be subjected to a comprehensive set of lighting metrics that would ensure desirable 3-D illumination because they were uniformly omnidirectional by nature and did not generate 2-D directional light. Consequently, it didn’t matter if manufacturers’ quantitatively demonstrated if the lamps’ 3-D luminous-intensity distribution covered all solid angles uniformly.
However, it should matter for the 2-D LED lamps, notwithstanding that such measurement, with adequate resolution for the near-field intensity data, is notoriously difficult to accomplish and subsequently interpret, especially for people who aren’t technically savvy.
Color temperature, color rendering acuity requirements for tasks, as well as color-spectrum suitability for day and night lighting, are important for our vision and general health. Although these various aspects remain under scientific investigation, strong evidence reinforces the adverse effects on our circadian rhythms from nighttime consumption of certain wavelengths of blue light.
Thermal challenges
A decade ago, many unknowing commentators said LEDs do not produce heat. Although “cool to the touch” and more efficient than incandescent and gas-discharge lamps, LEDs produce a fair amount of heat, but in small, light-generating areas. This heat creates thermal challenges because, if not conducted away from the diode’s p-n junction, it shortens the LED lifespan. Because various LED modules and chips suit numerous applications, thermal-management complexity ranges from low to very high. For traditional channel-letter illumination with sparsely positioned, low-power LED modules
in place, little or no heat management may be required.
In contrast, extensive thermal-management schemes – usually protruded, heat-dissipation fins – are required for LED replacement bulbs for A19, T5 or T8 fluorescent lamps, as well as high-bay lamps and luminaires.
Although certain groups recognize that decreasing the diode junction temperature increases lumen output and prolongs the lamp’s lifespan, my research shows that generated heat is dissipated via the lamp-engine construction and the extent of lumen-output deterioration during operation.
Unsurprisingly, the results show the dominant heat dissipation effect is conduction rather than radiation and convection, and that only through adequate heat conduction can substantial amounts of heat be quickly removed from the diode junction. This says that the time to reach thermal equilibrium can be reduced if heat-conduction paths are maximized. Doing so, however, places a restriction on how LEDs are mounted on heat sink boards, which further complicates thermal management for LED modules mounted on multiple vertical boards that are designed to generate omnidirectional illumination.
Fig. 1 shows measured and calculated results for two different categories of LED-based, Edison-style A19 bulbs (Samsung and Cree) and verifies that the Cree design, with LEDs arrayed on multiple boards, requires longer periods to reach thermal equilibrium, which causes it to undergo additional lumen depreciation. Details of this work can be found in my peer-reviewed article published in Applied Optics, February 2016 (www.osapublishing.org).
LED lamps need not be ensnared by the challenges discussed here, but recognizing why current LEDs remain unsatisfactory for different applications and which aspects need improvement is critical. Once the various pitfalls are diagnosed, manufacturers need to get serious about adopting solutions and enhanced technologies.